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Paradigms in Eastern Spruce Budworm (Lepidoptera: Tortricidae) Population Ecology: A Century of Debate
Abstract
Three main hypotheses have been postulated over the past century to explain the outbreaking population dynamics of eastern spruce budworm, Choristoneura fumiferana (Clemens). The Silviculture Hypothesis first arose in the 1920s, with the idea that outbreaks were driven by forestry practices favoring susceptible softwood species. In the 1960s, it was proposed that populations were governed by Multiple Equilibria, with warm weather conditions releasing low-density populations from the regulatory control of natural enemies. Dispersal from outbreak foci, or " epicenters, " was seen as causing widespread outbreaks that eventually collapsed following resource depletion. However, in the 1980s, following the re-analysis of data from the 1940s outbreak in New Brunswick, this interpretation was challenged. The alternative Oscillatory Hypothesis proposed that budworm population dynamics were governed by a second-order density-dependent process, with oscillations being driven by natural enemy–victim interactions. Under this hypothesis, weather and resource availability contribute to secondary fluctuations around the main oscillation, and weather and moth dispersal serve to synchronize population cycles regionally. Intensive, independent population studies during the peak and declining phases of the 1980s outbreak supported the principal tenet of the Oscillatory Hypothesis, but concluded that host plant quality played a more important role than this hypothesis proposed. More recent research on the early phase of spruce budworm cycles suggests that mate-finding and natural-enemy-driven Allee effects in low-density populations might be overcome by immigration of moths, which can facilitate the onset of outbreaks. Even more recent research has supported components of all three hypotheses attempting to explain spruce budworm dynamics. In the midst of a new rising outbreak (2006-present), we discuss the evolution of debates surrounding these hypotheses from a historic perspective, examine gaps in current knowledge, and suggest avenues for future research (e.g., intensive studies on low-density populations) to better understand and manage spruce budworm populations. The ecological mechanisms that drive periodic oscillations in animal populations have been a subject of intrigue and debate among population ecologists for at least a century (Elton 1924, Graham 1939, Berryman 1996, Liebhold and Kamata 2000). Outbreak-prone her-bivorous insects have attracted particularly intense study, because many of them have widespread and severe impacts on agricultural and forestry systems. This has included many empirical studies, of course, but herbivorous insects have also provided the foundations for theoretical debates about the mechanisms of animal population dynamics (reviewed in Berryman 2003). Such theoretical debates cry out for long-term data sets to help resolve them, and while these are unfortunately uncommon, they exist for a number of herbivorous insects. Long-term studies tend to generate a daunting quantity of data, and surprisingly often, the concordance between long-term data and theoretical expectations is not obvious. This can result in conflicting literature and in the lengthy persistence of conflicting hypotheses about population dynamics. The eastern spruce budworm (Choristoneura fumiferana (Clemens), Lepidoptera: Tortricidae; henceforth, just " spruce budworm " or " budworm ") exemplifies this conundrum. Despite more than 65 yr of intensive research, multiple long-term studies , and hundreds of published articles, our understanding of budworm population dynamics has seen paradigms shifting through the decades, but no broad consensus has been reached on the ecological factors that drive and modulate population oscillations (Sturtevant et al. 2015).
... They disperse by walking or ballooning to find food (Stuart and Polavarapu, 1998). After reaching the sixth instar, the larvae undergo pupation and adults emerge in mid-summer (Jaynes and Speers, 1949;Pureswaran et al., 2016). Fifty per cent of adults emerged at 467 degree-days (DD) on Abies balsamea (balsam fir) and 437 DD on Picea glauca (white spruce), above a development threshold of 8°C (Lysyk, 1989). ...
... A larger review of SBW outbreak periodicity and intensity is provided by Nealis (2016). Several hypotheses have been proposed to explain SBW outbreaks, focusing on the roles of climate, food availability and natural enemies in regulating population density (Morris, 1963;Pureswaran et al., 2016;Royama, 1981Royama, , 1984Royama et al., 2005). Forestry practices can trigger outbreaks, as severe defoliation has occurred in large stands of mature balsam fir regenerated after intensive logging. ...
... Forestry practices can trigger outbreaks, as severe defoliation has occurred in large stands of mature balsam fir regenerated after intensive logging. These practices may disrupt interactions between SBW and its natural enemies (Miller and Rusnock, 1993;Pureswaran et al., 2016). ...
In 2022, EFSA was mandated by the European Commission's Directorate‐General for Health and Food Safety (M‐2022‐00070) to provide technical assistance regarding the list of Union quarantine pests qualifying as priority pests, as specified in Article 6(2) of Regulation (EU) 2016/2031 on protective measures against plant pests. As part of Task C, EFSA conducted comprehensive expert knowledge elicitations on candidate priority pests, focusing on the lag period, rate of expansion and the impacts on production (yield and quality losses) and the environment. This report provides the rationale for the dataset on Choristoneura fumiferana and C. parallela, delivered to the European Commission's Joint Research Centre, to feed into the Impact Indicator for Priority Pest (I2P2) model and complete the pest prioritisation ranking exercise.
... This finding contrasts our prediction that increasing defoliation would increase consumer autochthony but rather emphasizes the differences between consumer responses to defoliation and forest harvesting as landscape disturbances, as the latter had been observed to increase consumer autochthony (England and Rosemond 2004). As discussed, we suspect that budworm carcasses (larvae and moths) become abundant during outbreaks (Miller 1977;Pureswaran et al. 2016) and provide an important terrestrial food source to stream predators--and this effect directly contrasts with forest harvesting, where terrestrial prey inputs to streams are reduced (Eros et al. 2012). In contrast to harvesting, which directly removes forest biomass, defoliation can be sustained over time in forest stands. ...
... Our demonstration of the downstream consequences of defoliation is also important from a forest management perspective. Although spruce budworm is a natural disturbance agent, fir and spruce trees are becoming more susceptible (Pureswaran et al. 2016;Bellemin-Noel et al. 2021) and outbreaks are becoming more widespread (Navarro et al. 2018;Kalamandeen et al. 2023). This, along with increasing stress on forest ecosystems due to climate change (Gauthier et al. 2023), may have consequences for the functioning of forest watersheds. ...
... Historically, the two main strategies for spruce budworm management in Canada have been "no intervention", allowing defoliation and tree mortality to occur unimpeded (a default strategy in the unmanaged boreal forest), versus "foliage protection", a reactive strategy with aerial insecticide applications targeting areas of severe defoliation to reduce cumulative defoliation, thereby minimizing mortality. Recently, a new proactive approach called early intervention strategy aims to minimize defoliation by targeting hotspots of rising spruce budworm populations in an attempt to prevent a large-scale outbreak altogether (Pureswaran et al. 2016). These management strategies often involve costbenefit analyses and differ primarily in the degree to which defoliation and mortality are allowed to progress; however, they also determine whether this natural disturbance is allowed to occur at local scales, such as forest watersheds. ...
Insect defoliation is a widespread cause of forest disturbance in Canada, and it has the potential to alter terrestrial carbon contributions to stream consumers and thus affect stream ecosystems. Although defoliation by spruce budworms (Choristoneura fumiferana) is considered a natural disturbance with importance to forest succession, the magnitude and duration of outbreaks are increasing, and there is a growing need to understand their downstream consequences. To examine this, algal productivity and autochthonous (in-stream) energy supporting macroinvertebrate and fish consumers (using δ¹³C, δ¹⁵N) were examined in 12 streams in the Gaspé Peninsula, Québec, Canada, that ranged in watershed defoliation from spruce budworm. Hierarchical partitioning models were used to assess the contributions of watershed and stream variables to these endpoints. Total seasonal algal production ranged fourfold among sites, and cumulative defoliation was the strongest predictor. Consumers from streams with greater defoliation (cumulative or annual) had lower reliance on autochthonous carbon, despite increased algal production, and this effect was greater for brook trout than macroinvertebrates. These findings demonstrate for the first time that insect outbreaks—which alter forest structure and function—also similarly affect downstream aquatic ecosystems by causing changes in the production and contributions of carbon sources. This suggests that insect control strategies and forest management decisions should also consider implications for downstream aquatic ecosystems.
... In addition to influencing ecosystem nutrient dynamics, it has been proposed that herbivores could also respond to these changes by increasing their fitness and subsequently population growth 2 . However, despite early work suggesting that bottom-up forces were a factor controlling spruce budworm (SBW) outbreaks, this idea was discredited in the SBW literature based on population models because outbreaks collapsed even when large patches of host species were still available in the forest 26 . Only recently has it been shown that specialist herbivore fitness may depend on both top-down (e.g., natural enemies) and bottom-up forces 27 . ...
... Only recently has it been shown that specialist herbivore fitness may depend on both top-down (e.g., natural enemies) and bottom-up forces 27 . For insect defoliators, nitrogen is the main factor limiting growth, thus, host nutritional quality could be the feedback mechanism that allows populations to grow quickly as soon as top-down pressure is relaxed 26 . It was hypothesized that coniferous forests are nutritional deserts that cannot support SBW population growth until some conditions, like a drought stress, raised the nutritional status of host trees 28,29 . ...
... Among multiple factors, the beginning of SBW outbreaks has been associated with masting of balsam fir cones that provide a nutrient boost to young larvae 56 and increased larval survival in low-density populations. The transition from low population densities to broad-scale outbreaks has been a topic of debate for decades with no broad consensus among ecologists 26,57 . Mortality of young lepidopteran larvae is usually high and extreme variation in mortality can be an important factor in modulating SBW population dynamics 58,59 . ...
Top-down effects, like predation, are drivers of insect outbreaks, but bottom-up effects, like host nutritional quality, also influence outbreaks and could in turn be altered by insect-caused defoliation. We evaluated the prediction that herbivory leads to a positive feedback on outbreak severity as nutrient concentration in plant tissues increases through improved soil nutrient availability from frass and litter deposition. Over seven years of a spruce budworm outbreak, we quantified litter nutrient fluxes, soil nitrogen availability, and host tree foliar nutrient status along a forest susceptibility gradient. As the outbreak progressed, both soil nutrient fluxes and availability increased which, in turn, improved foliage quality in surviving host trees. This is consistent with boosted insect fitness and increased population density and defoliation as outbreaks grow. Our results suggest that a positive bottom-up feedback to forest ecosystems from defoliation may result in conditions favorable to self-amplifying population dynamics in insect herbivores that can contribute to driving broad-scale outbreaks.
... Because of these diverse adverse effects, the spatiotemporal dynamics of spruce budworm populations have been the subject of intense ecological inquiry and debate over the last century (Pureswaran et al., 2016). The budworm's cyclic and typically spatially synchronous dynamics involve a complex interplay among (i) spatial correlation in environmental attributes such as temperature and precipitations (i.e., the Moran effect; Koenig, 2002;Moran, 1953), (ii) dispersal from areas with high population densities (epicentres) through mass exodus flights that colonize new, distant areas with low population densities (Boulanger et al., 2017;James et al., 2015;Larroque et al., 2019;Williams & Liebhold, 2000), and (iii) natural enemies (predators and parasitoids) that modulate their foraging behaviour as a function of the local densities of their hosts, which contributes to the regional synchronization of host densities (Eveleigh et al., 2007;McCann & Rooney, 2009) ...
... shown that dispersal is a key mechanism involved in driving population synchrony of spruce budworm outbreaks (Larroque et al., 2019). However, no consensus has yet been reached regarding the relative importance of each of the three main ecological mechanisms in explaining and predicting irruptive population dynamics in the spruce budworm system (Pureswaran et al., 2016;Royama, 1984;Sturtevant et al., 2015). Our objective in this paper is to explore in greater detail the role of parasitoids in driving large-scale, spatially synchronous population outbreaks. ...
... Spatiotemporal patterns in the population dynamics of eruptive species have long fascinated ecologists (Elton, 1924;Pureswaran et al., 2016;Schwartz et al., 2002). The role of natural enemies in driving these outbreaks is well acknowledged, although the population genetic consequences of outbreaks at higher trophic levels have seldom been examined. ...
Periodic and spatially synchronous outbreaks of insect pests have dramatic consequences for boreal and sub‐boreal forests. Within these multitrophic systems, parasitoids can be stabilizing agents by dispersing toward patches containing higher host density (the so‐called birdfeeder effect). However, we know little about the dispersal abilities of parasitoids in continuous forested landscapes, limiting our understanding of the spatiotemporal dynamics of host–parasitoid systems, and constraining our ability to predict forest resilience in the context of global changes. In this study, we investigate the spatial genetic structure and spatial variation in genetic diversity of two important species of spruce budworm larval parasitoids during outbreaks: Apanteles fumiferanae Viereck (Braconidae) and Glypta fumiferanae (Viereck) (Ichneumonidae). Using parasitoids sampled in 2014 from 26 and 29 locations across a study area of 350,000 km2, we identified 1,012 and 992 neutral SNP loci for A. fumiferanae (N = 279 individuals) and G. fumiferanae (N = 382), respectively. Using DAPC, PCA, AMOVA, and IBD analyses, we found evidence for panmixia and high genetic connectivity for both species, matching the previously described genetic structure of the spruce budworm within the same context, suggesting similar effective dispersal during outbreaks and high parasitoid population densities between outbreaks. We also found a significant negative relationship between genetic diversity and latitude for A. fumiferanae but not for G. fumiferanae, suggesting that northern range limits may vary by species within the spruce budworm parasitoid community. These spatial dynamics should be considered when predicting future insect outbreak severities in boreal landscapes.
... However, the development and further refinement of these models have proven to be challenging due largely to the lack of comprehensive knowledge of the many processes operating at various scales, as well as difficulties in tracking and quantifying SBW population across the landscape for model development (Nenzén et al. 2017a). Consequently, there currently is no generally accepted theory or generalized modeling framework summarizing SBW population dynamics across the landscape (Pureswaran et al. 2016). ...
... Despite the consideration of temporal dynamics, what is generally absent in the assessment of the dynamics of SBW outbreaks is the spatial component (Sturtevant et al. 2015). As growing evidence supports patterns of SBW outbreaks across the landscape such as the west to east development, synchrony, and hot spots of defoliation (Blais 1983;Irland et al. 1988;Pureswaran et al. 2016), it is reasonable to assume that defoliation at one location is affected not only by its biotic and abiotic characteristics but also defoliation at neighboring locations. This may be an important cause of the highly variable defoliation of similar stands observed in the same years (Chen et al. 2018a). ...
... The 11-year defoliation data from 1975 to 1985 covered most of the duration of the last SBW outbreak in Maine as an outbreak typically lasts about 15 years (Blais 1983). Since SBW is endemic to the region, the initiation of an outbreak is difficult to monitor and is considered a major data gap for SBW population ecology (Pureswaran et al. 2016). For example, there was an uptick of observed SBW population around 2016 in Maine but an outbreak has not ensued (Kanoti 2017). ...
Key message
Simulated spruce budworm (SBW; Choristoneura fumiferana (Clem.)) defoliation generally becomes ubiquitous in 3 years after its initiation in agreement with historical observations despite varying environmental and stand conditions over large ranges. Current-year defoliation has almost no correlation with defoliation more than 1 year ago at the same location, which may be related to the role of SBW dispersal in sustaining defoliation across space and time. Mitigation practices like insecticide spraying may be more efficient if applied early to initial spots (epicenters) of defoliation, while management probably should focus on improving forests’ resilience to withstand repeated defoliation by altering species composition.ContextSBW defoliation during its periodic and extensive outbreaks greatly affects forest productivity at large spatial and temporal scales. A generalized modeling framework that simultaneously accounts for both highly variable spatial and temporal dynamics of SBW outbreaks has not been developed.AimsTo develop a flexible parametric spatiotemporal model to explicitly predict defoliation in continuous space and time in order to evaluate the dynamics of SBW outbreaks across a complex forested landscape.MethodsA novel model was developed on extensive defoliation data covering approximately 50,000 km2 and 10 years of the last SBW outbreak during the 1970s and 1980s in Maine, USA. Simulations of various outbreak scenarios were performed using this model.ResultsThe developed model provided a sufficient fit of the data (R2 of 0.63 and mean bias of +0.3%) and was relatively consistent with expectations. Simulations show that defoliation generally becomes ubiquitous in 3 years despite varying environmental and stand conditions. Current-year defoliation has almost no correlation with defoliation more than 1 year ago at the same location, which may be related to the role of SBW dispersal in sustaining defoliation across space and time.Conclusion
Mitigation practices like insecticide spraying may be more efficient if applied early to initial spots (epicenters) of defoliation, while management probably should focus on improving forests’ resilience to withstand repeated defoliation by altering species composition. Our model provides quantitative information flexible in spatial and temporal scales yet directly usable in existing forest growth and yield modeling frameworks and management decision support systems. This generalized spatiotemporal model is readily extendable for evaluating spatial and temporal dynamics of other forms of defoliation across complex forest landscapes.
