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Nonoutbreak Species of Forest Lepidoptera
... The fluctuations of temperate, forest Lepidoptera have received an inordinately large fraction of the total hours devoted to the study of insect populations. This is so even though only 1-2% of species of forest Lepidoptera reach outbreak densities or show cyclic dynamics (Mason, 1987). Why are these fluctuations so fascinating? ...
... Mason (1987) gives several more examples of synchronous cycles of forest Lepidoptera. A study of western spruce budworm and associated Lepidoptera showed that non-outbreak species increased in density after the spruce budworm reached high density, increased at a faster rate, and declined at a slower rate than spruce budworm (Mason, 1987). Under crowded conditions the species of Lepidoptera that are normally rare may be better competitors, or escape some of the mortality agents that reduce the densities of the outbreak species, and therefore decline at a slower rate. ...
... nhimaki, 1985). Western spruce budmoth rarely causes defoliation in valley bottoms in British Columbia, Canada, even though many eggs can occur there laid by immigrating moths (Thompson et al., 1984). Higher temperatures at lower elevations are thought to disrupt the phenology of egg hatch and bud burst in this situation, but no data are available.Mason (1987)has speculated that the diversity of the fauna may be reduced in harsh environments, resulting in reduced predator and parasite pressure. In addition, non-outbreak species of Lepidoptera would be rare, and therefore the predators and parasites that were present would have few alternative hosts when the outbreak species declined. This cou ...
In this chapter, the characteristics of population cycles of forest Lepidoptera is described and mechanisms proposed to explain them are evaluated. Evidence for population cycles in forest lepidoptera is also discussed in the chapter. Characteristics of cyclic populations of forest lepidoptera is reviewed, wherein, characteristics of cyclic species, patterns of population change, the beginning of the decline, insect fecundity and population fluctuations, parasitoids and population fluctuations, cyclic and non-cyclic populations, and the impact of forest defoliators on the forests are described. The chapter then turns to the discussion of hypotheses to explain population cycles, wherein, it has described variation in insect quality, climatic release hypothesis, and variation in plant quality, disease susceptibility, and mathematical models. Evaluation of hypotheses is also done in the chapter. Population cycles of other organisms are also summarized in the chapter. Finally, the chapter closes with conclusions and speculations.
... On the contrary , the total herbivore load of the non - dominant species was positively correlated with the load of the dominant species in every year of the study ( Root and Cappuccino 1992 ) . Again , similar evidence comes to us from studies of the spruce budworm : non - outbreaking Lepidoptera have been shown to have higher rates of population growth in years when the western spruce budworm is out - breaking ( Mason 1987 ) . Such positive indirect interactions between herbivores may be more common than once thought ( Damman , 1993 , Cappuccino and Martin 1994 ) . ...
... On the contrary, the total herbivore load of the non-dominant species was positively correlated with the load of the dominant species in every year of the study (Root and Cappuccino 1992). Again, similar evidence comes to us from studies of the spruce budworm: non-outbreaking Lepidoptera have been shown to have higher rates of population growth in years when the western spruce budworm is outbreaking (Mason 1987). Such positive indirect interactions between herbivores may be more common than once thought (Damman, 1993, Cappuccino and Martin 1994). ...
Insect herbivores that irrupt to levels at which they severely damage their host plant tend to lay their eggs in clusters. Egg clumping is common among both forest pest insects and numerically dominant herbivores of goldenrod. Non-outbreak species tend to lay their eggs singly. I hypothesized that successful weed control agents, inasmuch as they can be considered outbreak species on their respective host plants, would, like other outbreak species, tend to lay eggs in clusters. A survey of 39 successful control agents, revealed that this is not the case; biocontrol agents are not more likely than randomly chosen herbivores to lay eggs in clusters.
... The following special feature papers present syntheses of the present knowledge about the population dynamics of eight foliage-feeding forest insect species. These selected reviews are by no means a random sample of all foliagefeeding forest insects because all these species exhibit outbreak behavior but most forest insect populations remain at largely unnoticeable densities (Mason 1987). Nevertheless, given the information presented here, there are some common characteristics in population behavior shared by most of the species covered. ...
... Certainly one of the reasons why these insects were studied was because their densities fluctuate considerably, sometimes reaching outbreak densities that have economic and sociological effects. There is little incentive to study insect populations that never reach outbreak levels (Mason 1987). It is also unlikely that an insect species that only sporadically reaches outbreak levels would be the object of a sustained research program. ...
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.
... Only a small fraction of insect species reach outbreak proportions (Mattson and Addy 1975, Hunter 1995 ). Most species maintain a relatively low, stable population size and do not become noticeable defoliators (Mason 1987). Although there have been numerous studies trying to find traits or factors to explain why some species outbreak, we are still left with some rather unsatisfying explanations that in some way contribute to population regulation, from weather and host plant chemistry, to life history traits and natural enemies. ...
... Furthermore, the questions of why some populations of species outbreak in certain geographical areas, but not others, are still the subject of much debate (Ruohomäki et al. 1997, Bjornstad et al. 2010). Several studies have used a comparative approach between outbreak and nonoutbreak species to examine important characteristics that set them apart (Mason 1987, Wallner 1987, Hunter 1995, Cappuccino et al. 1995, Price 2003). Hunter (1991) examined the Canadian Forest Insect Survey (CFIS) data supplemented with other sources to compare life history traits of outbreak and non-outbreak species of macrolepidoptera. ...
... The search for processes responsible for the increase and decline of animal populations is a central theme in ecology. Although many popu- lations oscillate at low magnitudes and appar- ently remain at low levels ( Mason 1987, Hunter 1995, a large number of animal species exhi- bit periodic behavior in population densities ( Kendall et al. 1999). Both exogenous and endo- genous processes have been proposed to explain animal oscillations. ...
