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Parasitoid ecology along geographic gradients: lessons for climate change studies
Abstract
One method to study the impact of climate change on host-parasitoid relationships is to compare populations along geographical gradients in latitude, altitude or longitude. Indeed, temperatures, which vary along geographic gradients directly shape the life traits of parasitoids and indirectly shift their populations through trophic interactions with hosts and plants. We explored the pros and cons of using these comparisons along gradients. We highlighted that the longitudinal gradients, although understudied, are well correlated to winter warming and summer heat waves and we draw attention to the impact of the increase in extreme events, which will probably be the determining parameters of the effect of climate change on host-parasitoid relationships.
... Exploring and understanding the geographical pattern of soil biodiversity is an important and long-standing task for biogeography and macroecology in the context of climate change in the Anthropocene. Patterns across latitudinal, longitudinal, and elevational gradients are of widespread concern for biodiversity, including plants [1], birds [2], mammals [3], and invertebrates [4] at genetic [5], species [6], community [7], and ecosystem [8] levels. ...
... Relevant studies show that the latitude factor is primarily associated with changes in temperature, which in turn affects the biomass and niche width of organisms [24]. Longitude often correlates with continentality gradients linked to absolute differences in precipitation and temperature [25], and also correlates with winter warming and summer heatwaves in the context of climate change [4]. There are good theoretical reasons to expect that longitude and its associated precipitation and temperature will likely be the determining factors for the spatial patterns of soil biodiversity [26]. ...
Soil biodiversity plays an important role in maintaining soil fertility and agricultural health. Exploring the patterns of soil fauna diversity across geographical gradients is a fundamental and crucial scientific topic for understanding the mechanisms of soil biodiversity in farmlands. However, the spatial pattern of soil fauna diversity across longitudinal gradients has received far less attention. In order to explore the longitudinal pattern (west to east) of the composition and diversity of the soil oribatid mite community in paddy fields along the middle and lower reaches of the Yangtze River (MLYR), an investigation was carried out in July 2021 at nine sites spanning a longitudinal range of 8.86° (906 km) in southeastern China. In total, 19 taxa and 2392 individuals were observed with a density of 1535.47/m². Protoribates and Ceratozetes were the most dominant and widely distributed genera along MLYR. The distribution patterns of the richness, abundance, and diversity index were obvious across the longitudinal and latitudinal gradients. The longitudinal pattern showed a quadratic distribution of first increasing and then decreasing, while the latitudinal pattern showed an increasing pattern with the increase in latitude (unimodal pattern). The influence of latitude on the abundance of the soil oribatid mite community was greater than that of longitude, and the influence of longitude on richness and the corresponding diversity index was greater. The dominance index did not show a distribution pattern in the longitudinal and latitudinal directions, while the evenness index showed only the distribution pattern in the longitudinal direction, and the latitudinal pattern was not significant. The results of this study suggest that the diversity of the soil oribatid mite community along MLYR displays both longitudinal and latitudinal patterns in paddy fields. Moreover, we highlighted the importance of integrating longitudinal and latitudinal patterns into spatial patterns of the soil fauna community in farmlands at a regional scale.
... Winter warming and summer heat waves correlate directly with the longitudinal geographical gradient. Thus temperature extremity affects the host-parasitoid relationships (Jego et al., 2023). American grapevine leafhopper (Scaphoideus titanus), is the main vector of grapevine diseases in Europe. ...
Global warming has been influenced by anthropogenic factors and the
global pattern of climate change. Warming temperatures have deleterious
effects on the insects worldwide. It has brought about massive changes in the
pattern of distribution of insects by influencing the biology and life cycle of
the insects. A change in the distribution, survival and reproductive behavior
of insects will have far-flung effects on the ecosystem. Many species of
insects have been estimated to be extinct by the end of the century. As insects
are dynamic players of agro-ecosystem, a change in population dynamics will
result in huge economic loss. While some of insects might go extinct, some
pest species have been predicted to witness a surge in population growth.