... In our study, we used simple population processes to assess how much additive mortality might be needed to slow or suppress outbreaks of eastern spruce budworm (hereafter spruce budworm, Choristoneura fumiferana Clemens) (Lepidoptera: Tortricidae), which is the major defoliating pest of spruce (Picea sp.) and balsam fir (Abies balsamea [L.] Mill.) in North America (Aakala et al., 2023;Pureswaran et al., 2016). Although spruce budworm has historically been managed via plant protection (aka Foliage Protection, Prebble, 1975;Pureswaran et al., 2016;Webb et al., 1961), during the past decade efforts have been underway in Atlantic Canada to implement a largescale outbreak suppression programme, the so-called Early Intervention Strategy MacLean et al., 2019). ...
... In our study, we used simple population processes to assess how much additive mortality might be needed to slow or suppress outbreaks of eastern spruce budworm (hereafter spruce budworm, Choristoneura fumiferana Clemens) (Lepidoptera: Tortricidae), which is the major defoliating pest of spruce (Picea sp.) and balsam fir (Abies balsamea [L.] Mill.) in North America (Aakala et al., 2023;Pureswaran et al., 2016). Although spruce budworm has historically been managed via plant protection (aka Foliage Protection, Prebble, 1975;Pureswaran et al., 2016;Webb et al., 1961), during the past decade efforts have been underway in Atlantic Canada to implement a largescale outbreak suppression programme, the so-called Early Intervention Strategy MacLean et al., 2019). This outbreak suppression programme was designed based on the best management practices established for vertebrate control programs to enhance additive mortality and limit compensation (e.g., Dell'Omo & Palmery, 2002;Fernández-Chacón et al., 2015;McCann & Garcelon, 2008;McColl et al., 2014;Pech et al., 1992;Robertson et al., 2016;Schwensow et al., 2014;Ueno et al., 2010). ...
The concepts of compensation and additive mortality form the ecological basis for understanding animal population responses to exploitation by humans. In the context of pest management, compensation is a density‐dependent response that allows populations to offset control‐related mortality, often via increased survival or reinvasion. Additive mortality, in contrast, accrues when a population's compensatory capacity is insufficient to offset losses, resulting in a net reduction in population size or growth rate. These concepts are rarely considered in forest insect pest management, which tends to emphasise short‐term plant protection over long‐term population control. We used published life table data for a major native forest insect defoliator, the spruce budworm (Choristoneura fumiferana [Lepidoptera: Tortricidae]) to simulate the amount of additive mortality required to suppress an outbreak. Simulations also assessed how the failure to account for different compensatory responses could hinder successful control. Our results suggest that only relatively modest amounts of additive mortality (perhaps as low as approximately 8%–18%) may be needed to stop spruce budworm from outbreaking, with immigration being the strongest potential compensatory hindrance to outbreak suppression. Many of the compensatory responses that thwarted outbreak suppression in the past (e.g., low detection efficiency, immigration, indiscriminate killing of predators and parasitoids) have contemporary solutions that could increase additive mortality and thereby enhance the feasibility of population control strategies for native forest insect pests. Our results suggest that some native forest insect pests may require relatively little additive mortality to suppress outbreaks if compensation‐limiting strategies are used. Incorporating theoretical and strategic frameworks used in vertebrate population management could advance the development of native insect population control programmes.
... However, the mechanisms generating local mortality patterns are not necessarily the same as those mechanisms involved at a landscape scale. SBW outbreaks in Eastern North America are long-lasting and cover vast areas (Jardon et al., 2003;Williams & Liebhold, 2000), with the outbreak in the 1970s causing upwards of 50 million ha of severe defoliation, lasting 15-20 years (Pureswaran et al., 2016). Outbreaks of this kind are known to have been occurring in Eastern North America with a 30-40 year recurrence for at least the past 400 years (Boulanger et al., 2012). ...
... The effects of landscape factors may also differ between early and late phases of an outbreak (Pureswaran et al., 2016). Defoliation onset in an area may be primarily influenced by features of the landscape that limit SBW dispersal to that area such as fragmentation or nonhost presence. ...
The stand‐level risk of both defoliation and mortality due to the spruce budworm ( Choristoneura fumiferana , Clem., SBW) is mainly linked to the quality of the resource such that stands dominated by the primary host species, balsam fir ( Abies balsamea [L.] Mill.), are considered the most vulnerable. At the landscape scale, insect dispersal suggests that both configuration and resource availability should influence the onset and subsequent mortality of an outbreak. However, outbreak risk at this scale is yet to be quantified for the SBW, one of the most studied forest pest defoliators in the world. We aim to determine the risk of both SBW defoliation onset and resultant tree mortality at a landscape scale due to factors that have been associated with SBW defoliation at the stand scale. We used Cox proportional hazard models to quantify the relative risk of different landscape configuration and species composition patterns on defoliation onset and risk of mortality of the current SBW outbreak in Eastern Canada. In particular, we measured the risk associated with primary host proportion, relative amounts of secondary host and hardwood trees to fir trees, the structure of balsam fir in the landscape, and the structure of the forest overall. We show that defoliation onset is more related to the configuration of the landscape than to the species composition while the risk of ensuing mortality increases as the proportion of the primary host increases. Defoliation onset risk is increased by a more complex configuration of fir patches in the landscape. Mortality risk, however, is unaffected by fir configuration but is reduced where there is more black spruce ( Picea mariana [Mill.] B.S.P.) or a decrease of balsam fir in the landscape. The usefulness of survival analysis for assessing risk for insect outbreaks has been overlooked and can quantify the effect of relevant factors to guide management and mitigation strategies. The results of our analysis suggest that forest management strategies should avoid clear cuts and the development of fir monocultures and instead maintain or increase the proportion of black spruce and improve forest connectivity across large forested landscapes to delay defoliation onset and reduce SBW‐related mortality.
... Recently, an unprecedented outbreak of the Mountain pine beetle in the western United States and Canada produced tree mortality over 374,000 km 2 from 2000-2020; the ensuing fires, decay and growth losses are estimated to have released 270 megatons (Mt) of carbon, contributing measurably to global carbon dioxide pools (Aukema et al. 2006;Kurz et al. 2008;Reed et al. 2014). Some species experience cyclical dynamics with peaks and troughs in abundance that occur at strikingly regular intervals ranging from a few years to multiple decades (Baltensweiler and Fischlin 1988;Tenow et al. 2013;Pureswaran et al. 2016). Others experience yearly fluctuations that can appear random or chaotic and are much more difficult to predict. ...
... Among forest insects, cyclical or outbreak dynamics are disproportionately common among defoliators, especially the Lepidoptera (moths and butterflies), though sawflies and some aphids/adelgids also exhibit similar densities and periodicities (Liebhold and Kamata 2000). Native lepidopterans such as the larch budmoth (Zeiraphera diniana), the autumnal moth (Epirrita autumnata), the winter moth (Operophtera brumata) in Europe, the eastern spruce budworm (Choristoneura fumiferana) and forest tent caterpillar (Malacosoma disstria) in North America have been extensively studied for their cyclic dynamics and their propensity to cause widespread defoliation during outbreak years (Varley et al. 1974;Ginzburg and Taneyhill 1994;Myers and Cory 2013;Pureswaran et al. 2016). Exotic species such as the spongy moth (Lymantria dispar) or the winter moth in North America (where both have been introduced) have also received considerable attention from population ecologists (Liebhold and Kamata 2000;Roland 2007). ...
Most insect species are rare most of the time, but populations of certain taxa exhibit dramatic fluctuations in abundance across years. These fluctuations range from highly regular, cyclical dynamics to mathematical chaos. Peaks in abundance, or “population outbreaks” are notable both for the damage they can cause in natural and planted forests and for the rich body of research and theory they have inspired focused on elucidating drivers of population fluctuations across time and space. This chapter explores some of the key mechanisms that explain the population dynamics of outbreaking species, including variation in intrinsic growth rates, lagged endogenous feedbacks linked to top-down and/or bottom-up effects, nonlinearities in the density dependent relationship, and the existence of multiple stable and unstable equilibria, among others. We explore some basic mathematical and graphical approaches to modeling and representing these dynamics and provide a suite of empirical examples from the recent and historical literature.
... Because of its wide distribution, it is considered to be an important tree species for research purpose, particularly, for recording past outbreaks (Simard et al., 2011). It has been affected by budworm defoliation (Lavoie et al., 2019;Pureswaran et al., 2016), and identified as a good host for budworm in terms of favorable environment (Blais, 1957), nutritional value , and high survival rate of late instar larvae in spruce stands (Nealis & Régnière, 2004). Nevertheless, the tree is more likely to survive multiple disturbances, and thus records the signals of outbreaks in its growth rings. ...
... The economic and ecological consequences of SBW disturbances have placed these events among the priority topics of Canadian forestry research. Decades of continuous research by various organizations and agencies have improved our understanding of the epidemic cycle (Boulanger & Arseneault, 2004;Girona et al., 2018b;Navarro et al., 2018;Ryerson et al., 2003), SBW effects on forest succession (Bergeron et al., 1995;Lavoie et al., 2021;MacLean, 1980MacLean, , 2016Martin et al., 2020), the synchrony of outbreaks over large areas (Bouchard et al., 2018;Sturtevant et al., 2015), and SBW population dynamics (Pureswaran et al., 2016;Régnière et al., 2019a;Royama, 1984). ...
Current ecological models predict profound climate change-related effects on the natural disturbance regimes of forests. Spruce budworm (Choristoneura fumiferana) (SBW) is the principal insect defoliator in eastern North America, and SBW outbreaks have a major impact on the structure and function of the Canadian boreal forest, as defoliation leads to decreased tree growth, increased mortality, and lower forest productivity. SBW outbreaks have become more severe over the last century with the changing climate; however, little is known about how the integrated effect of climate and SBW alters the growth of host species. Here we evaluate how climate and outbreak severity combined to affect black spruce (Picea mariana) growth during the SBW outbreak that occurred between 1968-1988 and 2006-2017. We compiled dendrochronological series (2271 trees), outbreak severity (estimated by observed aerial defoliation), and climate data for 164 sites in Québec, Canada. We used a linear mixed effect model to determine the impacts of climatic parameters, cumulative defoliation (of the previous five years), and their interaction effect on basal area growth. At maximum outbreak severity, basal area growth of black spruce was reduced by 14%–18% over five years due to the SBW effect. This outbreak growth response was affected by climate: warmer previous summer minimum temperatures and a higher previous summer climate moisture index (CMI) further decreased growth by 11% and 4%, respectively. In contrast, the negative effect of defoliation was attenuated by 9% for a warmer minimum temperature in the previous spring and 7% for a warmer maximum temperature in the previous summer. This study improves our understanding of combined insect–climate effects on growth and helps in the predictions of future SBW-related damage to forest stands to bolster sustainable forest management. We also recommend that projections of boreal forest ecosystems include several classes of SBW defoliation severity coupled with multiple climatic scenarios.
... (Lepidoptera: Tortricidae) is a key component of forest ecosystems in North America east of the Rocky Mountains. Its recurring outbreaks, caused by pronounced periodic population fluctuations, have been the object of much interest in insect ecology [1][2][3][4][5][6]. The ecological and economic consequences of these outbreaks are also of much concern in forest pest management [7][8][9]. ...
... A simple method to derive absolute density estimates of budworm populations would be helpful in better interpreting pheromone-trap catches, because they are influenced by male flight activity as affected by weather and responsiveness to the attractant, to the absolute density of the budworm population in a trap's vicinity, and to moth dispersal [18]. Absolute density estimates may also shed additional light onto long-term spruce budworm population dynamics, when stand structure changes as a result of reduced tree growth and mortality due to defoliation [5,9,28]. ...
Simple Summary
We measured the foliage area, weight and number of buds on young and mature balsam fir and white spruce trees. With these measurements, total amounts of foliage per tree and per unit area of forest land can be calculated. These estimates can be used to determine the absolute numbers of insects feeding on these trees, numbers that are important in understanding patterns and fluctuations of population abundance. We also discovered that spruce budworm larvae occur preferentially in buds arranged in clusters.
Abstract
We describe the distribution and amount of foliage, expressed as foliated branch surface area, weight, or number of buds in the live crown of healthy open-grown and closed-canopy balsam fir and white spruce trees. Balsam fir and white spruce have very similar total foliage surface area and weight. The live crown of white spruce trees contains fewer buds than balsam fir of similar dimensions. Thus, bud density per unit foliage weight or surface area is higher in balsam fir than in white spruce. We also observed that buds tend to grow in clusters more often on balsam fir than on white spruce, and that larvae of the spruce budworm preferentially attack buds that grow in clusters. Equations were developed to predict the total surface area and weight of foliage as well as number of buds in the live crown for estimation of absolute population density of spruce budworm. These equations use diameter at breast height (DBH) and the number of nodes in the live crown as predictors. When data on the number of live nodes are unavailable, it can be estimated from tree height. Equations were also developed from which to estimate foliage area, weight or bud numbers from DBH only.
... The Green River Project in New Brunswick, Canada, emerged as the foundational study on the dynamics of outbreaks (Morris 1963a). The project was notable for its application of basic ecology to management of a significant forest pest (Clarke et al. 1979) and launched a continuing debate on the dynamics of outbreaks (Pureswaran et al. 2016, Régnière et al. 2019. Two contrasting theories emerged. ...
... We see no theoretical inconsistency with including the resource as a food web element, conditioning the direction and amplitude of population fluctuations (Régnière and Lysyk 1995). Indeed, the inclusion of bottom-up processes reduces uncertainties impeding reconciliation of competing paradigms (Pureswaran et al. 2016). ...
We sampled outbreak populations of western spruce budworm, Choristoneura occidentalis (Lepidoptera: Tortricidae), between 1997 and 2016 in Douglas‐fir forests in the interior of British Columbia, Canada. Annual rates of change in population densities were correlated with generation survival, modulated by egg recruitment via dispersal of moths. Most temporal variation in generation survival was the result of variation in survival of small, non‐feeding larval stages. Survival rates of feeding larval and pupal stages determined the magnitude of generation survival but contributed little to temporal trend. Survival of small larvae was the product of density‐related losses of larvae dispersing to and from hibernation sites and weather‐related stress caused by warm temperatures in late summer and early spring. Overwinter survival improved in cooler years and at higher elevations. Survival of feeding larvae and pupae was associated with mortality by natural enemies, in particular a few species of dominant, ubiquitous endoparasitoids, and an episodic, virulent baculovirus. Recruitment of eggs indicated emigration of gravid moths independent of defoliation but proportional to density of local moths. During periods of increasing and decreasing population densities over a large outbreak area, a marked differential in adult densities among locations resulted in greater than expected per‐capita egg recruitment to areas of relatively low density and vice versa, indicative of net exchange of gravid moths from high‐ to low‐density populations. The result was homogenization of egg densities among locations, apparent synchrony of the outbreak at the large scale, and extended duration of the outbreak at some locations, despite declining generation survival. These results are compared with field studies of the closely related spruce budworm, Choristoneura fumiferana. We suggest our interpretation applies more generally and outbreaks of conifer‐feeding budworms are maintained for several years by compensatory survival of sequential life‐history stages and spatio‐temporal smoothing of densities via dispersal of gravid moths. Parasitoids and pathogens can cause sudden declines in densities, but populations inevitably wane as a result of cumulative degradation of their resource. The interaction and temporal sequence of bottom‐up and top‐down factors explain the similarities and differences in outbreak characteristics within‐ and among‐budworm species.
... The magnitude of this phenomenon is so important that in California, Patten and Burger (1998) have reported a positive correlation between observations of fall vagrant New World warblers (including all three budworm-linked species) and SBW densities in eastern Canada over a 23-year period. At a regional scale, SBW outbreaks may last from 10 to 20 years, whereas at a local scale, they may last from one to 10 years, depending on forest composition, environmental conditions, and SBW populations (Pureswaran et al. 2016). Such conditions appear much more unstable than those experienced by most other New World warblers, especially because many individuals may immigrate to new SBW epicenters, at least early in a regional outbreak. ...