The relative importance of bottom‐up, top‐down, and endogenous factors in population oscillations has been vigorously debated, yet few longer‐term studies of population dynamics have considered their potential nonadditive interactions. This study tests the hypothesis that oscillations can arise from the combined effects of bottom‐up, top‐down, and endogenous factors, with some of these factors being sensitive to anthropogenic modifications of habitats. Populations of Neodiprion abietis, a univoltine sawfly whose oscillations have strong first‐ and second‐order components, were monitored over six to eight generations in a natural and an adjacent managed (precommercially thinned) balsam fir (Abies balsamea) stand at each of three sites in Newfoundland, Canada. Endogenous and exogenous factors affecting N. abietis were evaluated to identify the factors contributing to first‐ and second‐order density dependence and uncover any differences in dynamics attributable to forest management. In both natural and managed stands, N. abietis populations remained at high densities for 2–4 yr before collapsing. At the beginning of outbreaks, immigration, high fecundity, female‐biased sex ratios, and high survival in later larval instars and during the cocoon stage allowed N. abietis populations to increase rapidly. After this increasing phase, top‐down (mortality by the baculovirus Neodiprion abietis nucleopolyhedrovirus [NeabNPV] and, under some circumstances, parasitism by Mesopolobus verditer) and bottom‐up (host‐plant) exogenous factors combined with endogenous factors associated with recruitment to initiate the localized collapse of N. abietis populations. Because first‐order density dependence was attributable to NeabNPV (and sometimes M. verditer) and second‐order density dependence was attributable to host‐plant effects and endogenous factors associated with recruitment, these factors are inferred to cause the periodic behavior of this species. Host‐plant effects, but not top‐down mortality factors, were more pronounced in natural than thinned stands, suggesting that recent increases in N. abietis outbreak severity and range are best explained by a management‐induced reduction of host‐plant effects. By considering nonadditive interactions between trophic forces, this study reveals the intricate relationships between the factors responsible for population oscillations that would otherwise remain hidden.
... Notes. Glyptapanteles pseudotsugae is an example of niche-based selection of caterpillar hosts by a parasitoid wasp, as all of the Lepidoptera species recorded here coexist on fir forests in North America (e.g., Mason 1987). That contrasts with the recorded information for most Microgastrinae wasps, which usually parasitize taxonomically related hosts. ...
Ten new species within four genera of Microgastrinae parasitoid wasps (Hymenoptera: Braconidae) are described from Canada and United States: Diolcogaster ichiroi, Diolcogaster miamensis, Glyptapanteles pseudotsugae, Microgaster archboldensis, Microgaster syntopic, Microplitis altissimus, Microplitis jorgeluisi, Microplitis juanmanueli, Microplitis julioalbertoi, and Microplitis mariamargaritae. The new taxa are significant because they represent the first North American records of a tropical group (species of the basimacula group in Diolcogaster), exemplify interesting ecological cases (niche-based host selection in Glyptapanteles, syntopic species in Microgaster), and showcase unique morphological features and/or altitudinal records (Microplitis). Most of the new species were collected in protected areas or areas with strong research programs (Archbold Biological Station and hammock forests near Miami, Florida; Great Sand Dunes National Park and Preserve, and Mount Evans Wilderness Area, Colorado; Sapelo Island, Georgia; Tonto National Forest, Arizona), and thus are also of value and interest for conservation and research efforts.
... In Canada, 18 species, representing 1%-2% of forest lepidopterans, are cyclically outbreaking (Faeth 1987;Mason 1987). From longterm abundance reconstruction studies of SBW in Quebec, we can exclude potentially introduced and western species. ...
Natural disturbances have a major impact on boreal forest landscape dynamics, and although fire history is well documented at the Holocene scale, spruce budworm (Choristoneura fumiferana (Clemens)) (SBW) dynamics have only been known for the last three centuries. This is likely due to the difficulty in using and interpreting existing indicators (cephalic head capsules and feces). In this methodological study, we present an original approach using lepidopteran wing scales to reconstruct insect abundance. We analyzed two sediment cores from the boreal forest in central Quebec and extracted wing scales at every stratigraphic level. The required quantity of sediment for paleoecological analysis is relatively small given the large quantity of wing scales produced by Lepidoptera and their small size. Scales are well preserved due to their chitinous structure and their great variety of shapes offer a high potential for taxonomic identification. A statistical model based on the shape of scales of the three major epidemic lepidopterans in Quebec discriminated 68% of SBW scales. This indicator allows a more efficient and more precise reconstruction of SBW history with respect to the use of cephalic head capsules or feces.
... 1). Chez les insectes herbivores, quelques espèces de coléoptères, de lépidoptères ou de pucerons sont particulièrement connues pour faire des cycles (Baltensweiler 1993, Faeth 1987, Mason 1987, Hanski 1990 ; voir exemple figure 5). ...
Cyclic dynamics are observed in many rodents. In a first step, single explanations were searched for explaining all cycles. However, more local studies have shown that causes of cycles are often different depending on the species or the habitat concerned. It is therefore important to work at a local level to understand the specific dynamics shown by cyclic populations. In this thesis, I sought to understand the cyclical dynamics of voles in the agricultural plains of western France. Three axes were mainly addressed: 1) the structure of density dependence; 2) The changes in demographic traits (survival and reproduction) responsible for the different phases of the cycles; 3) The relationships between demographic traits and agriculture. Modeling approaches, analysis of empirical data and experimental data were used. The main results are that the vole cycles show two density dependent phases. The decline phase takes place in winter and is due to a reproduction change through direct density dependence. The second is the low phase. The latter is probably multifactor and due to delayed density dependence mechanisms. Finally, we have shown that agricultural practices and crop type had a strong impact on the demographic traits and those depend on the season. This thesis gives a consistent pattern explaining how our common vole population enter and maintain cyclic dynamic. However, the underlying mechanisms remain unclear. In this context, some works have been initiated concerning research of maternal effects or resource depletion.
... Some species are known to reach outbreak densities often, while others never or rarely do so. Populations of species that rarely, or never, reach outbreak densities generally fluctuate much less than outbreak species (Mason, 1987). Those species appear to have life history strategies that keep them near their equilibrium density (MacArthur, 1960). ...
... Beyond the effects of climate change on a few serious pest species, there is a real possibility of the development of new pests. In a review of nonoutbreak species of lepidopteran defoliators of mixed fir forests in North America, Mason (1987) noted that for each major outbreak defoliator there are at least nine nonoutbreak species, whose roles in the forest ecosystem are not well known. Under significant climate change and accompanying changes in forest composition and disturbance levels, such defoliators may be released from control and become pests. ...
General circulation models (GCMs) predict dramatic future changes in climate for the northeastern and north central United States under doubled carbon dioxide (CO2) levels (Hansen et al., 1984; Manabe and Wetherald, 1987; Wilson and Mitchell, 1987; Cubasch and Cess, 1990; Mitchell et al., 1990). January temperatures are projected to rise as much as 12°C and July temperatures as much as 9°C over temperatures simulated at ambient CO2 (Kittel et al., 1997). Projections of precipitation are quite variable over the region, ranging from 71 to 177% of ambient levels in January and 29 to 153% of ambient in July among several GCMs (Kittel et al., 1997). Such climate changes clearly may affect the growth and species composition of our northern forests directly in ways discussed in previous chapters. In contrast with the discussions in previous chapters, this chapter steps up one trophic level to consider the effects of climate change on the populations of microorganisms, fungi, and insects that feed in and on forest trees.
... Their natural role in disturbance dynamics is normally positive cycling nutrients from foliage to soils, killing weak or noncompetitive trees, and decomposing dead trees (Haack and Byler 1993). Most insects and diseases rarely reach epidemic levels, but some insect species can cause significant damage when their numbers reach outbreak levels (Mason 1987). Between 1991 and 1996, native insects that reached outbreak levels on the ANF included cherry scallopshell moth, elm spanworm, forest tent caterpillar, and oak leaftier. ...
This publication describes the forest vegetation and health conditions of the Allegheny National Forest (ANF). During the past 15 years, the ANF has experienced four severe droughts, several outbreaks of exotic and native insect defoliators, and the effects of other disturbance agents. An increase in tree mortality has raised concerns about forest health. Historical aerial surveys (1984-98), the 1989 Forest Inventory and Analysis plot data, and the 1998/1999 Forest Health Monitoring plot data were analyzed to compare disturbed and undisturbed areas. Tree mortality and crown dieback levels were compared between undefoliated areas and areas defoliated by cherry scallopshell moth, elm spanworm, and gypsy moth. American beech mortality was compared inside and outside the beech bark disease killing front. This study illustrates the value of an intensified grid of Forest Health Monitoring plots and demonstrates the integration of aerial survey and plot data.
... Chey (1987) and Hutacharern (1993) reported many species of lepidopteran larvae defoliating acacia. However, despite the great diversity of forest Lepidoptera (over 120,000 species described worldwide), fewer than 15 species have been documented as forming outbreaks (Mason 1987). Many of the tropical lepidopteran larvae occur seasonally (Janzen 1993), which could be the reason for the very few larvae collected in this study. ...
The degree of defoliation in four different habitat types (primary forest, logged forest, acacia plantation and oil palm plantation) was investigated. Logged forest was most severely defoliated in terms of absolute leaf area eaten, percentage of leaf area eaten and number of holes. This was attributed to the many fastgrowing and short-lived tree species in the logged forest which were more palatable to the phytophagous insects. Phytophagous Coleoptera were more speciose and abundant in the forests compared with the plantations. Oil palm plantation recorded fewer Orthoptera compared with other sites. There were significant positive relationships between the abundance of Coleoptera and Orthoptera with the percentage of leaf area eaten. A significant positive relationship was also observed between the Coleoptera abundance and the number of holes on leaves. In this study, Coleoptera were the best predictor of defoliation compared with lepidopteran larvae and Orthoptera.
... In our study this is reflected in the rank-abundance curve for sites with no gypsy moth history from the first sampling year: less than a quarter of the community was taken up by the dominant species (Fig. 3).Table 5. Mixed effects models of the additive effects of gypsy moth (GM) outbreak history and sampling year on five characteristics of communities of macrolepidoptera collected from burlap bands on 50 hardwood trees at each of 10 northern temperate forest stands in north-central Ontario, Canada, with different histories of GM outbreak over 2 sampling years ( However, the rest of the rank-abundance plots show dominance by gypsy moth of up to 96% of the sample, representing a considerable shift in the dominance structure of the community. Extreme dominance by one species is common during outbreaks of forest caterpillars (Faeth 1987; Mason 1987 ), and periodic outbreak populations are a well-studied phenomenon in temperate-forest macrolepidoptera (e.g., Hunter 1991; Berryman 1996). Yet in a field dominated by the study of population dynamics of pest species, research on the community impacts of extremely abundant outbreak species is rare. ...
Little research has addressed the impacts of invasive-species establishment on native forest insect communities. Such information is lacking even for gypsy moth, Lymantria dispar (L.) (Lepidoptera: Erebidae), the most thoroughly studied invasive forest insect. We investigated the ecological impacts of gypsy moth on native species at sites in north-central Ontario, Canada, with and without significant histories of gypsy moth defoliation over a 2-year period. Patterns in native forest caterpillar communities are described using measures of species diversity and multivariate analysis. We documented a transition from low-level to dominant gypsy moth populations. Sites with different gypsy moth outbreak histories exhibited differences in rank-abundance distributions and dominance structures in the first year of the study; by the second year, gypsy moth was dominant at sites of both types irrespective of their previous defoliation history. Contrary to our predictions, we found that gypsy moth outbreak history had no significant effects on native caterpillar community diversity or structure. However, sites with currently high gypsy moth abundance demonstrated significant shifts in late-season caterpillar community structure. Our results suggest that observed community differences were due to the presence of a highly abundant folivore, and not to permanent shifts in the native community because of the introduction of an invasive species. Résumé*On a consacré peu de recherches aux impacts de l'établissement des espèces
... The conditions of our study were somewhat unusual insofar as spiders were unlikely to have been limited by food because of the large numbers of spruce budworm (>200 larvae per square meter) available as prey. Between outbreaks, insect-defoliator prey are usually rare «1.0 larva per square meter) and some spiders may have a shortage oHood (Mason 1987). Spiders also are highly sensitive to vegetation structure (Rypstra 1983, Gunnarsson 1988) and undoubtedly were affected adversely in this study by defoliation that degraded their habitat. ...
Spiders are acknowledged predators on all life stages of western spruce budworm, Choristoneura occidentalis Freeman, yet little is known about their communities in budworm infested forests. Systematic samples of arboreal spiders were taken from the mid-crowns of Douglas-fir, Pseudotsuga menziesii variety glauca (Beissner) Franco, and grand fir, Abies grandis (Douglas ex D. Don) Lindley, for 3 consecutive years during a budworm outbreak. Densities over the 3-yr period averaged 5.6 spiders and 288.0 fourth-instar budworms per square meter of branch area. Spider densities were significantly higher on Douglas-fir (7.8/m2) than on grand fir (4.6/m2); however, species composition of spiders was similar on both tree species. Overall densities of spiders did not differ significantly over the 3 yr. All sampled spiders were classified by family, but because of large numbers of juveniles only 20% could be identified to species. Nonetheless, 13 families and at least 26 species were represented in the samples. Relative abundance of both families and species followed a typical logarithmic-series distribution in which a relatively few taxa accounted for most of the individuals sampled. Total number of spiders sampled was about evenly divided between web-spinning (47.3%) and hunting (52.7%) guilds; however, there were almost twice as many web-spinning species as hunting species. Species-diversity indices also were consistently higher each year for web-spinners than for hunters. The salticid species Pelegrina aeneola (=Metaphidippus aeneolus) (Curtis), a well-known predator of defoliating insects, was clearly dominant each year and accounted for almost half of all hunting spiders. The results offer a convenient marker for comparison with the relative structure of arboreal spider communities in other forest ecosystems.