... Indeed, the fact that the current year's larval density was not correlated with cumulative defoliation (i.e., current year plus past years' defoliation) suggests that SBW density varied between years. The progression of a SBW outbreak is not linear, but rather characterized by interannual variability in defoliation and SBW density due to weather and parasitism (Bognounou et al. 2017, Pureswaran et al. 2016, 2018. ...
Most songbird species show some degree of fidelity to their previous breeding location, especially after successful reproduction. However, species associated with highly dynamic food sources (e.g., outbreaking insects) may have to adopt more flexible strategies. Three species (Tennessee Warbler, Leiothlypis peregrina ; Cape May Warbler, Setophaga tigrina ; and Bay-breasted Warbler, S. castanea ) show strong numerical responses to spruce budworm (Choristoneura fumiferana - SBW) outbreaks. These species, referred to as "budworm-linked warblers", might track SBW larvae through extensive natal dispersal. Then, the superabundance of food during outbreaks would be expected to lead to high breeding productivity which, in turn, should promote breeding site fidelity. Here, we aimed to determine whether budworm-linked warblers were faithful to their previous year's breeding season location and, if so, whether their probability of return was influenced by habitat characteristics such as the density of SBW larvae, stand structure, or landscape structure. We hypothesized that return rate of budworm-linked warblers will be high, as reported in other species of New World warblers, and we predicted that among habitat characteristics, return rate will increase with the density of SBW larvae. We banded 117 budworm-linked warblers (94 % being males) in 75 study plots distributed along a gradient of SBW density and searched for returning individuals within 50 m of their capture sites using song playbacks. Contrary to our hypothesis, resighting rate was very low (0-10.5%). This relative "infidelity" suggests that breeding dispersal of budworm-linked warblers was relatively extensive. Only habitat proportion within an 8-km radius had an important (negative) effect on the probability of resighting Bay-breasted Warbler. Budworm-linked warblers did not exhibit strong site fidelity as adults, but instead performed breeding dispersal movements, presumably to track SBW outbreaks. This strategy may reflect strong spatiotemporal variations in the density SBW larvae.
... The second case study involves the eastern spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), a major forest defoliator in eastern Canada. It undergoes outbreak cycles of 30-40 yr, sometimes extending over tens of millions of hectares (Pureswaran et al. 2016). In the province of Québec, Canada, only bioinsecticides are registered for use in forests (MFFP 2000), making Btk (Bacillus thuringiensis var. ...
... Although Btk is efficient for foliage protection, some forests or areas are less suitable for its use, including small private woodlots, protected areas, parks, residential areas, camping sites, etc. Although natural enemies such as parasitoids are a major mortality factor in its population dynamics (Eveleigh et al. 2007, reviewed in Pureswaran et al. 2016), they are not sufficient to prevent outbreaks. A large research study during the 1980s showed that proper deployment of the egg parasitoid T. minutum Riley-which is naturally present in North American forests-can cause up to 83% of egg parasitism, reducing larval population from 42 to 82%, thus providing significant foliage protection benefits (Smith et al. 1990). ...
The egg parasitoid Trichogramma spp. (Hymenoptera: Trichogrammatidae) is a widely used biocontrol agent against lepidopteran pests. Historically, Trichogramma were deployed either by plane or by using cardboard cards on which parasitized eggs are glued and manually installed at sites. Plane deployment is costly and card installation is time consuming, but the use of Trichogramma has been shown to be efficient against several pests. In 2016 and 2017, a research project investigated the potential use of unmanned aerial system for distributing Trichogramma as biocontrol agents against two major pests: an agricultural pest of maize, the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), and a forest pest, the eastern spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae). Exposure duration of parasitized eggs to field conditions (temperature, predation, etc.) in maize fields influenced the Trichogramma’s emergence rate, suggesting that timing of parasitoid releases with their emergence is essential. Although parasitism of naturally occurring eggs in maize fields could not be compared due to the low density of the European corn borer, parasitism of sentinel eggs by Trichogramma was more prominent in plots with unmanned aircraft systems (UAS)-releases compared to control plots. For spruce budworm, treatment with Trichogramma increased egg parasitism and there was no difference between the deployment by UAS and by Trichocards. We discuss these results in the context of pest biology and management. We also discuss the advantages and shortcomings of both methods and offer insights into where future work might go to further leverage the use of UAS in managing these important pests.
... Some studies suggest that predators are able to prevent or at least to diminish pest outbreaks (see, e.g., [137,138]). However, natural enemies can also become saturated in such situations and their population responses will lag behind [139]. Considering the variable results for tree diversity effects on predators, the impact of planting tree mixtures on the effectiveness of natural enemies in pest outbreak situations remains unclear (see also [80,136,139]. ...
... However, natural enemies can also become saturated in such situations and their population responses will lag behind [139]. Considering the variable results for tree diversity effects on predators, the impact of planting tree mixtures on the effectiveness of natural enemies in pest outbreak situations remains unclear (see also [80,136,139]. ...
Purpose of Review
Natural enemies are an important component for forest functioning. By consuming herbivores, they can be effective top-down regulators of potential pest species. Tree mixtures are generally expected to have larger predator and parasitoid populations compared to monocultures. This assumption is based on the “enemies” hypothesis, a classical ecological concept predicting a positive relationship between plant diversity (and complexity) and natural enemies, which, in turn, should increase top-down control in more diverse environments. However, the “enemies” hypothesis has mostly been tested and supported in relatively simple agricultural ecosystems. Until recently, research in forests was sparse. We summarize the upcoming knowledge-base for forests and identify forest characteristics likely shaping relationships between tree diversity, natural enemies (abundance, species richness, diversity), and top-down control. We further identify possible implications for mixed species forestry and key knowledge gaps.
Recent Findings
Tree diversity (almost exclusively quantified as tree species richness) does not consistently increase enemy abundance, diversity, or result in herbivore control. Tests of the “enemies” hypothesis are largely based on aboveground natural enemies (mainly generalists) and have highly variable outcomes across taxa and study systems, sometimes even finding a decrease in predator diversity with increasing tree diversity. Recurrent effects of tree species identity and composition indicate that a closer focus on tree functional and phylogenetic diversity might help to foster a mechanistic understanding of the specific circumstances under which tree diversity can promote top-down control.
Summary
Our review suggests that the “enemies” hypothesis may not unambiguously apply to forests. With trees as structurally complex organisms, even low-diversity forests can maintain a high degree of habitat heterogeneity and may provide niches for many predator and parasitoid species, possibly blurring correlations between tree and natural enemy diversity. Several further factors, such as latitude, identity effects, intraguild predation, or functional and phylogenetic components of biodiversity, may confound the predictions of the “enemies” hypothesis. We identify topics needing more research to fully understand under which conditions tree diversity increases natural enemy diversity and top-down control—knowledge that will be crucial for forest management.
... Currently, the mixed boreal forest of Quebec is dominated by 2 major forest disturbances: the spruce budworm and fire. The spruce budworm [Chorisoneura fumiferana Clemens] is a native lepidopteran defoliator and is the major biotic disturbance in the mixed boreal forest (MacLean, 2016;Nealis, 2016;Pureswaran et al., 2016). As a larva, the spruce budworm preferentially feeds on current year's needles of mature balsam fir [Abies balsamea (L.) Mill], its primary host, also feeding on older needles when necessary (Piene, 1989;Hennigar et al., 2008) along with the needles of secondary hosts (Picea spp.; Hennigar et al., 2008). ...
Characterizing millennial and multi-millennial variability in disturbance regimes will be crucial in improving knowledge within the context of a changing climate and the development of sustainable forest management practices in the eastern Canadian mixed boreal forest. The major biotic and abiotic disturbances in the mixed boreal forest are the spruce budworm, and fire, respectively. The ability to reconstruct the variability of these disturbance agents under different climate conditions over long time periods will help elucidate the interaction between the agents and their dynamics in the mixed boreal forest. The objective of this observational study was to reconstruct the frequency of large spruce budworm population (LSBP) and fire disturbance events, and describe their interaction in the mixed boreal forest over the course of the Holocene within the context of changing vegetation and climatic conditions. Lepidopteran scales and sedimentary charcoal were used to reconstruct the local/extra-local disturbance history from lake sediment along with pollen to reconstruct changes in tree species composition. Spruce budworm and fire disturbance events were determined using the CharAnalysis software. Regime shifts in disturbance event frequencies along with changes in tree composition were detected using Sequential T-test Analysis of Regime Shifts. Spearman’s correlation was used to determine the relationship between spruce budworm and fire event frequencies. Over the course of the Holocene, 57 LSBP events and 76 fire events were detected with event frequencies ranging between 0.75-6.30 events*kyr⁻¹ and 1.71-10.5 events*kyr⁻¹ respectively. Nine and 7 regime shifts in LSBP and fire event frequencies were detected respectively, along with 2 shifts in vegetation. A significant negative correlation was observed between LSBP and fire event frequencies from 6000-1000 BP suggestive of a linked disturbance interaction. The first local lake sediment multi-millennial disturbance regime reconstruction comprising both spruce budworm and fire in the mixed forest revealed a very peculiar oscillation in disturbance event frequencies. Each disturbance seemingly establishes a positive disturbance-vegetation feedback that favors itself and inhibits the occurrence of the other. Further, rapid climate change events may act as a key trigger in establishing the respective feedback loops resulting in the observed disturbance event frequency oscillation.
... En tant que ravageur forestier le plus dévastateur de l'est de l'Amérique du Nord, la tordeuse des bourgeons de l'épinette n'a besoin que de peu d'introduction. Les épidémies ont tendance à se reproduire à intervalles semi-réguliers ; cependant, la cause du cycle et de la variabilité spatiale et temporelle autour de ce modèle de base a fait l'objet de nombreuses études et débats au cours des 100 dernières années , Pureswaran et al. 2016. ). ...
Une histoire brève de mes experiences en cherchant la TBE.
... As the most devastating forest pest in eastern North America (MacLean 1980), the spruce budworm, Choristoneura fumiferana Clem., needs little introduction. Outbreaks tend to recur at semiregular intervals every few decades (Royama 1984); however, the cause of the cycle, and of spatial and temporal variability around that basic pattern, have been the subject of much study and debate over the last 100 years Sturtevant et al. 2015;Pureswaran et al. 2016). ...
I outline the “definitional problem” in forest insect outbreak analytics and show how it is related to the “counting problem” in dendroentomology and the “forecasting problem” in forest insect population dynamics, through the ubiquitous presence of non-stationary complex periodicity. Using real-world examples from the spruce budworm (Choristoneura fumiferana Clem.) system, I show that regardless how outbreak patterning is characterized—whether by peak impact, cycle frequency, interval duration, or interval severity—the distribution in pattern attributes appears to be extremely variable, regardless how the data are processed through definitional filters. I show that this extreme variability is an unavoidable and key feature of the system’s dynamics and argue that it needs to be viewed as an object of study, instead of a nuisance problem to be swept under the rug. The single biggest opportunity for rapid gains in spruce budworm predictive ecology is determining the environmental and ecological factors that separate high-intensity from low-intensity outbreak cycling.
... It has often been argued that predators and parasitoids eff ectively control fl uctuations in the abundance of eruptive species during the latency phase of the outbreak cycle, eliminating a signifi cant proportion of the population [Isaev et al., 2001;Klemola et al., 2002Klemola et al., , 2010Pureswaran et al., 2016;Mlynarek et al., 2017]. However, the mortality of black-veined white larvae and pupae from parasitoids remained relatively low during the latency phase (2014-2020) near Fomino (with the exception of 2016, when an increase in tachinid infestation of caterpillars and pupae was observed, Figs 2, 3). ...
This study investigates the long-term dynamics of preimaginal mortality of the black-veined white at different stages of its life cycle (II–III instar larvae during the diapause and overwintering, V instar larvae, prepupae and pupae). The research was conducted in two locations: in the south of the Sverdlovskaya Oblast near Fomino village and in the suburban forests of the City of Novosibirsk. The mortality of black veined white larvae during overwintering is shown to be independent of winter weather conditions and reaches maximum values (up to 89%) at the end of outbreaks. The decline of the black-veined white outbreak in the southern Sverdlovsk region was significantly contributed to by mass mortality of V instar larvae due to the nuclear polyhedrosis virus and II–III instar larvae during overwintering. In Novosibirsk, the infection rate of V instar larvae, prepupae, and pupae by tachinid flies (Diptera: Tachinidae) and the high mortality of larvae
during diapause and overwintering were found to have a similar effect. The parasitoid hymenoptera Cotesia Cameron, 1891 sp. (Hymenoptera: Braconidae) (specialised parasitoids of larvae) and ichneumonids (Hymenoptera: Ichneumonidae (pupal parasitoids) did not significantly contribute to reducing the density of the studied black-veined white populations.
... This in turn greatly affects forest dynamics while impacting the forest sector and regional economy (Chang et al., 2012). A major outbreak occurred between 1967 and 1991 and spread over 55 Mha over eastern Canada and northeastern USA (Pureswaran et al., 2016). The climate envelope of the species is mostly regulated at its northern edge by the length of the growing season while at its southern edge, it is limited by warm conditions and associated energy depletion during the winter diapause (Régnière et al., 2012). ...
Climate change is redefining the dynamics of forest ecosystems globally, particularly through its impact on forest pest populations such as the spruce budworm (SBW, Choristoneura fumiferana [Clem.]), a major defoliator in North American boreal forests. This study investigates the shifts in the population dynamics of spruce budworm across its range in response to recent climate change. We used a process-based, temperature-dependent ecophysiological model combined with the ERA5 reanalysis to assess changes in SBW phenology, reproduction rate, winter survival and population growth rates from 1950 to 2022 across North America. Our findings demonstrate a pronounced northward expansion of suitable climate conditions for SBW, accompanied by earlier phenological events and increased reproduction rates in northern regions. Conversely, the southern parts of its range are experiencing increased winter mortality due to warmer temperatures. This study highlights the significant impact of elevated temperatures, particularly during critical developmental windows such as spring and summer, which are pivotal for spruce budworm survival and reproduction. Additionally, our results reveal that the observed shifts in pest dynamics are more strongly driven by climate change than by changes in landscape composition and structure. We estimated that suitable growth rates have shifted northward by over 68 km on average, but this shift reached more than 200 km in the easternmost portions of its range. Climate-induced shift in suitable conditions for SBW underscores the need for adaptive forest management strategies that consider the rapid ecological changes and the potential for increased forest vulnerability due to climatic and biotic stressors. This study provides vital insights that can inform adaptive management ensuring the sustainability of forest ecosystems in the face of ongoing climate change.
... For example, the growth and death rates for parasitoids could not be parameterised accurately from empirical studies as the parasitoid community is so diverse (Eveleigh et al., 2007) leading to difficulties in empirically establishing growth rate and death. These parameters were assigned values so that the model output would match known SBW outbreak patterns, i.e. 40 year recurring cycle with top down control and secondary bottom up effects (Pureswaran et al., 2016). ...
... Many species are characterized by eruptive dynamics, where they maintain low densities over a long time before outbreaking at the landscape scale. This includes species such as spruce budworm (Ludwig et al. 1978;Pureswaran et al. 2016), spongy moth (Elkinton and Liebhold 1990), and bark beetles such as spruce beetle (Werner et al. 2006) and mountain pine beetle (Safranyik and Wilson 2006). Outbreaks are therefore particularly hard to predict due to their irregular intervals and dependence on stand conditions and low population densities that are challenging to measure (Berryman and Stark 1985). ...
Insects, especially forest pests, are frequently characterized by eruptive dynamics. These types of species can stay at low, endemic population densities for extended periods of time before erupting in large-scale outbreaks. We here present a mechanistic model of these dynamics for mountain pine beetle. This extends a recent model that describes key aspects of mountain pine beetle biology coupled with a forest growth model by additionally including a fraction of low vigor trees. These low vigor trees, which may represent hosts with weakened defenses from drought, disease, other bark beetles, or other stressors, give rise to an endemic equilibrium in biologically plausible parameter ranges. The mechanistic nature of the model allows us to study how each model parameter affects the existence and size of the endemic equilibrium. We then show that under certain parameter shifts that are more likely under climate change, the endemic equilibrium can disappear entirely, leading to an outbreak.
... For example, the growth and death rates for parasitoids could not be parameterised accurately from empirical studies as the parasitoid community is so diverse (Eveleigh et al., 2007) leading to difficulties in empirically establishing growth rate and death. These parameters were assigned values so that the model output would match known SBW outbreak patterns, i.e. 40 year recurring cycle with top down control and secondary bottom up effects (Pureswaran et al., 2016). ...