... Most natural populations of herbivorous insects do not reach outbreak densities (Mattson and Addy 1975; Mason 1987; Hunter 1995) while a few others (Baltensweiler et al. 1977; Ginzburg and Taneyhill 1994; Myers 1993; Berryman 1995 ) display irregular outbreaks or regular cycles, an outbreak being defined as an " explosive increase in the abundance of a particular species that occurs over a relatively short period of time " (Berryman 1987). The geometrid Epirrita autumnata (Borkhausen) (earlier called Oporinia autumnata) displays outbreaks in mountain birch [Betula pubescens ssp. ...
Creating multiyear cycles in population density demands, in traditional models, causal factors that operate on local populations
in a density-dependent way with time lags. However, cycles of the geometrid Epirrita autumnata in northern Europe may be regional, not local; i.e., successive outbreaks occur in different localities. We review possible
causes of cycles of E. autumnata under both local and regional scenarios, including large-scale synchrony. Assuming cyclicity is a local phenomenon, individual
populations of E. autumnata display peaks but populations all over the outbreak range fluctuate in synchrony. This concept assumes that the peaks at
most localities are so low that they do not lead to visible defoliation and easily remain unnoticed. In this scenario, populations
are able to start recovery a few years after the crash, i.e., at the time of the mitigation of detrimental delayed density-dependent
factors, such as delayed inducible resistance of the host plant or parasitism. In that case, the same factors that lead to
crashes also explain the periodicity of cyclic fluctuations. According to the regional cyclicity scenario, different factors
can be important in different phases of the cycle. The key is to identify the factors that tend to produce outbreaks with
a periodicity of about 10 years. Initiation of the increase phase seems to coincide with maxima in sunspot activity, but causal
connections remain unclear. Climatic factor(s) associated with the solar cycle could contribute to the large-scale geographic
synchrony.
... The potential mechanisms for such regional differences comprise environmental conditions and genetic differences, as well as multitrophic interactions ( Berryman, 1988;Cappucino & Price, 1995). Although delayed density-dependent interactions with host plants and specialized natural enemies are the most frequently suggested factors for causing forest insect population cycles, which often culminate in outbreak densities for a few successive years ( Berryman, 1988;Ruohomäki et al., 2000), predation by generalists may act as a principal force in maintaining low population densities ( Mason, 1987;Klemola et al., 2002). In particular, pupal predation is considered to have a regulating influence on many lepidopteran populations ( East, 1974;Bauer, 1985;Cook et al., 1994;Elkinton et al., 1996;Tanhuanpää et al., 1999;Raymond et al., 2002). ...
One of the unresolved questions in studies on population dynamics of forest Lepidoptera is why some populations at times reach outbreak densities, whereas others never do. Resolving this question is especially challenging if populations of the same species in different areas or of closely‐related species in the same area are considered.
The present study focused on three closely‐related geometrid moth species, autumnal Epirrita autumnata , winter Operophtera brumata and northern winter moths Operophtera fagata , in southern Finland. There, winter and northern winter moth populations can reach outbreak densities, whereas autumnal moth densities stay relatively low.
We tested the hypothesis that a lower vulnerability to pupal predation may explain the observed differences in population dynamics. The results obtained do not support this hypothesis because pupal predation probabilities were not significantly different between the two genera within or without the Operophtera outbreak area or in years with or without a current Operophtera outbreak.
Overall, pupal predation was even higher in winter and northern winter moths than in autumnal moths. Differences in larval predation and parasitism, as well as in the reproductive capacities of the species, might be other candidates.
... Fluctuations in the densities or relative abundances of Lepidoptera are well known, and have received widespread attention in studies of insect population dynamics (Myers 1988; Hunter 1995a; Price 1997 Price , 2003). However, in spite of several attempts to understand why some 1–2% of species (Faeth 1987; Mason 1987) undergo outbreaks whereas the vast majority of Lepidoptera possess latent population dynamics (Myers 1988; Redfearn and Pimm 1988; Price 1990; Hunter 1991 Hunter , 1995a; Ginzburg and Taneyhill 1994; Tammaru et al. 1995; Tammaru and Haukioja 1996; Ruohomäki et al. 2000; Haukioja 2005), no definitive links between life-history traits and eruptive population dynamics have yet been identified. Several methodological issues may have contributed to the failure to detect life-history correlates of outbreaks, or to the inconsistency of results obtained in different studies. ...
Records of 232 moth species spanning 26 years (total catch of ca. 230,000 specimens), obtained by continuous light-trapping in Kevo, northernmost subarctic Finland, were used to examine the hypothesis that lifehistory traits and taxonomic position contribute to both relative abundance and temporal variability of Lepidoptera. Species with detritophagous or moss-feeding larvae, species hibernating in the larval stage, and species pupating during the first half of the growing season were over-represented among 42 species classified as abundant during the entire sampling period. The coefficients of variation in annual catches of species hibernating as eggs averaged 1.7 times higher than those of species hibernating as larvae or pupae. Time-series analysis demonstrated that periodicity in fluctuations of annual catches is generally independent of life-history traits and taxonomic affinities of the species. Moreover, closely related species with similar life-history traits often show different population dynamics, undermining the phylogenetic constraints hypothesis. Species with the shortest (1 year) time lag in the action of negative
feedback processes on population growth exhibit the largest magnitude of fluctuations. Our analyses revealed that only a few consistent patterns in the population dynamics of herbivorous moths can be deduced from life-history characteristics of the species. Moreover, the diversity of population behaviour in one moth ssemblage challenges any conventional wisdom suggesting predictable patterns. Our results raise several questions about perceptions and paradigms in insect population dynamics and stress the need for research on detritivorous insect population dynamics, as well as the need for more assemblage-wide studies using common trapping methods to provide comparative data on related and unrelated species with different life-history traits.