... To conclude, our current study has underscored the pivotal role of defense chemistry in the intricate interactions between white spruce and ESB. Earlier investigations have outlined four key factors within the same system (Pureswaran et al. 2016;Régnière et al. 2019;Berguet et al. 2021). First, budburst and leaf flush timing, integral to host phenology, can substantially impact spruce budworm outbreaks by influencing their survival and development (Pureswaran et al. 2015(Pureswaran et al. , 2019. ...
Main conclusion
Monoterpenes and phenolics play distinct roles in defending white spruce trees from insect defoliators. Monoterpenes contribute to the toxicity of the foliage, deterring herbivory, whereas phenolics impede budworm growth. This study demonstrates the complex interplay between monoterpenes and phenolics and their collective influence on the defense strategy of white spruce trees against a common insect defoliator.
Abstract
Long-lived coniferous trees display considerable variations in their defensive chemistry. The impact of these defense phenotype variations on insect herbivores of the same species remains to be thoroughly studied, mainly due to challenges in replicating the comprehensive defense profiles of trees under controlled conditions. This study methodically examined the defensive properties of foliar monoterpenes and phenolics across 80 distinct white spruce families. These families were subsequently grouped into two chemotypes based on their foliar monoterpene concentrations. To understand the separate and combined effects of these classes on tree defenses to the eastern spruce budworm, we conducted feeding experiments using actual defense profiles from representative families. Specifically, we assessed budworm response when exposed to substrates amended with phenolics alone or monoterpenes. Our findings indicate that the ratios and amounts of monoterpenes and phenolics present in the white spruce foliage influence the survival of spruce budworms. Phenotypes associated with complete larval mortality exhibited elevated ratios (ranging from 0.4 to 0.6) and concentrations (ranging from 1143 to 1796 ng mg⁻¹) of monoterpenes. Conversely, families characterized by higher phenolic ratios (ranging from 0.62 to 0.77) and lower monoterpene concentrations (ranging from 419 to 985 ng mg⁻¹) were less lethal to the spruce budworm. Both classes of defense compounds contribute significantly to the overall defensive capabilities of white spruce trees. Monoterpenes appear critical in determining the general toxicity of foliage, while phenolics play a role in slowing budworm development, thereby underscoring their collective importance in white spruce defenses.
... Harmonic oscillation theory-advanced by Royama (1984) in the case of SBW and generalised to a broader array of animal systems by Royama (1992)-was inspired by Lotka-Volterra, emphasising the regularity of recurring population cycling events (Wangersky, 1978). The two theories have been contrasted at a high level in terms of their ability to explain population cycling in SBW (Pureswaran et al., 2016), but considerably less attention has been given to how they might relate to observed patterns of synchronisation in actual systems (see Box 1). ...
Defoliators cause extensive damage in boreal and temperate forests of the world. Considerable effort has been invested to understand their individual population dynamics, and despite ample theorising, there is little empirical evidence on factors causing spatial synchrony of pest eruptions at landscape scales.
We report on the landscape‐level effect of forest configuration and composition on the intensity of outbreaks of spruce budworm and forest tent caterpillar in a mixedwood boreal forest in northern Minnesota (USA) and adjacent Ontario (Canada), and how this is related to the degree of spatial synchrony in each species' outbreak cycling.
Using a large spatiotemporal tree‐ring reconstruction of outbreak impacts across these two systems, we evaluate two contrasting theories governing defoliator outbreaks: harmonic oscillation (a.k.a. ‘clockwork’) and relaxation oscillation (a.k.a. ‘catastrophe’), each with consequences linked to top‐down versus bottom‐up influences on outbreak behaviour in time and space.
We find synchrony varies temporally, among outbreak cycles and in direct proportion to cycle peak intensity; however, cycle peak intensities are distributed bimodally in time, and so, therefore, are synchrony coefficients. Spatially, the area where each pest species currently cycles with the greatest peak intensity and synchrony is where their preferred host trees are currently found in greatest proportion.
Despite overall synchrony in cycling, we found, in both systems, a persistent negative spatial correlation among successive eruptive pulses of defoliation. Many of these eruptions failed to spread spatially and to coalesce with other spot eruptions to form extensive area‐wide outbreaks. Eruptions often fail to spread at the hardwood‐conifer interface, resulting in outbreak pulses that systematically bounce back and forth between landscape types, particularly when systems were cycling at low amplitude. These over‐dispersed spatial patterns of pulse impact are consistent with a contagious theory of eruption and outbreak spread. They could be considered consistent with harmonic oscillation theory only for populations cycling at different frequencies, with cycling frequency determined by host forest landscape structure.
Synthesis. We find that defoliator outbreak dynamics across systems include spatiotemporal signatures of each theoretical paradigm—suggesting a hybrid approach will better characterise outbreak behaviour. Host concentration influences which paradigm dominates the spatial dynamics in any given forest landscape context. Because of the synchronising effect of host concentration on forest insect spatial dynamics, mixedwoods appear to be less prone to intense, synchronised defoliator attack than forests of pure hardwood or pure conifer.
... Mature larvae feed on buds, flowers, and newly developing foliage of host trees, and can cause up to 87% defoliation in affected trees [6]. Dendrochronological evidence from ancient timbers suggests that outbreaks of C. fumiferana have a long history in Nova Scotia, New Brunswick, Newfoundland, Quebec, and Ontario [7]. More recently, in Quebec, the current budworm outbreak has damaged approximately 7.2 million hectares since in 2006 [8]. ...
Simple Summary
The spruce budworm (Choristoneura fumiferana Clemens) is a major defoliating pest of coniferous trees in North America. In recent decades, substantial advances in pheromone-mediated trapping and mating disruption technologies have provided researchers with renewed hope for novel population control strategies of C. fumiferana. While the chemical ecology of spruce budworm is continually being studied, a detailed study of the antennal sensilla in adults has yet to be completed. In this research, we review the state of knowledge of C. fumiferana chemical ecology and behavioral responses to chemical stimuli. Further, extracellular single sensillum recordings (SSR) were used to determine the response of olfactory receptor neurons (ORNs) in the antennal sensilla of male and female C. fumiferana to host plant volatiles, and female sex pheromones with a range of concentrations. Together, these data will improve knowledge of mechanisms by which adult C. fumiferana respond to pheromone and host plant volatiles and will provide insights that may improve development of integrated pest management strategies based on the chemical ecology of spruce budworm.
Abstract
Spruce budworm, Choristoneura fumiferana Clemens, is an ecologically significant defoliator of spruce and balsam fir in North America. Optimization of semiochemical-mediated control is needed to improve the existing integrated pest management systems such as mating disruption and population estimation. This study used single sensillum recordings (SSR) to identify the responses of olfactory receptor neurons (ORNs) in the antennal sensilla of adult male and female C. fumiferana to host plant volatiles, and female sex pheromones. There have been few SSR studies done on spruce budworm, and to our knowledge, the present study represents the first attempt to examine the responses of ORNs from antennal sensilla in response to a range of host and conspecific stimuli. A total of 86 sensilla were characterized and sorted into 15 possible sensillum categories based on odor responses. We observed that specialist sensilla responding to few ligands were more abundant in both male and female than sensilla exhibiting more generalized odorant responses. (E/Z)-11-tetradecenal elicited responses from ORNs from any sensilla which were sensitive to pheromones in both males and females. Female C. fumiferana ORNs were able to detect and physiologically respond to female-produced sex pheromones with the same degree of sensitivity as their male counterparts. Together, these data improve our knowledge of mechanisms by which adult budworms respond to pheromone and host plant volatiles and provide insights that may be complementary to existing integrated pest management (IPM) strategies based on the chemical ecology of spruce budworm.
... The economic and ecological consequences of SBW disturbances have placed these events among the priority topics of Canadian forestry research. Decades of continuous research by various organizations and agencies have improved our understanding of the epidemic cycle (Boulanger and Arseneault, 2004;Girona et al., 2018;Navarro et al., 2018;Ryerson et al., 2003), SBW effects on forest succession (Bergeron et al., 1995;Lavoie et al., 2021;MacLean, 1980MacLean, , 2016Martin et al., 2020), the synchrony of outbreaks over large areas (Bouchard et al., 2018;Sturtevant et al., 2015), and SBW population dynamics (Pureswaran et al., 2016;Régnière and Nealis, 2007;Régnière et al., 2019a;Royama, 1984). The population density of this lepidopteran moth increases periodically to reach an outbreak level (Boulanger and Arseneault, 2004;Régnière and Nealis, 2007;Royama, 1984). ...
Current ecological models predict profound climate change-related effects on the natural disturbance regimes of forests. Spruce budworm (Choristoneura fumiferana) (SBW) is the principal insect defoliator in eastern North America, and SBW outbreaks have a major impact on the structure and function of the Canadian boreal forest, as defoliation leads to decreased tree growth, increased mortality, and lower forest productivity. SBW outbreaks have become more severe over the last century with the changing climate; however, little is known about how climate fluctuations affect the growth of SBW host species during the outbreak period. Here we evaluate how climate and outbreak severity combined to affect black spruce (Picea mariana) growth during the SBW outbreak that occurred between 1968-1988 and 2006-2017. We compiled dendrochronological series (2271 trees), outbreak severity (estimated by observed aerial defoliation), and climate data for 164 sites in Québec, Canada. We used a linear mixed effect model to determine the impacts of climatic parameters, cumulative defoliation (of the previous five years), and their coupled effect on basal area growth. At maximum outbreak severity, basal area growth of black spruce was reduced by 14%-18% over five years. This outbreak growth response was affected by climate: warmer previous summer minimum temperatures and a higher previous summer climate moisture index further decreased growth by 11% and 4%, respectively. In contrast, a preceding year's warmer spring minimum temperatures (9%) and summer maximum temperatures (7%) attenuated the negative SBW effect. This study adds knowledge to our landscape-level understanding of combined insect-climate effects and helps predictions of future SBW-related damage to forest stands to bolster sustainable forest management. We also recommend that projections of boreal forest ecosystems include several classes of SBW defoliation and multiple climatic scenarios in future simulations.
... These parasitoids also play an important role during periods between outbreaks by maintaining populations at almost imperceptibly low densities (Bouchard et al. 2018;Régnière et al. 2019). Additional factors that regulate budworm populations include viruses, birds predation, food depletion, and climate (Régnière and Duval 1998;van Frankenhuyzen et al. 2007;Pureswaran et al. 2016;Régnière et al. 2021). ...
Narrow-spectrum insecticides are currently used to control populations of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), in eastern Canada. However, these could have nontarget impacts on other caterpillars – some of which may serve as alternative or alternate hosts to key parasitoids – that are also susceptible to control tactics. This study was conducted to determine how the insecticides, Bacillus thuringiensis variety kurstaki ( Bt k) and tebufenozide, used to control spruce budworm populations, impact caterpillar communities and associated parasitism rates. Post-treatment field sampling of caterpillars was conducted in 2018 and 2019 in New Brunswick, Canada, at sites treated with either Bt k or tebufenozide and at control sites. Caterpillar species richness and abundance, community structure, and parasitism rates were assessed using molecular analyses for 659 collected caterpillars. We found that insecticide applications had no significant impact on abundance, species richness, or parasitism rate relative to the measurements made in the control sites. Nonetheless, a significantly higher caterpillar abundance and lower parasitism rate occurred in Bt k-treated sites than in tebufenozide-treated sites. Overall, however, Bt k and tebufenozide treatments did not negatively affect the non-budworm caterpillar community under the present conditions of low caterpillar densities, suggesting that parasitoids have alternative and alternate hosts after treatments that target the spruce budworm.
... SBW populations reach epidemic levels according to a cycle of 30 to 40 years [7][8][9]. During an outbreak, SBW density increases by several orders of magnitude, with hundreds of larvae per branch [10], whereas during an endemic period, SBW populations are nearly undetectable [11]. An outbreak may last several years, during which host trees are continually defoliated [12]. ...
Eastern spruce budworm (Choristoneura fumiferana Clem.) is considered the most important disturbing insect in coniferous stands in eastern North America. During an outbreak, spruce budworm can cause severe defoliation in balsam fir (Abies balsamea (L.) Mill.), which can affect wood properties such as moisture content and mechanical properties. This project aimed to assess the influence of the duration of spruce budworm defoliation on the wood quality of mature balsam fir trees. To do this, we studied sapwood proportion, decay, moisture content, mechanical properties and tracheid dimensions in stands that had suffered three, four or five years of defoliation. We also compared living and dead balsam firs and evaluated the change in wood properties with time. Our results showed that dead balsam firs suffered from a loss of wood quality rapidly after their death, particularly in terms of moisture content and decay in the sapwood. Sapwood proportion was similar between living and dead trees, but the sapwood of dead trees contained more decay and had a lower moisture content than living trees. Mechanical properties and tracheid dimensions were 10% and 4% lower in dead trees than in living trees. We did not observe any major differences in wood properties between the three durations of defoliation, suggesting that wood degradation occurs before that. The study did not make it possible to determine the optimal duration of defoliation to harvest the stands.
... Consequently, it may be possible to slow outbreak spread and to mitigate the damage caused by outbreaks by limiting dispersal from attacked to unattacked areas, provided that potential source patches can be detected early and suitably treated (e.g., using Btk, Fuentealba et al. 2019). However, current knowledge regarding SBW dispersal is considered fragmented and insufficient to guide effective management interventions (Pureswaran et al. 2016;Johns et al. 2019). ...
Context
Dispersal has a key role in the population dynamics of outbreaking species such as the spruce budworm ( Choristoneura fumiferana ) as it can synchronize the demography of distant populations and favor the transition from endemic to epidemic states. However, we know very little about how landscape structure influences dispersal in such systems while such knowledge is essential for better forecasting of spatially synchronous population dynamics and to guide management strategies.
Objectives
We aimed to characterize the spatial environmental determinants of spruce budworm dispersal to determine how these features affect outbreak spread in Quebec (Canada). We then apply our findings to predict expected future landscape connectivity and explore its potential consequences on future outbreaks.
Methods
We used a machine-learning landscape genetics approach on 447 larvae covering most of the outbreak area and genotyped at 3562 SNP loci to identify the main variables affecting connectivity.
Results
We found that the connectivity between outbreak populations was driven by the combination of precipitation and host cover. Our forecasting suggests that between the current and next outbreaks, connectivity may increase between Ontario and Quebec, and might decrease in the eastern part, which could have the effect of limiting outbreak spread from Ontario and Quebec to the eastern provinces.
Conclusions
Although we did not identify any discrete barriers, low connectivity areas might constrain dispersal in the current and future outbreaks and should in turn, be intensively monitored. However, continued sampling as the outbreak progresses is needed to confirm the temporal stability of the observed patterns.
... During outbreaks, SBW populations are limited by food supply. This type of population regulation has been labeled a multiple-equilibrium system [9,10]. ...
Simple Summary
Cages preventing access to birds were used to measure the rate of predation by birds in a spruce budworm population during the decline of an outbreak. Three species of budworm-feeding warblers were involved in this predation on larvae and pupae. It was found that bird predation is a very important source of mortality in declining spruce budworm populations, and that bird foraging behavior changes as budworm prey become rare at the end of the outbreak.
Abstract
The impact of avian predation on a declining population of the spruce budworm, Choristoneura fumifereana (Clem.), was measured using single-tree exclosure cages in a mature stand of balsam fir, Abies balsamea (L.), and white spruce, Picea glauca (Moench.) Voss. Bird population censuses and observations of foraging and nest-feeding activity were also made to determine the response of budworm-linked warblers to decreasing food availability. Seasonal patterns of foraging. as well as foraging success in the declining prey population was compared to similar information from birds observed in another stand where the spruce budworm population was rising. Avian predation was an important source of mortality between the 4th instar and moth emergence in the declining outbreak population. Mortality by predation increased from negligible to over 98% as budworm density dropped from 100 to <1 larva/kg of host foliage, over 3 years. Calculations based on nest-feeding activity and basic metabolic demands support these observed rates. Seasonal and yearly differences in predation rates observed between the two host-tree species correspond to equivalent shifts in bird foraging behavior in response to dropping insect density. In particular, a preference for searching on white spruce disappeared, although budworm-linked birds remained more efficient at finding food on this plant. The ability to change foraging behavior as prey density dropped differed between bird species.
... Budworm has massive and relatively predictable outbreaks every 35 years, followed by periods of budworm rarity (Royama et al. 2005). This cycle is considered to be a predator-prey cycle, in which the predator is a complex of natural enemies, including insects that parasitise, then kill, caterpillar hosts (parasitoids; Pureswaran et al. 2016;Royama et al. 2017). These parasitoids collectively cause 30-90% mortality, depending on the surrounding forest composition and the point in the budworm cycle (Dowden et al. 1950;Cappuccino et al. 1998;Seehausen et al. 2014;Royama et al. 2017). ...
The world is astoundingly variable, and organisms – from individuals to whole communities – must respond to variability to survive. One example of nature’s variability is the fluctuations in populations of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), which cycle every 35 years. In this study, we examined how a parasitoid community altered its parasitism of budworm and other caterpillar species in response to these fluctuations. Budworm and other caterpillar species were sampled from balsam fir (Pinaceae) in three plots for 14 years in Atlantic Canada, then were reared to identify any emerging parasitoids. We found that the parasitoid community generally showed an indiscriminate response ( i.e., no preference, where frequencies dictated parasitism rates) to changes in budworm frequencies relative to other caterpillar species on balsam fir. We also observed changes in topology and distributions of interaction strengths between the parasitoids, budworm, and other caterpillar species as budworm frequencies fluctuated. Our study contributes to the hypothesis that hardwood trees are a critical part of the budworm–parasitoid food web, where parasitoids attack other caterpillar species on hardwood trees when budworm populations are low. Taken together, our results show that a parasitoid community collectively alters species interactions in response to variable budworm frequencies, thereby fundamentally shifting food-web pathways.
... In early intervention strategies, spraying must be conducted at very low SBW densities (i.e. less than 4 to 5 larvae per branch) to effective. Although detection of this key transition from low to rising populations at broad spatial scales is challenging with current techniques (Pureswaran et al., 2016;Bouchard et al., 2018), it is essential for successful implementation of an early intervention strategy (MacLean et al., 2019). ...
Outbreaks of eastern spruce budworm (Choristoneura fumiferana; hereafter SBW) are a major natural disturbance in coniferous forests of eastern North America. These outbreaks provide a superabundant source of food for insectivorous birds. Three species, referred to as budworm-linked warblers, exhibit strong positive numerical responses to early increases of SBW density: Tennessee Warbler (Leiothlypis peregrina), Cape May Warbler (Setophaga tigrina), and Bay-breasted Warbler (S. castanea). Their abundance increases even before defoliation is visible from aerial surveys. Budworm-linked warblers may detect new epicenters of SBW outbreaks through natal dispersal, as this movement is typically much more extensive than subsequent (breeding dispersal) movements. Our main objectives were, thus, (1) to determine whether sudden increases in the abundance of budworm-linked warblers could be used to detect early stages of SBW outbreaks, and (2) to examine age-specific responses of budworm-linked warblers to local and landscape-level habitat characteristics, in order to investigate the potential role of natal dispersal in the detection of new epicenters. To do so, we estimated the abundance of each species of budworm-linked warbler in 75 study plots sampling a gradient of SBW density and related them to 7 stand variables and landscape metrics with generalized additive mixed models. We also compared the responses of yearling (second-year; SY) and older (after-second-year; ASY) individuals to the density of SBW larvae and habitat variables at different spatial scales. We captured 31 Tennessee Warblers, 27 Cape May Warblers, and 57 Bay-breasted Warblers. The abundance of all three species of budworm-linked warblers increased with SBW larval density, but the numerical response of Bay-breasted Warbler was initiated earlier and it varied with age. SY individuals tended to be associated with stands supporting lower larval densities than ASY individuals and, as suggested by other authors, Bay-breasted Warbler appeared to be more efficient at exploiting SBW larvae at low density. For that reason, this species represents an early indicator of stands undergoing SBW outbreaks and we propose to use its abundance as an indicator to orient labour-intensive ground surveys of SBW larvae.
... The recording of outbreak periods therefore depends on the spatial scale under consideration. It is likely that at a small spatial scale, recorded outbreaks are often shorter in duration because the insect changes location on the basis of available resources (Pureswaran et al., 2016). The shorter duration of outbreaks at a local scale may explain why, at the local level, outbreaks are generally considered as punctual events or as peaks of disturbance (Martin et al., 2019). ...
Spruce budworm (SBW) outbreaks are a major disturbance in North American forests. In Quebec, Canada, the recent history of SBW outbreaks at the local scale is well-known. Studies at the Québec scale nonetheless remain rare despite the need to better understand the dynamics of SBW outbreaks at a larger scale. This study aimed to reconstruct the spatiotemporal dynamics of SBW outbreaks during the 20th century across the insect's range in southern Quebec. To this end, we sampled 83 stands throughout southern Quebec. These stands were selected according to their age and the presence of black, white, and red spruce. In fact, spruce, unlike balsam fir, survives the SBW outbreaks and can record them in these growth rings. In each stand, cores were taken from 20 spruce trees. The dendrochronological series of more than 1,600 trees were analyzed, and we identified, through the k-means grouping of stands, the spatial patterns of tree growth for the three previously documented 20th century SBW outbreaks. The outbreaks were not homogeneous across the distribution range of the insect. Two groups of stands showed early- (1905–1930) and late-century (1968–1988) outbreaks of high severity and a mid-century (1935–1965) outbreak of moderate severity. This pattern is explained mainly by the presence of the insect within the balsam fir–yellow birch and balsam fir–white birch bioclimatic domains, areas where outbreaks tend to be most severe because of the abundance of balsam fir, the main SBW host species. However, these two models differ in terms of the duration of outbreaks. A third, more northern, cluster of stands experienced lower severity outbreaks over the 20th century, a pattern explained by a lower proportion of balsam fir trees in these landscapes. Our study shows that, on the one hand, these three groups of stands are defined by outbreaks of specific duration (an outbreak period beginning when more than 20% of the trees are affected and ending when <20% of the trees are affected) and severity (in terms of percentage of affected trees), and on the other hand they are spatially distinct and subject to different climatic conditions.
... The budworm is the most important forest pest in eastern Canada (MacLean 2016). Outbreaks often last 10 years [15,16], causing high levels of tree mortality and large reductions in growth of surviving trees [17]. Results from the manipulative studies described above suggest that inoculation of white spruce trees with P. scopiformis could improve tree performance in a budworm outbreak if reduced budworm survival resulted in lower levels of host tree defoliation. ...
Wild eastern spruce budworm (Choristoneura fumiferana Clemens) were reared on white spruce (Picea glauca (Moench) Voss) trees, half of which had been previously inoculated with a native endophytic fungus, Phialocephala scopiformis DAOM 229536 Kowalski and Kehr (Helotiales, Ascomycota). Survival up to pupation and up to adult emergence was approximately 27% higher for budworm juveniles that developed on control trees compared to trees inoculated with the endophyte. The endophyte did not influence the size or sex of survivors but did reduce defoliation by approximately 30%. Reductions in defoliation on endophyte-inoculated versus control trees, due to reductions in survival of juvenile budworms, suggests that tree inoculations with P. scopiformis could play an important role in integrated management programs against the eastern spruce budworm.
... Response of insects and pathogens to climate change is likely to increase plant mortality (4), with variable impacts on growth and recruitment (Fig. 3N). Tree mortality can result from girdling of the phloem and xylem by bark beetles (74) and from repeated defoliation events that exhaust the capacity of trees to recover (109). Tree mortality during outbreaks is usually partial at the stand level because many biotic agents preferentially attack trees of specific size or health classes or are host-specific (16). ...
Shifting forest dynamics
Forest dynamics are the processes of recruitment, growth, death, and turnover of the constituent tree species of the forest community. These processes are driven by disturbances both natural and anthropogenic. McDowell et al. review recent progress in understanding the drivers of forest dynamics and how these are interacting and changing in the context of global climate change. The authors show that shifts in forest dynamics are already occurring, and the emerging pattern is that global forests are tending toward younger stands with faster turnover as old-growth forest with stable dynamics are dwindling.
Science , this issue p. eaaz9463
... The eastern spruce budworm (SBW) [Choristoneura fumiferana (Clemens)] is a major defoliator of balsam fir and black spruce in this study area. Cyclical SBW outbreaks during the 20th century caused a > 50% annual loss of productivity and widespread mortality in spruce and fir species (Morin, 1994;Pureswaran et al., 2016). Since 2006, a new outbreak has been observed in Quebec, Canada, affecting >9 M ha of forest in 2019 (Bouchard et al., 2018; Ministère des forêts de la faune et des parcs, 2019). ...
Defoliation can enhance tree water status by reducing canopy transpiration under drought. During long-lasting insect outbreaks however, this effect can be transient as reduced foliage affects not only transpiration but also the entire soil-plant-atmosphere continuum. In this study, we investigated the effects of defoliation and vapor pressure deficit (VPD) on plant and soil water status in balsam fir and black spruce defoliated by spruce budworm, Choristoneura fumiferana (Clemens). We sampled 48 fir trees and 36 spruce trees subjected to differing severities of defoliation. In May–September 2014 and 2015, we monitored the relative shoot water content (RWC) and soil volumetric water content (VWC), and midday shoot water potential (Ψmd, only in 2015). We applied linear mixed models (LMMs) to assess changes in RWC, Ψmd, and VWC to defoliation and VPD and we ran structural equation models (SEM) to determine the causal relationships between the measured variables in relation to defoliation and VPD. In LMMs models, defoliation and VPD, as individual factors, reduced Ψmd in both balsam fir and pooled species models but did not affect RWC. Defoliation alone increased VWC in balsam fir and in pooled models. We observed no interaction between VPD and defoliation on tree water status, but significant effect on VWC (in balsam fir and pooled models), indicating that both factors had independent and additive effects on plants but not on soil. However, in SEM models, RWC was negatively correlated to defoliation, suggesting a hydraulic safety margin. Under conditions of multiple-years of natural defoliation during a spruce budworm outbreak, the decrease in Ψmd reflects the amount of internal water capacitance that could be caused by both a lower Ψmd due to larval feeding and a negative feedback between defoliation and xylem vulnerability.
Climate change is redefining the dynamics of forest ecosystems globally, particularly through its impact on forest pest populations such as the spruce budworm (SBW, Choristoneura fumiferana [Clem.]), a major defoliator in North American boreal forests. This study investigates the shifts in the population dynamics of spruce budworm across its range in response to recent climate change. We used a process-based, temperature-dependent ecophysiological model combined with the ERA5 reanalysis to assess changes in SBW phenology, reproduction rate, winter survival and population growth rates from 1950 to 2022 across North America. Our findings demonstrate a pronounced northward expansion of suitable climate conditions for SBW, accompanied by earlier phenological events and increased reproduction rates in northern regions. Conversely, the southern parts of its range are experiencing increased winter mortality due to warmer temperatures. This study highlights the significant impact of elevated temperatures, particularly during critical developmental windows such as spring and summer, which are pivotal for spruce budworm survival and reproduction. Additionally, our results reveal that the observed shifts in pest dynamics are more strongly driven by climate change than by changes in landscape composition and structure. We estimated that suitable growth rates have shifted northward by over 68 km on average, but this shift reached more than 200 km in the easternmost portions of its range. Climate-induced shift in suitable conditions for SBW underscores the need for adaptive forest management strategies that consider the rapid ecological changes and the potential for increased forest vulnerability due to climatic and biotic stressors. This study provides vital insights that can inform adaptive management ensuring the sustainability of forest ecosystems in the face of ongoing climate change.
Young plantation trees are often highly vulnerable to insect herbivory in ways that are difficult to predict as underlying mechanisms linked to plant traits and natural enemy pressure interact in context-dependent ways. We compared bottom-up and top-down forces acting on spruce budworm ( Choristoneura fumiferana ) on young white spruce ( Picea glauca ) trees in plantations vs in natural regeneration under hardwood canopy. Recognized as the most important outbreaking conifers defoliator in Eastern Canada, we aim to better understand how its herbivory on young trees can affect post-outbreak forest succession. We conducted a 4-year field survey in Northwestern Québec, Canada, to compare plant phenology, budworm density, defoliation rates, predator populations, and parasitism between two habitats. We also designed manipulative experiments with sentinel larvae to assess bottom-up and top-down forces in these habitats. The field survey showed earlier budburst phenology in plantation trees, which improves synchronization with a model (BioSIM) predicted timing of budworm emergence from diapause. The field survey showed higher budworm density and lower larval parasitism in plantations, but no significant difference in current-year growth defoliation during the initial outbreak phase. The bottom-up experiment showed slightly better budworm biological performance, indicated by higher pupal mass, in plantations. The top-down experiment showed greater predator and parasitoid pressure in the understory. Together, our results show how mechanisms controlling insect defoliator populations are context-dependent. In plantations both bottom-up and top-down forces on the spruce budworm are relaxed in these open habitats, leading to better biological performance and higher population density of this forest pest.
Insects, especially forest pests, are frequently characterized by eruptive dynamics. These types of species can stay at low, endemic population densities for extended periods of time before erupting in large-scale outbreaks. We here present a mechanistic model of these dynamics for mountain pine beetle. This extends a recent model that describes key aspects of mountain pine beetle biology coupled with a forest growth model by additionally including a fraction of low-vigor trees. These low-vigor trees, which may represent hosts with weakened defenses from drought, disease, other bark beetles, or other stressors, give rise to an endemic equilibrium in biologically plausible parameter ranges. The mechanistic nature of the model allows us to study how each model parameter affects the existence and size of the endemic equilibrium. We then show that under certain parameter shifts that are more likely under climate change, the endemic equilibrium can disappear entirely, leading to an outbreak.
Aim
While many studies on ectotherm thermal tolerance consider temperature exposure, the frequency of temperature exposures is emerging as an important and generally overlooked driver of survival and fitness which may influence species ranges. We use a physiologically-informed species distribution model to evaluate the influence of temperature fluctuations on the historical distribution and intensity of defoliation of a Lepidopteran forest pest, and on predicted future defoliation.
Location
Eastern Canada.
Time period
2006-2016, projections to 2041-2070.
Major taxa studied
Choristoneura fumiferana (Lepidoptera: Tortricidae, spruce budworm).
Methods
We combined publicly-available maps of spruce budworm-induced defoliation between 2006-2016 in the Canadian province of Quebec with climate, forest composition, and de novo temperature fluctuation predictors to train a species distribution model. Our model evaluated how predictor categories influence spruce budworm defoliation and compared these results to a model trained without temperature fluctuations. Additionally, we predicted future spruce budworm defoliation under 2041-2070 climate change conditions using the models trained with and without temperature fluctuation predictors to determine the impact of temperature fluctuations on future defoliation predictions.
Results
We found that the inclusion of temperature fluctuation predictors improved model performance, and these predictors ranked highly in importance relative to predictors in other categories. The model trained with temperature fluctuation predictors also predicted vastly different defoliation distribution and severity across Quebec, Ontario, and Labrador than the model trained without them under future climate.
Main conclusions
Our study reveals the previously overlooked importance of temperature fluctuations on landscape-scale spruce budworm defoliation and demonstrates the importance of including physiologically-informed predictors in species distribution models. It also provides a novel framework for including thermal variation in correlative species distribution models of ectotherms.
Competition between parasitoids for available hosts is common in insects. These interactions are often negative and are believed to reduce the overall impact of top‐down population regulation. In the spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), two parasitoid species are very common and often interact during the period of low‐density populations between outbreaks: the koinobiont endoparasitoid Tranosema rostrale (Brishke) (Hymenoptera: Ichneumonidae) and the idiobiont ectoparasitoid Elachertus cacoeciae (Howard) (Hymenoptera: Eulophidae). Data collected from field‐implanted larvae of spruce budworm exposed in summer during a 35‐year period between 1987 and 2022 were used to estimate the response of these two parasitoids to the density of their spruce budworm host, and the nature and importance of competitive interactions between them. It was found that both parasitoids are most effective at very low host population density, and that a strong, density‐dependent, and negative correlation of frequencies exists between them. This negative correlation is evident both among and within years and indicates that the parasitoids have developed seasonal history and host choice behaviors that tend to minimize the negative impact of direct competition on their respective performance. It appears that the koinobiont T. rostrale manages, through those adaptive strategies, to avoid direct competition inside a larval host with the competitively superior idiobiont E. cacoeciae .
Integrated pest management (IPM) is perhaps best described as “…the maintenance of destructive agents, including insects, at tolerable levels by the planned use of a variety of preventative, suppressive or regulatory tactics that are ecologically and economically efficient and socially acceptable.