Extreme weather events dramatically impact populations of individual insect species but the consequences of such events for entire insect communities are not well documented. We present evidence that mid spring frosts and summer drought negatively affect the community of insects found on Missouri oaks (Quercus alba and Q. velutina), amounting to a 23–186 fold decrease depending on the oak species, insect body size and feeding guild, and the specific weather event. Depending on the event, spring faunas required 1–5 years and summer faunas 1–4 years following spring frosts to reach pre-event levels. The impact of summer drought on leaf tying caterpillars also lasted over an extended period of time; it was 5 years before numbers of leaf ties reached pre-drought levels. Smaller-bodied species of leaf tying Lepidoptera took longer to recover than larger-bodied species following the drought. Overall, we found no evidence for a general decline in abundance, even a modest one, during the 20 years of study of faunas on oak trees in southeastern Missouri. However, the risk of mid-season frost damage to trees is expected to increase with predicted earlier onset of spring. Similarly, the effects of drought reported here are likely to increase with time, as the climate in the Midwest U.S. is only expected to become warmer and drier during the summer months. Understanding the impact of such weather events on insect communities influences our ability to predict how habitat and landscape management, or lack thereof, will influence future patterns of insect abundance and diversity.
The chapter deals with the interactions of animal populations and their habitats in a more general view. The many factors controlling these interactions are considered in the following sections.
Agroforestry systems most often harbor more pests than monocrop systems. Parasitism is increased by management practices that increase light availability in agroforestry systems, together with connectivity between these agroforestry systems and the forest. In cocoa farms, the trees used to provide shade also host the fungus Phytophthora megakarya, which causes brown rot in cocoa plants. In Malawi, some insects such as Brachyplatys testudonigro, Mesoplatys ochroptera, Exosoma sp. and Ootheca sp. feed on the sap of Sesbania sesban, and are associated with other agroforestry species. Four types of caterpillar that defoliate Ricinodendron heudelotii were identified in the Democratic Republic of Congo, namely Lobobunaea phaedusa, Imbrasia petiveri, Imbrasia epimethea, Imbrasia obscura and probably Imbrasia melanops. Parasitism in agroforestry systems is also due to the integration of germplasms from species with high nutritional and commercial value in local agroforests, and that germsplasms’ integration can be problematic if the genetic base of the species being domesticated is reduced. The integration of trees in agricultural landscapes can also contribute to pest control. Trees can serve as a barrier to insect movement, thereby reducing crop infestation. Integrated pest management in agroforestry can be achieved by: (i) the identification and use of host plants that are resistant to pests and pathogens, (ii) crop rotation between host plants and plants that do not harbor pests, (iii) the biological control of pest abundance, and (iv) the use of farming practices that do not increase light intensity in agroforestry systems.
Describes forest vegetation and health conditions on the Allegheny National Forest (ANF). During the past 20 years, the ANF has experienced four severe droughts, several outbreaks of exotic and native insect defoliators, and the effects of other disturbance agents. An increase in tree mortality has raised concerns about forest health. Historical aerial surveys (1984-98), Forest Inventory and Analysis plot data collected in 1989, and FHM plot data collected 1998-2001 were analyzed to compare disturbed and undisturbed areas. Tree mortality and crown dieback levels were compared between undefoliated areas and areas defoliated by cherry scallopshell moth, elm spanworm, and gypsy moth. American beech mortality was compared inside and outside the beech bark disease killing front. This study illustrates the value of an intensified grid of P3 plots and demonstrates the integration of aerial survey and plot data.
We conducted a two-year predator-exclusion study to assess the magnitude and timing of larval predation in non-outbreaking populations of a geometrid moth, Epirrita autumnata. Laboratory-produced newly hatched larvae were placed on the experimental trees which were assigned to five treatments within two larval densities: (1) all predators, including parasitoids, excluded by mesh bag, (2) birds excluded by cage, (3) ants excluded by glue ring, (4) birds and ants excluded, and (5) control without any predator exclusion. Thereafter, larvae were censused every 3-4 d throughout the five-instar larval period. Mortality of E. autumnata larvae in these populations was high and mostly due to natural enemies. In control trees, only ∼10% of larvae survived, while survival was ∼90% in mesh bags preventing all natural enemies. Bird exclusion significantly improved larval survival, as survival was almost three times higher in trees with cages than in those without cages. On the other hand, ant exclusion did not have any overall effects on larval survival, mostly because ants were only detected in about half of the trees without glue rings. Larvae survived longer in high-density trees from which ants were excluded, but the effect was masked by high mortality, Unrelated to ant exclusion, in the late larval season. The results suggest that the effect of ant predation on survival of E. autumnata larvae may be spatially restricted and not important at a larger scale. The same result applies for crab spiders, as they caused high mortality in ∼20% of the study trees. Our results emphasize the importance of considering the spatial scale as well as assessing the impact of multiple predators in order to detect predators affecting survival at the population level. Exclusion of all predation had a significantly stronger effect on larval survival than exclusion of birds alone. Further, mortality was highest during the late larval period, when parasitoids emerge. Thus, a large proportion of larval mortality was most likely due to parasitism. Our results suggest that predation by passerine birds and parasitism may contribute to maintenance of low E. autumnata densities by strong suppression of the number of larvae entering the pupal stage.
While synchrony among geographically disjunct populations of the same species has received considerable recent attention, much less is known about synchrony between sympatric populations of two or more species. We analyzed time series of the abundance of ten species of spring foliage feeding Lepidoptera sampled over a 25-year period at 20 sites in the Slovak Republic. Six species were free-feeders and four were leaf-rollers as larvae. Twenty-nine percent of interspecific pairs were significantly synchronous and correlations were highest among species exhibiting similar feeding strategies. Similar patterns of interspecific synchrony have been previously demonstrated in several other taxa, and the synchronizing effects of weather and/or specialist predators have been proposed as mechanisms. As an alternative explanation, we explored a model in which two species within the same feeding guild were synchronized by the functional response of generalist predators. In this model, species remained unsynchronized in the complete absence of predation or when predatory pressures were applied to only one species. Pairs of prey species projecting relatively similar search images to predators were more highly synchronized than species with relatively different search images. Prey handling time only influenced synchrony when it was very high relative to the total time prey was exposed to predators. Our model's prediction of greater synchrony among species that project similar search images to predators was in agreement with our field study that showed greater synchrony among species sharing similar larval feeding strategies and morphologies.
Trembling aspen (Populus tremuloides Michx.) is the most widely distributed tree in North America, occurring over a variety of climatic, soil and topographical conditions.1,2 Interactions between aspen and its biotic and abiotic environments play pivotal roles in the ecological dynamics of many early-suc-cessional ecosystems. These interactions, in turn, influence and are influenced by the chemical composition of aspen.
Among‐population differences in pupal mass were studied in a geometrid, Epirrita autumnata. Some Epirrita autumnata populations regularly reach outbreak densities while others are never known to do so. Because adults do not feed, pupal mass of females correlates strongly with fecundity.
Larvae were collected from twelve field sites. Ten of our sample populations originated within the outbreak range of the species and represented different phases of outbreaks. Two populations originated outside the outbreak range.
Pupal mass of field‐collected E. autumnata varied significantly among populations. The peak phase populations had the smallest pupae and the biggest were found in low density populations outside the outbreak range.
Offspring of moths from each population were reared under identical conditions in two larval densities. Significant differences were not found in pupal mass among populations. That is, the inherent size, correlated with fecundity of moths, was not different between populations originating within and outside the outbreak range, nor among collections from different densities or phases of the outbreaks.
Rearing density did not interact in a consistent way with population.
As far as size and fecundity are concerned, the results do not support Chitty's hypothesis that differences in genetic composition of the population at low and high density phases generate cyclic fluctuations of population density.
Because no hereditary or maternal differences were found in size and fecundity between E.autumnata originating within and outside the outbreak range, variation in reproductive capacity cannot explain why outbreaks occur only in some populations.
1 The use of native natural enemies to combat native pests has received proportionately less consideration than other viable approaches to biological control. Successful attempts at augmentation, conservation, preservation, and enhancement of the effectiveness of natural enemies have been limited.
2 However, this approach to biological control should be re-examined, placing emphasis on intentional manipulation of population dynamics to encourage the natural tendencies of populations to grow in the absence (or during the delay in resurgence) of certain regulating forces.
3 We discuss theoretical and empirical evidence that suggests native herbivorous insects have the potential to be manipulated successfully as biological control agents for native competing vegetation.
Many species of birch‐feeding Lepidoptera make leaf‐shelters by tying leaves together with silk. Several species, or several instars of a single species, may be found together within a single leaf‐shelter.
Shelters made in June by the birch tube‐maker Acrobasis betulella (Pyralidae) are colonized throughout the season by other Lepidoptera. Artificial A.betulella shelters, made by tying leaves together with string, were colonized at a greater rate than nearby control foliage, indicating that secondary species are indeed responding to the presence of the shelter, and not to some other aspect of plant quality.
Several species in the families Oecophoridae, Gelichiidae and Stenomidae make ‘leaf‐sandwiches’ by tying two leaves together; these are later colonized by oviposition from adult A.betulella. Artificial sandwiches made by joining leaves together with paperclips were colonized by several species of sandwich‐makers, as well as by A.betulella. Colonization occurred mainly via oviposition, although some sandwiches may have been colonized by wandering larvae.
First instar A.betulella , which cannot form their own shelter, enjoyed greater survivorship when placed in artificial leaf‐sandwiches than when placed on leaves without sandwiches.
1. Most insect species occur at low abundance but a greater research effort has been devoted to so‐called outbreak species and little research is available on scarce (low abundance) species that are typical of most insect species.
2. Larval free‐feeding macrolepidoptera of two riparian trees Salix nigra (Marsh) (black willow) and Acer negundo L. (box elder) were sampled and sorted by species and abundance.
3. Data collected established that the majority of species in the assemblages in each tree species occurred at low abundance in each of the 5 years when larvae were sampled.
4. Species in the Noctuidae and Geometridae dominated both assemblages.
5. On both trees, assemblages were dominated numerically by relatively few species, a pattern that has been observed for insect assemblages on plants in managed and unmanaged habitats.
1. Geographical distributions of individual foliage-feeding forest herbivore species have been found to be aligned closely with the distribution of their host trees, however little is known about the extent to which broad herbivore communities are geographically associated with distinct host communities.
2. Large-scale geographic variation in canopy herbivore communities in a 80 000 km2 area (the state of Maine, U.S.A.) was characterised using historical insect survey data. Variation in insect communities was compared with corresponding variation in forest over-storey composition, which was quantified using data from a regional forest inventory survey.
3. Principal components analysis was used to characterise associations among herbivore and tree species. Analysis of the herbivore data identified three main insect groups: group A corresponded to a single species (Choristoneura fumiferana), group B corresponded to pine-feeding species including Semiothisa sp., and group C corresponded to the spruce-feeding species Gilpinia hercyniae and Pikonema alaskensis. Analysis of the forest inventory data characterised three important forest types: northern hardwoods, eastern white pine, and northern spruce-fir forest types.
4. Spatial analyses were carried out on the first two components of each of the principal components analyses. Factor 1 of the insect data showed a trend of decreasing values from south to north, while factor 2 of the forest inventory data showed an opposite trend. These inverse trends reflected the distribution of the main contributing species to the principal components analysis, C. fumiferana and Pinus strobus respectively. These distributions were highlighted further by the significant negative cross-correlations that were found between the two factors up to distances of 120 km.
5. Analyses indicated a parallel between the geographic variation in the insect guilds associated with conifers and the geographic pattern of their conifer hosts. Hardwood-feeding caterpillars, on the other hand, did not form well-defined guilds and showed varying geographical distributions. The survey data showed varying quality in defining large-scale associations in the structures of herbivore and host communities.
6. Implications for biodiversity management are discussed.
1. We studied the effect of pupal predation on Epirrita autumnata populations within and outside the outbreak range, in northern and southern Finland, respectively. The purpose of the study was to evaluate the potential of pupal predation to contribute to the contrasting dynamics of northern and southern populations. In addition to overall pupal survival, we separately estimated the influence of small mammal and invertebrate predation.
2. Each summer for a period of 5 years, pupae inside their cocoons were buried in the ground. The pupae were re-collected before the flight season and checked for damage. Direct and delayed density dependence of pupal survival was tested using the current and the previous year larval density indices, respectively.
3. Annual survival of pupae from both small mammal and invertebrate predation was lower in the south. Daily survival rates did not differ between the south and the north indicating that the difference in annual pupal survival is most likely due to the longer pupal period in the south.
4. In the south, pupal survival was negatively density dependent and resulted from small mammal predation. The results indicate that small mammals may prevent E. autumnata populations from reaching high densities in the south.
5. In the northern study area with a current outbreak, total pupal survival was density independent. Also small mammal predation was density independent, most likely due to predator satiation. Moreover, there was no difference in pupal survival between northern areas with and without a current outbreak. The results indicate that pupal predation was not a significant factor in termination of the outbreak, nor is it likely to contribute to the maintenance of the cyclic density fluctuations.
6. We conclude that low pupal survival and its density dependence are likely to contribute to stability of E. autumnata populations in southern Fennoscandia. On the contrary, shorter pupal period and the lack of density dependent pupal predation may make northern populations more prone to outbreaks. The potential regulating effect of pupal predation in southern populations is due to small mammals being able to respond to changes in E. autumnata densities.