One of the most significant categories of insect that cause damage to trees are the defoliators. While many orders of insects feed on tree foliage, in this chapter we will focus on Lepidoptera, as there are so many Lepidopteran larvae (caterpillars) that are known for their extensive tree damage. In this chapter we review the impact of foliage feeders on forest trees and stand composition, and the ways in which densities of these species or the defoliation they cause are monitored. We do not cover insects attacking ornamental trees in the landscape, nor do we cover insects feeding exclusively on foliage tips or buds.
Eastern Spruce Budworm (ESBW) is a major agent of disturbance in Eastern Canada's boreal forests. Outbreaks have historically led to widespread defoliation of its preferred host trees, fir and spruce species, leading to high rates of mortality. This in turn can result in significant economic losses and enhancement of fire potential in the region. Representation of such biotic disturbance has rarely been included in Dynamic Global Vegetation Models (DGVM), which have become essential tools in understanding and predicting forest dynamics in present and future contexts. We present novel representation of host-specific defoliation in a DGVM (LPJ-LMfire), to better represent disturbance regimes in the boreal forest of eastern Canada. Using host foliage density to trigger outbreak, we were able to calibrate and simulate general spatial patterns of defoliation relative to historical aerial sketch map data. Return intervals were thus sensitive to the growth rates of host trees. Modeled return intervals tended to be significantly longer than 30 years, the approximate observed return interval. A factorial experiment was performed on the interactions of ESBW with wildfire, which was found to be slightly enhanced in terms of burned areas after outbreaks due to increased fuel loads. Interactions between ESBW and fire were found to have 36 higher interaction strength in the drier Western region of the boreal forest. Our study demonstrates that biotic disturbance and its interaction with wildfire can be effectively simulated in a DGVM. We show that bottom-up climatic controls are sufficient to drive simulated spatiotemporal patterns of ESBW that can be calibrated to generally match historical observations.
Insects are declining, but the underlying drivers and differences in responses between species are still largely unclear. Despite the importance of forests, insect trends therein have received little attention. Using 10 years of standardized data (120,996 individuals; 1,805 species) from 140 sites in Germany, we show that declines occurred in most sites and species across trophic groups. In particular, declines (quantified as the correlation between year and the respective community response) were more consistent in sites with many non-native trees or a large amount of timber harvested before the onset of sampling. Correlations at the species level depended on species’ life-history. Larger species, more abundant species, and species of higher trophic level declined most, while herbivores increased. This suggests potential shifts in food webs possibly affecting ecosystem functioning. A targeted management, including promoting more natural tree species composition and partially reduced harvesting, can contribute to mitigating declines.
Context: The stand level risk of defoliation and mortality due to the spruce budworm (Choristoneura fumiferana, SBW) is mainly linked to the host species content such that stands dominated by the primary host species, balsam fir (Abies balsamea), are considered the most vulnerable. At the landscape scale there is much less information but some work suggests that risk may decrease when landscapes are dominated by secondary hosts, such as black spruce(Picea mariana) or when landscapes are fragmented.
Objectives: We aim to determine risk of defoliation onset and of tree mortality due to SBW at a landscape scale due to factors that have been associated with SBW defoliation at the stand scale. We hypothesise that outbreak onset will occur earlier in landscapes dominated by host species and that fragmentation and black spruce content will not affect onset but will be associated with subsequent lower mortality.
Methods: We employed Cox hazard models to estimate the relative risk of different landscape configuration and species composition patterns on the onset and risk of mortality of a spruce budworm outbreak in Cote Nord, Quebec, Eastern Canada, using government data bases of forest tree species composition and of SBW defoliation across Québec. In particular, we measured the risks associated with primary host proportion, the relative amounts of secondary host and hardwood trees to fir trees, the structure of balsam fir in the landscape, and the structure of the forest overall.
Results: Our results show that defoliation onset is more related to the configuration of the landscape than to the species composition while mortality is ultimately influenced by the tree species present. Defoliation onset risk is increased the most by a more complex configuration of fir patches in the landscape. Mortality risk on the other hand is unaffected by fir configuration, with the greatest change being a decrease in risk due to increased black spruce presence in the landscape.
Conclusions: Maintaining black spruce abundance and forest connectivity across forested landscapes could reduce SBW related mortality and delay defoliation onset. Forestry management strategies should avoid clear cuts and the development of fir monocultures in the forest.
We formulate and analyze a general reaction-diffusion equation with delay, inspired by age-structured spruce budworm population dynamics with spatial diffusion by matured individuals. The model has its particular feature for bistability due to the incorporation of a nonlinear birth function (Ricker's function) and a Holling type function of predation by birds. Here we establish some results about the global dynamics, in particular, the stability of and global Hopf bifurcation from the spatially homogeneous steady state when the maturation delay is taken as a bifurcation parameter. We also use a degree theoretical argument to identify intervals for the diffusion rate when the model system has a spatially heterogeneous steady state. Numerical experiments presented show interesting spatialtemporal patterns.
Incidence of immigration in spruce budworm Choristoneura fumiferana (Lepidoptera: Tortricidae) was inferred with rules of thumb and numerical thresholds contrasting closed populations (residents >> migrants) versus populations subjected to major immigration event, based on attributes of daily time series of abundance at light traps (abundance, statistical mode, sex ratio and protandry level). The data set included time series with known migration event (Bas St‐Laurent, QC, 2013), which were used to calibrate numerical thresholds associated with immigration.
The study site on west coast of Newfoundland provided a natural laboratory to infer long‐range immigration due to geographic isolation from the mainland, proximity to the coast and low population density at onset of the study. Time series between 2014 and 2016 exhibited trends consistent with scenario of closed population, including low numbers of budworms, limited variation in day‐to‐day abundance and strong level of protandry (males fly seasonally earlier than females).
Time series with candidate major immigration event (Bas St‐Laurent 2013, Sally's Cove 2017, 2018) displayed specific combinations of parameters, including: (a) sharp increment (>one order of magnitude) in abundance of both males and females from one day to the next (statistical mode, approximately corresponding to timing of migration); (b) pure (100%) convergence of pulse detection interval (post‐immigration period when migrants with nonzero survival rate are collected at light traps) across traps for males and females and (c) overall absence of protandrous flight.
Parameters related to immigration thresholds highlighted that females are more likely to migrate than males, as indicated by increased ratio of females per male during pulse detection interval. Aerial collection of adult spruce budworms with helikite in 2019 confirmed the preponderance of females among true migrants.
Spruce budworm is a major defoliator of forests in North America and its periodic outbreak can cause severe economic growth loss. In 2008, Vaidya and Wu proposed a delayed spruce budworm population model and studied the impact of physiological structure on outbreak control (Vaidya and Wu, 2008 [27]). In this paper, we revisit this model and study its global dynamics that has not been analyzed previously. By carefully dealing with the major difficulty caused jointly by the time delay and the nonlinearity, we give a rather complete stability analysis for the model, including global asymptotic stability, bistability, semi-stability and Hopf bifurcation. Moreover, we show that the time delay can induce interesting dynamics including multitype bistability and long transients.
Forest landscape disturbances are a global phenomenon. Simulation models are an important tool in understanding these broad scale processes and exploring their effects on forest ecosystems. This book contains a collection of insights from a group of ecologists who address a variety of processes: physical disturbances such as drought, wind, and fire; biological disturbances such as defoliating insects and bark beetles; anthropogenic influences; interactions among disturbances; effects of climate change on disturbances; and the recovery of forest landscapes from disturbances—all from a simulation modeling perspective. These discussions and examples offer a broad synopsis of the state of this rapidly evolving subject.
Abstract. 1. Limited empirical support is available for mate-encounter Allee effects
in invasive insects due to the logistical challenges of studying demographic trends in
low-density populations.
2. Traps baited with pheromone and spruce volatiles were used to monitor the
abundance of female Tetropium fuscum F. (Coleoptera: Cerambycidae) at multiple sites
in Nova Scotia in 2011 and 2012. Each female was dissected to determine the presence
or absence of sperm in its spermatheca (mated or virgin female, respectively).
3. Both male and total T. fuscum abundance declined with increasing distance to
the focal point of T. fuscum’s invasion. Female mating probability declined with male
abundance and with distance from the invasion focus, and mating probabilities were very
low at the most peripheral sites. Difficulty in encountering mates may thus contribute to
limiting the spread of T. fuscum.
4. The approach outlined here could be integrated into existing surveys of wood borers
using traps baited with semiochemicals to improve our understanding of the role of the
mate-encounter Allee effect in invasion dynamics.
The spruce budworm (Choristoneura fumiferana Clemens) is perhaps the single most important disturbance agent in Canada’s eastern forests. Climate and forest composition are dominant factors in spruce budworm outbreak dynamics through their direct influences on the pest, its natural enemies, and its hosts, and through their influence on the multitrophic interactions that are important in outbreak dynamics. A combination of four climate variables, three forest composition variables, and one location variable explained 60% of the multivariate variability in outbreak characteristics (duration and severity) in eastern Canada. Outbreak duration was most strongly influenced by April–May accumulation of degree-days; outbreak severity was most strongly influenced by the extreme maximum temperatures of April–May. The basal area of balsam fir had a stronger influence than that of black spruce on duration and on severity. Both outbreak characteristics declined in more northerly locations. Under a projected future (2011–2040) climate scenario the largest increases in outbreak duration and severity are predicted to occur on the Gaspé Peninsula and the north shore of the St. Lawrence River (Quebec). The largest decreases in duration and severity are predicted to occur in southern Ontario and along the Bay of Fundy in New Brunswick and Nova Scotia. The predicted average change in outbreak duration is around –1.3 years. The predicted average change in outbreak severity is only slightly different from zero (around –1.5% defoliation).
Recurrent and synchronous spruce budworm (SBW) outbreaks have important impacts in boreal and sub-boreal forest ecosystems of North America. This study examines the early phase of an outbreak that was developing across a 268,000 km2 area over a period of 9 years (2003–2011). The territory was subdivided in 225 km2 cells, and the relative influence of forest composition, elevation, forest age, average degree-days and soil drainage were examined during three development phases of the outbreak: initial epicenter location, relatively long-distance spread (cell-to-cell expansion), and expansion inside individual cells (within-cell expansion). The results indicate that elevation is the most determinant variable for initial epicenter location. Other variables that were identified as important for outbreak development by previous studies, such as forest composition and average degree-days, were not so important during this phase. However, forest composition and average degree-days were important factors during the cell-to-cell and within-cell expansion phases. Separating outbreak development in distinct phases also allowed to integrate phase-specific spatial and temporal covariates that were highly significant in the models, such as distance from previous year defoliations during the cell-to-cell expansion phase, and the proportion of defoliated stands during the preceding year for the within-cell expansion phase. Overall, this study provides limited evidence that patterns of SBW outbreak expansion could be altered by reducing host tree species abundance in the forest [mainly balsam fir (Abies balsamea) in this region]. More generally, this study suggests that the influence of environmental variables on SBW outbreak development is clearly phase-dependent, and that this landscape-level, process-based approach could be useful to forecast insect outbreak development in forest ecosystems.
RESUMELa modelisation dynamique est souvent utilisee afin de prevoir les reactions des populations animales et de plantes a differentes strategies de gestion. Les modeles sont typiquement bases sur des etudes sur le terrain intensives des facteurs principaux affectant la survie et la reproduction des individus dans la population d'interet. Ces modeles, implicitement developpes a l'echelle spatiale fine de l'etude experimentale, sont alors “transposes” aux echelles spatiales plus grossieres, auxquelles les decisions de gestion sont prises. Un processus, appele transmutation spatiale, par lequel le comportement de la dynamique d'une population semble changer avec le niveau de l'echelle de la recherche, peut serieusement fausser l'interpretation des resultats de la transposition aux plus grandes echelles. Ce probleme est repandu dans la plupart des aspects des sciences de la foresterie, et est largement ignore ou meconnu.Les effets de la transmutation spatiale sur le developpement et sur l'interpretation des...
Dispersal can play an important role in the population dynamics of forest insects, but the role of long-distance immigration and emigration remains unclear due to the difficulty of quantifying dispersal distance and direction. We designed an agent-based spruce budworm flight behavior model that, when interfaced with temperature, wind speed, and precipitation output from a high-resolution atmospheric model, produces detailed flight trajectories and deposition patterns over large landscapes. Rules and relationships describing budworm adult (moth) lift-off, ascent, horizontal flight, and descent were parameterized using a detailed empirical study of budworm dispersal behavior and corresponding meteorological conditions during a 1970s outbreak in New Brunswick, Canada. Simulated moth landings were assumed to be dependent in part on the availability of suitable host tree species. We applied the model to a 6.4 million ha landscape at the border between northern Minnesota (USA) and Ontario (Canada) during an eight-day flight window in late June 2007. Specimens collected during and after this flight window indicated moths emerging from an inland source of outbreak populations dispersed over 150 km to trap sites near the north shore of Lake Superior, where localized cooling was predicted to have delayed emergence of locally-produced budworm moths. Simulations suggested immigration of moths to lakeshore sites from the outbreak source was plausible on three of eight dates within the flight window, but the relatively narrow deposition footprints implied immigration occurred on different dates across lakeshore sites. Apart from wind speed and direction, precipitation and low temperatures limited dispersal to substantially shorter distances for a few dates within the simulated flight window. Key uncertainties limiting our understanding of atmospheric transport of spruce budworm include behavioral responses to vertical heterogeneity in the air temperature profile, the precipitation threshold required for the forced descent of moths from the air column, and search mechanisms affecting host and/or mate location during long-distance flight.
V.G. Nealis and J. Régnière Abstract: Demographic data from a 15-year outbreak of the spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae), in a boreal mixedwood forest in Ontario, Canada, are used to interpret stand-level ecological disturbance in terms of susceptibility and vulnerability (mortality) of balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (Mill.) BSP). All three host-tree species are highly susceptible for oviposition by the spruce budworm and all are suitable for completion of the budworm life cy- cle. Host-related differences in susceptibility arise from the degree of synchrony between spruce budworm phenology during the feeding stages and host-tree phenology. Spruce budworm density was highest on white spruce throughout the budworm's life cycle and over the course of the outbreak, but more rapid flushing and growth of current-year buds in white spruce reduced damage relative to that on balsam fir. Conversely, later flushing of current-year buds on black spruce led to a reduction in budworm density early in the season and a corresponding reduction in defoliation. The combination of high budworm densities and severe defoliation caused mortality first on balsam fir. By the end of the outbreak, 89% of the balsam fir component >10 cm DBH was eliminated compared with 49% of the white spruce in the same size class. The lower susceptibility of black spruce resulted in survival of all but the smallest size classes of that species. Nonhost species such as trembling aspen (Populus tremuloides Michx.) nearly doubled their basal area during the outbreak. The results link processes inherent in the insect-host relationship with the population ecology of the insect and the disturbance ecology of the forest.
Field data from outbreaks of the spruce budworm, Choristoneura fumiferana (Clem.) (Lepidoptera: Tortricidae), in Ontario and Quebec show that current-year defoliation is the single greatest determinant of variation in fecundity. The species of tree that served as the foodplant and infections by the microsporidian Nosema fumiferanae (Thomson) (Microsporida) had little effect on mean lifetime fecundity in field populations. During a prolonged outbreak at one location (Black Sturgeon Lake, Ontario), annual lifetime fecundity was inversely related to observed defoliation in the same year, with the highest mean fecundity recorded at the beginning and the end of the outbreak when defoliation was least severe. The overall trend in yearly rate of change in egg density in a population time series at Black Sturgeon Lake was associated more closely with generational survival than with fecundity. Measured fecundity of local moths was greater than estimated per capita rates of recruitment in most years of the outbreak at Black Sturgeon Lake. This suggests that in these years of the outbreak at Black Sturgeon Lake there was a net emigration of egg-carrying moths.