We examined how plant genetic variation and a common herbivore (the leaf-galling aphid, Pemphigus betae) influenced leaf litter quality, decomposition, and nutrient dynamics in a dominant riparian tree (Populus spp.). Based on both observational studies and a herbivore exclusion experiment using trees of known genotype, we found four major patterns: 1) the quality of galled vs non-galled or gall-excluded litter significantly differed in the concentration of condensed tannins, lignin, nitrogen and phosphorus; 2) the difference in litter quality resulted in galled litter decomposing at rates 34 to 40% slower than non-galled litter; 3) plant genotype and herbivory had similar effects on the magnitude of decomposition rate constants; and 4) plant genotype mediated the herbivore effects on leaf litter quality and decomposition, as there were genotype-specific responses to herbivory independent of herbivore density. In contrast to other studies that have demonstrated accelerated ecosystem properties in response to arthropod herbivory, our findings argue that herbivore-induced secondary compounds decelerated ecosystem properties though their “after-life” effects on litter quality. Furthermore, these data are among the first to suggest that genotype-specific responses to herbivores can have a major impact on decomposition and nutrient flux, which likely has important consequences for the spatial distribution of nutrients at the landscape level. Due to the magnitude of these effects, we contend that it is important to incorporate a genetic perspective into ecosystem studies.
The population dynamics and the relative importance of bottom-up and top-down effects in a plant-leafminer-multiparasitoid interaction was studied between 1981 and 1990 in a natural forest in Kyoto, Japan. The leafminer, Chromatomyia suikazurae (Agromyzidae, Diptera), passed two generations (G1 and G2) on Lonicera gracilipes (Caprifoliaceae). The G1 population in February was free from parasitoid attack, and the mortality in G1 was mainly caused by resource limitation. Intraspecific competition for resources occurred at the larval stage in G1, and the larval mortality was density-dependent. The G1 adult density was resource-limited (the number of newly opened leaves), and its variability was lower than that of G2. The G2 population in April was not resource-limited but subject to intense attack by a species-rich parasitoid complex, and thus total mortality was much larger than that in G1. Significant density dependence was detected not in larval but in pupal mortalities, which were mainly caused by parasitism by parasitoids that attacked the pupa. The host population alternately experienced bottom-up effects during the larval stage in G1 and top-down effects during the pupal stage in G2. Overall population fluctuation was non-cyclic and mainly due to climatically-induced fluctuation of available plant resources in G1.
In this article we review the use of natural enemies in crop pest management and describe research needed to better meet information needs for practical applications. Endemic natural enemies (predators and parasites) offer a potential but understudied approach to controlling insect pests in agricultural systems. With the current high interest in environmental stewardship, such an approach has special appeal as a method to reduce the need for pesticides while maintaining agricultural profitability. Habitat for sustaining populations of natural enemies occurs primarily at field edges where crops and edge vegetation meet. Conservation and enhancement of natural enemies might include manipulation of plant species and plant arrangement, particularly at these edges; and consideration of optimum field sizes, number of edges, and management practices in and near edges. Blending the benefits of agricultural and forestry (windbreak) systems is one promising approach to field edge management that has additional benefits of wind protection and conservation of desirable wildlife species.
The hypothesis is developed that there are causal linkages in evolved insect herbivore life histories and behaviors from phylogenetic
constraints to adaptive syndromes to the emergent properties involving ecological interactions and population dynamics. Thus
the argument is developed that the evolutionary biology of a species predetermines its current ecology.Phylogenetic Constraints refer to old characters in the phylogeny of a species and a group of species which set limits on the range of life history
patterns and behaviors that can evolve. For example, a sawfly is commonly limited to oviposition in soft plant tissue, while
plants are growing rapidly.Adaptive Syndromes are evolutionary responses to the phylogenetic constraints that minimize the limitations and maximize larval performance.
Such syndromes commonly involve details of female ovipositional behavior and how individuals make choices for oviposition
sites relative to plant quality variation which maximize larval survival. Syndromes also involve larval adaptations to the
kinds of choices females make in oviposition. The evolutionary biology involved with phylogenetic constraints and adaptive
syndromes commonly predetermines the ecological interactions of a species and its population dynamics. Therefore, these ecological
interactions are calledEmergent Properties because they are natural consequences of evolved morphology, behavior, and physiology. They commonly strongly influence the
three-trophic-level interactions among host plants, insect herbivores, and carnivores, and the relative forces of bottom-up
and top-down influences in food webs. The arguments are supported using such examples as galling sawflies and other gallers,
shoot-boring moths and beetles, budworms, and forest Macrolepidoptera. The contrasts between outbreak or eruptive species
and uncommon and rare species with latent population dynamics are emphasized.
We studied the effects of predation and oviposition activity on reproductive success of a late-season moth, Epirrita autumnata by exposing adult females and eggs to predation in their natural habitat in two successive years. Daily survival rates of adult females ranged from 0.4 to 0.8, average being 0.7. Most predation occurred during nights and was caused by harvestmen and other invertebrate predators. Avian predation did not have an effect on adult survival rates, most likely because of the lateness of E. autumnata flight season. Eggs were also preyed upon by invertebrate predators, although a notable proportion of egg mortality was attributable to causes other than predation. Daily survival rates of eggs were more than 0.99. Using modeling based on empirical data on eclosion of female adults, their oviposition behavior and survival rates of adults and eggs, the daily survival rates were translated into population level consequences. Adult predation was estimated to decrease reproductive success of non-outbreaking E. autumnata by 60-85 percent and egg mortality by 20-40 percent. Predation on adult lepidopterans is a mortality factor potentially as relevant as predation in any other life history stage and thus, should not be ignored in studies of population regulation.
Preface Part I. Introduction: 1. The scope of insect ecology Part II. Behavioral Ecology: 2. Behavior, mating systems, and sexual selection 3. Social insects: the evolution and ecological consequences of sociality Part III. Species Interactions: 4. Plant and herbivore interactions 5. Lateral interactions: competition, amensalism, and facilitation 6. Mutualisms 7. Prey and predator interactions 8. Host and parasite interactions Part IV. Population Ecology: 9. Demography, population growth and life tables 10. Life histories 11. Population dynamics Part V. Food Webs and Communities: 12. Community structure 13. Multi-trophic interactions Part VI. Broad Patterns in Nature: 14. Biological diversity 15. Planet Earth: patterns and processes Glossary References Taxonomic index Author index Subject index.