Foliage-feeding forest insects have served as model systems in the study of animal populations for more than 50 years. Early
studies emphasized identification of "key" mortality agents or density-dependent sources of mortality. However, these efforts
became burdened by rhetorical ambiguity, and population ecologists are increasingly focusing on characterizing population
behavior and identifying the processes that generate that behavior. Two types of behavior seem to be common in forest insect
populations: periodic oscillations ("population cycles") and spatial synchrony (synchronous fluctuations over large geographic
areas). Several population processes (e.g., host–pathogen interactions) have been demonstrated to be capable of producing
periodic oscillations, but the precise identity of these processes remains uncertain for most forest insects and presents
a challenge to future research. As part of these efforts, a greater emphasis is needed on the use of statistical methods for
detecting periodic behavior and for identifying other types of population behavior (e.g., equilibrium dynamics, limit cycles,
transient dynamics). Spatial synchrony appears to be even more ubiquitous in forest insect populations. Dispersal and regional
stochasticity ("Moran effect") have been shown to be capable of producing synchrony, but again more research is needed to
determine the relative contribution of these processes to synchrony observed in natural populations. In addition, there is
a need to search for other types of time–space patterns (e.g., traveling waves, spiral waves) in forest insect populations
and to determine their causes.
We investigated the spatial synchrony of outbreaks of the spruce budworm, Choristoneura fumiferana, over much of its outbreak range in eastern North America during the period 1945-1988. Spatial synchrony decreased with distance between local populations and approached zero near 2000 km. Investigation of the synchrony of local population time series with cluster analysis revealed a pattern of geographically distinct blocks of clusters oriented along an east-west axis. Spatial synchrony also was identified in monthly temperature and precipitation time series at 18 weather stations over the same time period and geographical range as the spruce budworm outbreaks. Cross correlations decreased linearly with distance between stations and approached zero near 3000 km and 1800 km, respectively. We developed a spatially explicit lattice model for a single species occupying multiple patches. Within patches, the model had first order logistic dynamics, and patches were linked by dispersal that depended upon their separation distances. Both local and regional stochasticity (i.e., a Moran effect) were present The modeled lattice had the Same spatial configuration as the outbreak region to facilitate investigating the relative effects of a Moran effect and dispersal on spatial synchrony. Simulations with and without a simple region-wide Moran effect and three levels of dispersal did not produce the decrease in spatial synchrony with distance observed with spruce budworm time series. However, when run at the highest dispersal rate, those simulations produced cluster maps similar to that observed for spruce budworm defoliation. Simulations with a spatially autocorrelated disturbance that had either zero or high local variability and three levels of dispersal produced decreases in spatial synchrony with distance similar to that observed in the historical data. When run at the highest dispersal rate, simulations yielded cluster maps similar to the cluster map for defoliation. We discuss the potential significance of the spatially autocorrelated disturbance factor in understanding regional insect outbreaks. We also consider the plausibility of dispersal rates used in our simulations. We suggest in conclusion that spruce budworm outbreaks were synchronized by a combination of a spatially autocorrelated Moran effect and a high dispersal rate.
It is shown that trapping figures for the lynx are definitely related to weather conditions. The significance of this for the various theories of the origin and synchronization of the cycle is discussed.
In this study we used dendrochronology to reconstruct the history of eastern spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks over the last 450 years in the Bas-Saint-Laurent region of southeastern Quebec. In total, 260 tree cores were sampled from 204 beams in seven historic buildings and 12 trees in a virgin forest stand. Eight previously documented outbreaks (1975-1992, 1947-1958, 1914-1923, 1868-1882, 1832-1845, 1805-1812, 1752-1776, 1710-1724) and three presumed previous outbreaks (1678-1690, 1642-1648, 1577-1600) were identified based on periods of growth reduction. Of these 11 confirmed or presumed outbreaks, six were documented for the first time in eastern Quebec. Such data suggest that outbreak frequency has remained quite stable, with a mean interval of about 40 years between the midpoint of successive outbreaks since the mid-16th century. In addition, together with previous studies, our results indicate a strong spatial synchrony of spruce budworm outbreaks across central and eastern Quebec during the last 300 years. Consequently, our study does not support the hypothesis that spruce budworm outbreak frequency and synchrony increased during the 20th century.
Long-term (17–31 year) density estimates of jack pine budworm, Choristoneurapinuspinus Free. (Lepidoptera: Tortricidae), populations from 31 townships in northwestern Wisconsin were analyzed to investigate variation in population behaviour. Populations had varying combinations of cyclic components with periods of 5, 6, and 10 years. Populations that fluctuated with higher frequencies (shorter periods between outbreaks) tended to be found in locations where the habitat type was indicative of nutrient-poor and extremely dry soils. Many populations in these habitat types had high mean densities, and the coefficient of variation in density was smaller than that in other populations. Just under half (14) of the populations were regulated by statistically significant second-order density-dependent processes. Nevertheless, second-order processes were present to some extent in all populations examined. These results demonstrated that detection of density dependence and population regulation of jack pine budworm depends on the local site where studies are undertaken. Population fluctuations encountered in these populations are of the phase-forgetting quasi-cyclic kind. Studies of mechanisms that account for these cycles and cause populations to fluctuate at three different characteristic frequencies promise to be rewarding.
The “silvicultural hypothesis” of spruce budworm (Choristoneura fumiferana Clem.) dynamics postulates that increasing severity of spruce budworm outbreaks over the last century resulted from forest conditions created by past management activities. Yet, definitive tests of the hypothesis remain elusive. We examined spruce budworm outbreak dynamics (synchrony, periodicity, and intensity) in the 20th century using historical reconstruction from tree-ring chronologies sampled within 19 sites in a large ecoregion located on the border of Minnesota and Ontario. The study encompassed three areas affected by contrasting management legacies: a fine-grained area (Minnesota, six sites, average cut size = 17 ha), a coarse-grained area (Ontario, six sites, average cut size 10 times that of Minnesota), and a conservation zone (seven sites) with little recent harvest activity overlapping the border. Results suggest important differences in outbreak dynamics between the forest management zones that cannot be explained by differences in climate among sample sites. Budworm outbreaks within the conservation zone were more synchronous, with more trees per site affected and less frequent outbreaks than sites sampled within fine-scale managed areas. Outbreak dynamics within forests managed at coarser scales suggest a mixture of the conservation and fine-scale management zone outbreak patterns. Potential factors affecting differences in the observed outbreak patterns include forest pattern, composition, and age. Our study generally supports the silvicultural hypothesis and emphasizes that management legacy effects on spruce budworm dynamics should be observable at landscape scales, as well as at local scales.
A knowledge of animal population dynamics is essential for the proper management of natural resources and the environment. This book, now available in paperback, develops basic concepts and a rigorous methodology for the analysis of animal population dynamics to identify the underlying mechanisms.
Our review highlights research during the past century focussed on the population ecology of outbreak-prone insect defoliators in Canadian forests. Based on reports from national and provincial surveys that began in the 1930s, there have been at least 106 insect defoliators reported to outbreak, most of which are native Lepidoptera, Hymenoptera (sawflies), or Coleoptera (in order of frequency from most to least). Studies comparing life-history traits of outbreak versus non-outbreak species to better understand why certain species are more outbreak-prone indicate several traits especially common among outbreak species, including egg clustering and aggregative larval feeding. There have been at least 50 time-series studies examining the spatiotemporal population behaviour of 12 major defoliator species. These studies provide evidence for both regular periodicity and spatial synchrony of outbreaks for most major species. Life-table studies seeking to understand the agents causing populations to fluctuate have been carried out for at least seven outbreak species, with the majority identifying natural enemies (usually parasitoids) as the major driver of outbreak collapse. Our review concludes with several case studies highlighting the impact and historical underpinnings of population studies for major defoliator species and a discussion of potential avenues for future research.
Recurrent outbreaks of forest insect populations have been discussed extensively in the ecological literature but rarely from the perspective of disturbance ecology. The reason lies, in part, in the traditional focus of quantitative animal ecologists on the species of interest contrasted with the traditional emphasis of plant ecologists on whole communities. This chapter presents the argument that herbivorous insects constitute a class of forest disturbance that is distinct from fire, windthrow, or flooding. Insect outbreaks tend to be spatially synchronized and temporally periodic (periods of high impact followed by periods of low impact over extensive areas), and insect outbreaks are predictably selective, and therefore result in different legacies than do abiotic disturbances. The chapter suggests that recognition of the distinct nature of disturbances caused by insect outbreaks may shed light on the analysis of other disturbances that are, by nature, aperiodic and difficult to forecast. It reviews the dynamics of several outbreak systems. To study these relationships, the chapter uses a comparative, process-oriented approach and illustrates the approach by comparing the ecologies of four species of insect defoliators.
Forest landscape disturbances are a global phenomenon. Simulation models are an important tool in understanding these broad scale processes and exploring their effects on forest ecosystems. This book contains a collection of insights from a group of ecologists who address a variety of processes: Physical disturbances such as drought, wind, and fire; biological disturbances such as defoliating insects and bark beetles; anthropogenic influences; interactions among disturbances; effects of climate change on disturbances; and the recovery of forest landscapes from disturbances-all from a simulation modeling perspective. These discussions and examples offer a broad synopsis of the state of this rapidly evolving subject.
When Keith (1963) published his ‘Wildlife’s 10-year cycle’, available information on the theme was minimal. Many theories were no more than conjectures. In 1961, realizing that further theorizing would get him nowhere, Keith and a team of researchers from the Wisconsin school of wildlife ecology, launched a long-term field study on snowshoe hare (Lepus americanus) populations near Rochester, Alberta. A number of important papers from this study have appeared since then, including the monograph (Keith and Windberg, 1978) that provides a nearly complete 15-year set of demographic data. I shall call this work ‘the Rochester study’.
Nothing has so intrigued and fascinated many naturalists and ecologists as the persistent 10-year cycle of Canada lynx, Lynx canadensis. The well-known archives of the fur trade between the Hudson’s Bay Company and the Canadian trappers in the past two centuries have been a rich source of speculation about the cause of the cycle. As a result, diverse theories exist in the literature. However, many of these ideas were not substantiated by observations, and some statistical analyses ignored ecological mechanisms.
Climate change is altering insect disturbance regimes via temperature-mediated
phenological changes and trophic interactions among host trees, herbivorous insects, and their
natural enemies in boreal forests. Range expansion and increase in outbreak severity of forest
insects are occurring in Europe and North America. The degree to which northern forest
ecosystems are resilient to novel disturbance regimes will have direct consequences for the
provisioning of goods and services from these forests and for long-term forest management
planning. Among major ecological disturbance agents in the boreal forests of North America
is a tortricid moth, the eastern spruce budworm, which defoliates fir (Abies spp.) and spruce
(Picea spp.). Northern expansion of this defoliator in eastern North America and climateinduced
narrowing of the phenological mismatch between the insect and its secondary host,
black spruce (Picea mariana), may permit greater defoliation and mortality in extensive
northern black spruce forests. Although spruce budworm outbreak centers have appeared in
the boreal black spruce zone historically, defoliation and mortality were minor. Potential
increases in outbreak severity and tree mortality raise concerns about the future state of this
northern ecosystem. Severe spruce budworm outbreaks could decrease stand productivity
compared with their occurrence in more diverse, southern balsam fir forest landscapes that
have coevolved with outbreaks. Furthermore, depending on the proportion of balsam fir and
deciduous species present and fire recurrence, changes in regeneration patterns and in nutrient
cycling could alter ecosystem dynamics and replace black spruce by more productive mixedwood
forest, or by less productive ericaceous shrublands. Long-term monitoring, manipulative
experiments, and process modeling of climate-induced phenological changes on
herbivorous insect pests, their host tree species, and natural enemies in northern forests are
therefore crucial to predicting species range shifts and assessing ecological and economic
impacts.
The comparative ecology of conifer-feeding budworms in the genus
Choristoneura
Lederer (Lepidoptera: Tortricidae) in Canada is reviewed with emphasis on publications since 1980. Systematics and life history are updated and historical outbreak patterns and their current interpretation summarised. Recent evidence is analysed in the context of ecological interactions among three trophic levels; host plant, budworm herbivore, and natural enemies. The influence of weather and climate are viewed as modulating factors. The population behaviour of budworms is interpreted as the result of tri-trophic interactions that vary at different scales. The result of these multi-scale interactions is that despite shared phylogenetic constraints and common adaptations, different budworm species display different population behaviour because of specific ecological relationships with their respective hosts and natural enemies.
IntroductionMethodological considerationsReconstructions of outbreak historiesConclusions
AcknowledgmentsReferences
Using the latest observations, experiments, and theoretical studies, I have reanalyzed spruce budworm data from the Green River Project, and now propose a new interpretation of the population dynamics of the species. Spruce budworm populations in the Province of New Brunswick have been oscillating more or less periodically for the last two centuries, the average period being @?35 yr. Local populations over the province tend to oscillate in unison, though their amplitudes and mean levels are not always the same. The local population process in the spruce bud worm is composed of two major parts, a basic oscillation, and secondary fluctuations about this basic oscillation. The basic oscillation is largely determined by the combined action of several intrinsic (density-dependent) mortality factors during the third to sixth larval instars. These factors include parasitoids and, probably, diseases (e.g., microsporidian infection), and, most important, an intriguing complex of unknown causes, which I term @'the fifth agent@' (a large number of larvae with no clear symptoms died during the population decline in the late 1950s). Other mortality factors, including predation, food shortage, weather, and losses during the spring and fall dispersal of young larvae, are not causes of the basic, universally occurring oscillation. Because of immigration and emigration of egg-carrying moths, the ratio of all eggs laid to the number of locally emerged moths (the E/M ratio, or the apparent oviposition rate) fluctuates widely from year to year but independently of the basic oscillation of density in the local populations that were studied. The fluctuation in this ratio is the main source of the secondary fluctuation in density about the basic oscillation, and is highly correlated with the meteorological conditions that govern the immigration and emigration of moths. The E/M ratio is the major density-independent component of budworm population dynamics. Contrary to common belief, there is no evidence to indicate that invasions of egg-carrying moths from other areas upset the assumed endemic equilibrium state of a local population and trigger outbreaks. Moth invasions can only accelerate an increase in a local population to an outbreak level, but this happens only when the population is already in an upswing phase of an oscillation caused by high survival of the feeding larvae. In other words, the @'seed@' of an outbreak lies in the survival of feeding larvae in the locality, and moth invasions can act only as @'fertilizers.@' The weight of evidence is against the idea that an outbreak occurs in an @'epicenter@' and spreads to the surrounding areas through moth dispersal. Rather, the egg mass survey data in New Brunswick since 1952 favor an alternative explanation. If the trough of a population oscillation in a certain area stays comparatively high, as in central New Brunswick in the 1960s, or if the area is more heavily invaded by egg-carrying moths when the populations in that area are in an upswing phase, these populations might reach an outbreak level slightly ahead of the surrounding populations, all of which are oscillating in unison. If a local population oscillates because of the action of density-dependent factors intrinsic to the local budworm system, it may appear to be difficult to explain why many local populations over a wide area oscillate in unison. However, Moran's (1953) theory shows that density-independent factors (such as weather) that are correlated among localities will bring independently oscillating local populations into synchrony, even if weather itself has no oscillatory trend. I illustrate this with a simple time-series model. The same model also illustrates a principle behind the fact that outbreaks occurred fairly regularly in New Brunswick and Quebec during the past two centuries but rather sporadically in other regions of eastern Canada. Finally, I review the commonly accepted theory of outbreaks based on the dichotomy of endemic and epidemic equilibrium states and argue that the theory does not apply to the spruce budworm system.
Ecosystems that are managed for resource production are under continual structural change. Changes imposed by local management aggregate to produce regional patterns and new regionwide responses. Anthropogenic influences on hemispheric and global processes add another level of change. The result is a bewildering variety of real or anticipated changes unique to experience. For example, in the spruce/fir and budworm interaction of eastern North America, a syndrome of causes affects the vulnerability of renewable resources, and the triggers of change can never be predicted. Yet, it is possible to identify key features that affect resilience of ecosystems and robustness of regulation and to reject other possibilities. This approach provides a way to assign priorities for research and for contingency planning to adapt to change.
This paper reformulates the notion of density dependence and shows how this notion plays an important role in constructing appropriate models for data analysis. The regulation and persistence of population processes are interpreted as a close resemblance to the behavior of a series of random variables in which the second moments are bounded. On this basis the formal criteria of persistence are deduced. General structural models of population processes are set up and translated into discrete single-variable difference equations, ranging from the simplest linear first-order process to more complex nonlinear second-order processes. The discussion includes the derivation of general conditions for the second-order limit cycles, a reanalysis of the Canadian lynx 10-yr cycle, and models for population outbreaks. Based on the results of the preceding study of models, the notion of density dependence is reformulated. First, the meaning of the word 'dependence' is discussed. In the context of 'density dependence,' the word has two meanings; the causal dependence of a factor on density, and the statistical dependence. Statistical dependence is defined as a converse of statistical independence, the latter being a process in which the rate of change in density has zero correlation with density; this is a very special class of processes and is unlikely to occur in natural population processes. Therefore, the test of density dependence against the null hypothesis of statistical independence will not provide much insight. It is also argued that a deduction from the persistence criteria shows that a negative correlation between density and its rate of change is a necessary outcome of regulation and hence that the notion of 'density-dependent regulation' in statistical dependence is an uninspiring tautology. As opposed to statistical density independence, which necessarily generates an unbounded population process, causal density independence may satisfy the persistence conditions and hence may regulate populations. However, such a causally 'density-independent regulation' tends to be 'fragile' against perturbations by random exogenous factors. It is a particular class of causally density-dependent processes that can ensure regulation more durable against such perturbations. The inference of generating mechanism from observation is discussed. Although regression analysis is an essential method of inference, simple regression analysis will not work unless the observed processes are known to be a simple Markov chain. Statistical inference of generating mechanisms in observed systems depends largely on the choice of appropriate models, and it is in the construction of such models that the notion of causal density dependence plays an important role.
An individual animal may suffer simultaneously from several different maladies, which lead to its eventual death. Such @'overlaps@' between contemporaneous mortality factors create some problems in partitioning, in a consistent manner, the animals killed among several specific causes. The aim of this paper is to develop the notion of @'killing power@' of a mortality factor, so that we can properly evaluate and incorporate in a model the effect of any given factor on the dynamics of the animal population concerned. I first introduce the basic notion of killing power under the assumption that the mortality factors involved are operating independently of each other. The killing power of a given factor is then shown to be equivalent to the marginal probability of an animal being killed by that factor. The assumption of independence of the mortality factors is relaxed, and the more general notion of conditional killing power is introduced. This is equivalent to the conditional probability of an animal being killed by a given mortality factor, when that individual has not been simultaneously affected by other contemporaneous factors. The concept of mutually exclusive mortality factors is discussed to clarify an apparent confusion in some published literature. Typical examples of this type of mortality factor are an extended drought and excessively wet weather, both of which may kill some animals, but which will not occur simultaneously. Therefore, unless the stage division in the life tables so broad that these types of factors can occur in the same stage, we do not need to consider mutually exclusive mortality factors. How to evaluate the killing power of a mortality factor is illustrated with some examples taken from the literature. Also discussed is a more complicated case, in which the annual fluctuation in the proportion of animals killed by a given factor is correlated with those killed by other contemporaneous mortality factors. Concluding remarks include some notes on the division of stages in a life table so as to facilitate the evaluation of the killing powers of as many mortality factors as possible.
Allee effects can cause populations to decline due to decreasing population growth rates with decreasing density and play a major role in population dynamics. Mate‐finding failure, a common mechanism contributing to demographic Allee effects, is usually difficult to demonstrate because of the arduous nature of sampling individuals at very low densities. In a rising outbreak of the eastern spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), we used caged and tethered virgin females in traps to measure mating success over population densities ranging from deep endemic to outbreak conditions. We found that mating success increased with increasing population density, and that at endemic population densities, females experienced difficulties attracting males and mating, demonstrating for the first time a mate‐finding Allee effect in the spruce budworm. The relationship between population density and mating success is nonlinear. As population density increased, the proportion of mated females eventually reached a plateau and mating success was not 100% even at the highest moth densities, probably due to female reluctance to mate and perhaps interference competition by males for access to females. Both laboratory‐reared and wild females were equally effective in synthesizing pheromone, attracting males, and mating. Our results strongly suggest that a mate‐finding Allee effect is involved in maintaining low‐density spruce budworm populations below an Allee threshold where they fail to grow. Factors such as changes in predation pressure and immigration could help populations overcome this Allee threshold.
The spruce budworm ( Choristoneura fumiferana (Clem.)) is a forest pest in all Canadian Provinces and Territories, and in the northeastern, midwest, and northwestern Unired States. It is by iar the most destructive insect affeiting the extensive balsam fir-spruce forest types in Ontario, Quebec, the Maritime Provinces, and Maine. Outbreaks of varying extent have been reported from these regions almost every year for the past half-century (deGryse, 1947). Periods in which outbreaks were particularly severe and widespread occurred from about 1909 to 1920 and in the 1940's and 1950's. In both periods, outbreaks showed a tendency to shift from west to east, intensifying first in parts of northern Ontario and Quebec and dying out in the Atlantic region south of the St. Lawrence River.
When spruce budworm larvae emerge in the spring, they either mine the needles of old foliage or feed upon the freshly opened staminate flowers of balsam fir. As soon as the vegetative buds begin to expand, the larvae abandon the needle mines for this newer and more succulent growth. Later, when the pollen is shed, the staminate flowers are in turn abandoned in favour of new shoots. Usually, the larvae continue to feed on the new shoots until pupation. When this insect reaches epidemic proportions, however, the current year's growth is often totally destroyed prior to the completion of the larval stage. Late spring frosts have also been known to destroy, in part, or even completely, the shoots of the current year's growth. Under these conditions spruce budworm larvae must resort to feeding on old foliage.
Spruce budworm populations in New Brunswick have been surveyed annually since 1952 by sampling egg masses (later, overwintering larvae) as part of the insecticide application program. Although not designed for an ecological investigation, we extracted as much information from the survey data as we could with respect to several ecological issues. (1) All populations across the province tended to cycle in unison, although three major regions were distinguished by dissimilarity in peak and trough levels. We found that these regional distinctions were a result of random variation in the egg recruitment rate, rather than due to factors associated with comparatively fixed ecoregional (e.g., topographic, climatic, or forest type) characteristics. (2) We found, among all regions, a significant correlation in the rate at which eggs were recruited to each generation, thus providing evidence for the Moran effect as the mechanism underlying population synchrony that caused the province-wide outbreaks. (3) We discuss, with the aid of simulations, the nature and significance of random variations in the egg recruitment rate to explain observed differences in the spatial and temporal patterns of population cycles. Finally, we remark on problems in forecasting.
A high-resolution macrofossil analysis was conducted to reconstruct spruce budworm abundance in an 8600-year-old mire in Saguenay, Québec, Canada. Abundant spruce budworm (Choristoneura fumiferana [Clem.]) faeces recovered in the peat profile suggested endemic and epidemic presence of the insect in the study site since 8240 cal. BP. Important variations in the abundance of faeces were observed, and two exceptional periods of insect activity were delineated, from 6815 to 6480 cal. BP and during the twentieth century. Lepidoptera head capsules were also found in the Lac des Îlets peat profile. They were less abundant and more altered than spruce budworm faeces, but they offered complementary information on insect activity. The long-term perspective achieved with this macrofossil analysis strongly suggests that intense periods of spruce budworm activity were rare events during the Holocene.
The history of spruce budworm (Choristoneurafumiferana (Clem.)) outbreaks for the past 200 to 300 years, for nine regions in eastern Canada, indicates that outbreaks have occurred more frequently in the 20th century than previously. Regionally, 21 outbreaks took place in the past 80 years compared with 9 in the preceding 100 years. Earlier infestations were restricted to specific regions, but in the 20th century they have coalesced and increased in size, the outbreaks of 1910, 1940, and 1970 having covered 10, 25, and 55 million ha respectively. Reasons for the increase in frequency, extent, and severity of outbreaks appear mostly attributable to changes caused by man, in the forest ecosystem. Clear-cutting of pulpwood stands, fire protection, and use of pesticides against budworm favor fir–spruce stands, rendering the forest more prone to budworm attack. The manner and degree to which each of these practices has altered forest composition is discussed. In the future, most of these practices are expected to continue and their effects could intensify, especially in regions of recent application. Other practices, including large-scale planting of white spruce, could further increase the susceptibility of forest stands. Forest management, aimed at reducing the occurrence of extensive fir–spruce stands, has been advocated as a long-term solution to the budworm problem. The implementation of this measure at a time when man's actions result in the proliferation of fir presents a most serious challenge to forest managers.
Wind tunnel experiments confirmed the optimum blend of the two major components of the sex pheromone of the spruce bud worm Choristoneura fumiferana (Clem.), (E)- and (Z)-11-tetradecenal, to be close to a ratio of 95(E):5(Z). The addition of two minor components, (E)-11-tetradecenyl acetate and the saturated tetradecanal, previously identified in the effluvia of calling virgin females, showed that the acetate alone reduced the level of some responses, while the addition of tetradecanal to the acetate:aldehyde blend restored response to previous levels. Addition of tetradecanal alone showed only minor effects on responses. Males responding to calling females showed higher levels of response and sustained flight longer than did males responding to the four-component blend, and this fact suggests that the blend is incomplete.
New Brunswick outbreaks of the spruce budworm which began in 1912 and 1949 are considered in relation to the theory of climatic release. Studies on a natural population show that larval development is more rapid in dry and sunny weather than in humid and cloudy weather. Polar air masses bring the favorable conditions, and tropical air masses and cyclones the unfavorable. June precipitation and temperature records, analyzed in conjunction with weather maps, show that climatic changes took place in regions where the outbreaks developed. The outbreaks were preceded by dry and sunny summers during four or five consecutive years. Although direct mortality of the budworm due to adverse weather conditions has not been observed, favorable climatic conditions may have indirectly promoted population increase from the endemic to the outbreak level in the following ways. Flower production became more frequent in periods of dry years and larvae which fed on staminate flowers developed more rapidly. Larvae that developed early in the season gave rise to more fecund females than larvae that developed late. The age of current foliage consumed is also related to fecundity. The rate of development of balsam fir relative to the budworm varies from year to year. It is postulated that the average fecundity of the budworm increases in the pre-outbreak years as the result of favorable climatic conditions and greater than usual flower production. Larval mortality is greater in years with a prolonged developmental period although the increase is not statistically significant. The theory that outbreaks in New Brunswick resulted from the spread of populations from outbreak areas to the west will be considered in the second part of this two-part paper.
A method for preparing life tables is described and two examples are presented, based on field data from the Green River Watershed in northwestern New Brunswick. Column headings for age interval (x), survivorship (lx), deaths (dx), and death rate (qx) are adopted from human life tables. An additional column, dx factor, should be added to insect life tables so that the different mortality factors can be tabulated. Life tables for the spruce budworm are based on intensive population sampling on permanent plots, complemented by data on fecundity, natural and applied control factors, and dispersion. Techniques for these basic studies will be detailed in subsequent papers. Life tables for consecutive generations of the budworm in different forest types should provide fundamental information on the epidemiology of the species and on the possible reduction of damage through forest management and through direct control measures.
The species composition of stands and surrounding forest have been suggested as important factors influencing the amount of spruce budworm (Choristoneura fumiferana [Clem.]) defoliation and, consequently, budworm-induced growth loss and mortality. We measured spruce budworm defoliation from 1989-1993 in 40 spruce (Picea sp.) and balsam fir (Abies balsamea [L.] Mill.) stands in north-central New Brunswick, Canada, and evaluated the influence of surrounding forest type (softwood, mixedwood), species group (balsam fir, spruce), and site quality (wet/nutrient poor, moist/nutrient rich) on defoliation. Surrounding forest type had a significant effect on the amount of defoliation in white spruce (Picea glauca [Moench] Voss) stands; stands in softwood forests sustained 11% more (P = 0.0485) defoliation than those in mixedwood. There was evidence that hardwood species have to be within, rather than surrounding, a spruce-fir stand to significantly influence defoliation. Site quality had a significant effect (P = 0.0039) on balsam fir defoliation, with stands on moist/rich sites sustaining 19% more defoliation than those on wet/poor sites. In softwood forest, white spruce stands sustained an average of 16% more defoliation than red-black spruce (Picea rubens Sarg.-Picea mariana [Mill.] B.S.P.) stands.
In Part I of this paper consideration was given to the role of climate in the initiation of outbreaks of the spruce budworm in New Brunswick. Analysis of the available weather data showed that the 1912 and 1949 outbreaks developed after several consecutive dry summers. Support was given to the theory of climatic release, which explains the time and place of outbreaks on a climatic basis. However, the recorded history of the spruce budworm also shows that high populations appeared in New Brunswick shortly after "spreading" through Quebec, and this suggests that the New Brunswick outbreaks are also a continuation of this spread. In the present part of the paper consideration is given to the role of dispersal. Moth dispersal is a more effective agent of spread than larval dispersal. Moths may be transported by convectional and turbulent air currents for long distances. Light traps used to detect the incidence of moth movements, showed that large segments of a population may be transferred from one area to another. Unspent females often predominate in these movements. Moth invasion was not detected before the 1949 outbreak although there is evidence from other sources that it occurred in 1948. When deposited in dense, mature, softwood stands, the moths can create outbreaks, but when deposited in young, open, or mixed-wood stands the ensuing high populations soon decline unless bolstered by repeated invasions. Populations in New Brunswick showed gradual and general increases as early as 1947. It is thought probable that these increases resulted from the build-up of local populations through climatic release. The nearest highly-populated centers were over 100 miles to the west in 1947. Later, invasion of moths from centers outside of the Province may have hastened the process.
Tree mortality caused by spruce budworm (Choristoneurafumiferana (Clem.)) defoliation was assessed annually from 1976 to 1985 in 20 mature balsam fir (Abiesbalsamea (L.) Mill.) stands on Cape Breton Island, Nova Scotia, and was related to defoliation and to tree, stand, and site characteristics. Ten to 12 years after the start of the budworm outbreak, fir mortality averaged 87% of the merchantable volume (range 60–100%) among the stands. Timing of mortality was similar to that found in studies of previous outbreaks. In the first 4 years of the outbreak, virtually all the trees that died had more than 90% cumulative defoliation but, overall, 64, 21, and 14% of the dead trees had cumulative defoliation >90, 76 to 90, and 51 to 75%, respectively. Early in the outbreak, fir mortality was generally negatively correlated with tree vigor, relative crown position, or diameter at breast height, but in later years, trees were killed irrespective of these factors. Fir mortality was evenly distributed among different sized trees, and 73 to 86% of the trees in each 5 cm diameter at breast height class died. A linear regression equation between dead fir volume and total fir volume explained 89% of the variability in mortality among stands. Percent fir mortality was correlated (r = 0.84) with visual estimates of cumulative defoliation (including all age-classes of foliage) in 1981, but mortality was not correlated with cumulative current annual defoliation or with site characteristics. Using regression equations, fir mortality during this budworm outbreak was predicted to within ±6 m2/ha in 14 of 18 (78%) of the stands, with a relative accuracy of 17.7%.
The annual Forest Insect and Disease Survey reports of the Canadian Forestry Service were used to develop a jack pine budworm (Choristoneurapinus Freeman) defoliation severity index for a 50-year span. The region covered was the western half of the host's (Pinusbanksiana Lamb.) range. An interpretation of this record permitted the construction of an annual time series of the total area moderately to severely or severely defoliated. The area of outbreaks has increased over the period. This trend was removed from the data to obtain a stationary time series. Analyses of the time series showed that there was a statistically significant periodicity to the size of outbreaks. An examination of the sample autocorrelation function revealed that only the past year's outbreak area was significantly correlated with that of the current year's outbreak. The model identified by applying the Box–Jenkins methodology to these results was inadequate, indicating that the series itself does not contain sufficient information for predictions. Outbreak area and the total area burned in Manitoba and Saskatchewan 4–7 years previously were highly correlated. Despite the crudity of the data, these relations could be exploited to develop predictors of outbreak size and occurrence. The significance of these results for forest management in the region is discussed.
A time series analysis was performed on 32 dendrochronological chronologies using Dutilleul's multi frequential periodo gram. Our aim was to estimate the periodicities of spruce budworm (Choristoneura fumiferana (Clem.)) outbreaks in the province of Quebec. Our analysis estimated a signal of 25-38 years corresponding to spruce budworm outbreaks. Analyses conducted in the 20th century indicated the existence of true periodicities in half of the series suggesting local population dynamics. These periodicities were estimated to be 29-34 years. For the other series, outbreaks were explained by the addition of a second periodic component to the fundamental periodicity. These combined periodicities (pseudoperiodicities or complex periodicities) described most of the 130-year series analysed. The lack of several outbreaks explained part of the combined periodicity. The results suggest a migration effect and complex local population dynamics. A third periodic component explains the double epidemic waves. These estimated periodicities are often similar and in phase. We believe that a common regional effect (the Moran effect) acts on the minor fluctuations of the insect populations and helps synchronize the outbreaks. This research shows that different types of outbreak dynamics can be found within a same insect species.