This study investigated the temporal patterns of parasitoid attack and diversity within and among years and the potential ecological factors influencing the incidence of parasitism experienced by the larvae of a bivoltine moth, Psilocorsis quercicella Clemens. To do this, field- collected larvae were reared throughout both generations during 1995—1997 in eastern Missouri. Annual variation in percentage parasitism was extremely low, ranging from 16.2 to 18.1%. Low annual variation in parasitism may contribute to host population stability. In contrast, intergenerational differences in percentage parasitism were highly significant in 2 of the 3 yr of study, with higher rates in the 1st generation (G1) than in the 2nd generation (G2). Instar at collection was a significant predictor of percentage parasitism during G1 but not during G2; this difference was likely caused by variation in the frequency of attack by idiobiont parasitoids in the 2 generations. Ten species of parasitoids were positively identified from the rearings. Of these, 3 were new records and 1 was a new species. A consistent increase in parasitoid species richness from G1 to G2 was found for all 3 yr, with several generalist parasitoids emerging only from larvae collected during G2. Host-switching behaviors related to seasonal differences in the abundance of a congener, P. reflexella Clemens, are hypothesized to contribute to the increased parasitoid diversity in G2. Although parasitoids were more likely to visit high-density ties, the risk of parasitism for individual larvae was independent of the density of both conspecifics and heterospecifics occupying a leaftie. A field experiment exam- ining the effects of prior tie occupancy on parasitism found that larvae placed in previously occupied ties were almost twice as likely to be parasitized (24.4%) as larvae placed in newly created ties (13.6%).
Thesis (Ph. D.)--West Virginia University, 2003. Vita. Includes abstract. Includes bibliographical references . Photocopy.
The acridian species Melanoplus bivittatus (Say), Melanoplus differentials (Thomas) and Melanoplus lakinus (Scudder) occur in the same habitat, utilizing the same foods. Such coexistence suggests that these species are in the same niche and in competition for a common food supply. Their food usages were investigated by offering samples of dominant and semi—dominant vegetation from the common habitat to caged populations of each species, then estimating the amount consumed. The overall usual of foods of each species formed a preferential pattern sufficiently different from the patterns of the other two species to indicate that the three grasshopper populations occupy separate niches in the community and are not in complete competition for food. See full-text article at JSTOR
MacAthur's model was derived to predict the relative abundance of species in adequately sampled sympatric associations in which niches are contiguous and nonoverlapping, the species are comparable in size and physiology and are maintained in equilibrium so that relative abundances are constant. The model is applicable only to homogeneously diverse biotopes in which the range of the environmental mosaic is small in relation to the requirements of the inhabitant species. These factors require that the niche arrangement be the outcome of competitive exclusion. Data are presented on the adequacy of the MacArthur model in describing the relative abundance of fish, ophiuroids, gastropods, pagurid crabs, and ciliates. In addition, summaries of previously reported tests are included. By comparing groups which are adequately and inadequately described by the MacArthur model, it is possible to examine the characteristics which may establish the equilibrium. It is hypothesized that characteristics of importance are length of life cycle, duration and frequency of reproduction, and relative (to generation time) constancy of environmental conditions. It is shown that when fish or gastropod species are tested, the goodness-of-fit to the model's predictions is related to the taxonomic affinity of the group being tested. Stable associations of species with high taxonomic affinity must be highly organized. A high degree of organization is required to maintain the random distribution of abundances. Density dependent competition for food is the most likely bases of this organization.
When data are collected so that they can be treated as coming from a variety of sample sizes, it is possible to use the same data to test the effect of altering sample size on the fit to any model. The method is applied to nonpredatory soil mites and their fit to the @'broken-stick@' model of MacArthur. It is found that by choosing a quadrat size of 4 in.^2 (25.8 cm^2), a good fit is obtained. Smaller and larger quadrats involving the same data do not permit the data to fit the model. It is concluded that the model lacks ecological meaning.
Lepidopteran species richness was strongly related to sampling intensity as measured by the number of collections made by the Canadian Forest Insect Survey of insects on broad-leaved trees in Ontario. Local abundance of the host, independently estimated by Beschel et al. (1962), and ecological and geographical distribution of the host were unrelated to the local species richness of Lepidoptera when the effect of sampling intensity was accounted for. The importance of collecting effort in this analysis of an extensive sampling program suggests that the effect of sample size must be considered before it is possible to test the effects of host characteristics on herbivore species richness.
The investigations of the bionomics and parasitoid-host interactions of some gypsy moth parasitoids provide an opportunity to consider the role of r and K strategies in the life history of some tachinid and hymenopterous parasitoid species. Available historic data as well as results of recent studies are utilized in an attempt to evaluate the degree to which the data conform to this paradigm. Few studies have provided the information required to evaluate this concept particularly in a parasitoid complex. The concept of r and K also is discussed in relation to biological control strategies, and the implications of this analysis for those arguments are discussed.
It is proposed that for many if not most animals — both herbivore and carnivore, vertebrate and invertebrate — the single most important factor limiting their abundance is a relative shortage of nitrogenous food for the very young. Any component of the environment of a plant, by varying the amount of adequately nutritious plant tissue available to herbivores, may consequently affect the abundance of food through all subsequent trophic levels; in this regard weather may be important more often than is immediately obvious.
The hypothesis proposes that animals live in a variably inadequate environment wherein many are born but few survive, and leads to a concept of populations being “limited from below” rather than “controlled from above”. And it may lead to a reappraisal of the role of predation, competition and social and territorial behaviour as factors likely to influence the numbers of animals in the environment, the response of “pests” to manipulation of populations of their food plants by Man, and the likely effectiveness of agents of biological control.
Phytophagous insect larvae (Lepidoptera and Tenthredinoidea) in the leaf-chewing guild were sampled from accessible parts of all the 18 species of common woody plants in a mature upland forest in New York State, in early and in late June. Among the procedures used to describe patterns of associations between plants and insects are principal components analysis and the clus- tering techniques of numerical taxonomy, whereby the faunistic similarity among plant species is described. We introduce a weighting factor for cluster analysis that weights a character (in this case the density of an insect species on a plant) in proportion to its deviation from the norm. During June, both the abundance and variety of insects declined on most plant species, which vary greatly in both these aspects of their fauna. Insect species that are either quite highly specialized or generalized in diet appeared to be prevalent; those with intermediate diet breadth were less so. Clustering plant species by faunal similarity revealed some clusters of taxonomically related species, but the correspondence between faunal similarity and taxonomic affinity is not strong. Few plant species have a highly distinct, specialized fauna. The complexity of distribution of insects over plants indicates that insects are responding to many factors that differentiate plant species.