Hence, proper prediction models have to be used to understand the changing
ecosystem and develop appropriate management measures for the
conservation of environment as well as management of pest species.
... Applying these general concepts to host-parasitoid relationships, we can expect to see over-dispersion of data, resulting from a higher-than-expected rate of parasitism in less profitable hosts or habitats. Parasitoids, however, do not always follow the typical biogeographic pattern of increasing species richness at lower latitudes (Burington et al., 2020), probably due to the intimate interaction with their hosts (Jego et al., 2023). ...
Bet-hedging occurs when unreliable environments select for genotypes exhibiting a lower variance in fitness at the cost of a lower mean fitness for each batch of progeny. This means that at the level of the genotype, the production of mostly non-optimal phenotypes may be favored when at least some phenotypes are successful. As extreme unreliable climatic events are increasing because of climate change, it is pertinent to investigate the potential of bet-hedging strategies that allow insects to cope with climate change. Evidence for bet-hedging is scarce in most insects, including parasitoids, but the unique lifestyle and biology of parasitoids leads to the expectation that bet-hedging may occur frequently. Here, we evaluate a range of parasitoid traits for which a bet-hedging strategy could be envisioned even if bet-hedging has not been identified as such yet. Under-identification of bet-hedging in nature could have resulted from a major focus of studies on parasitoid life history evolution and foraging behavior on optimality models, predicting how mean fitness can be maximized. Most environmental factors, however, vary unpredictably. Life history and behavioral adaptations are thus expected to be affected by environmental stochasticity. In this paper, we review different aspects of parasitoid behavior, physiology, and life histories and ask the question whether parasitoid traits could have evolved under selection by environmental stochasticity.
The prevalence of crop insect pests, which damage crops and reduce their yield, is increasing globally owing to changes in climate and land use, posing a threat to food security. In this Review, we synthesize evidence on how tropical, temperate, migratory and soil crop pests respond to changes in climate, land use and agricultural practices. In general, crop pests are responding to warming with expanded geographic ranges, advanced phenological events and increased number of reproductive generations per year. Increased pest damage under warming is projected to exacerbate yield losses of 46%, 19% and 31% under 2 °C warming for wheat, rice and maize, respectively. Pests at mid–high latitudes respond more positively to warming than those in the tropics. Moderate drought can increase pest damage to crops owing to enhanced feeding on plants as a water source and decreased resilience of plants and natural enemies of pests. Increased precipitation reduces small pests through washing them away, but favours pests in general through buffering thermal-hydro stresses. Land use change, such as deforestation and conversion to cropland, enhances warming and reduces biodiversity, leading to enhanced crop damage. Agricultural intensification, particularly fertilization and irrigation, increases the quality and quantity of host plants and buffers pests from environmental extremes, favouring proliferation. Globalization of trade networks increases pest invasions, with associated damage exceeding US $423 billion in 2019. Future research should examine the mechanisms underlying changes in pest status and develop monitoring and prediction systems to inform management approaches.
Body size has been used thoroughly in arthropod ecology as a reliable trait to assess fitness responses to changes in environmental factors. Among these, spiders represent a large and diverse group, colonizing almost all terrestrial habitats. Here, we propose a review on intraspecific body size variation in arthropods over two main macroecological spatial gradients—latitude and elevation—both of high interest in a global warming context. We found that more species with a direct than with an indirect development present a converse Bergmann cline along both gradients. Focusing on spiders, we propose that life history traits such as voltinism, mobility, and brood care influence intraspecific body size patterns—potentially hiding large-scale patterns. Further, we assessed interspecific sexual size dimorphism (SSD) in spider species. We found that extreme SSD—the most original feature in spider biometry—is influenced by hunting guild rather than phylogeny of spider families, suggesting that ecological factors prevail over evolutionary drivers in shaping SSD.
Climate warming is considered to be among the most serious of anthropogenic stresses to the environment, because it not only has direct effects on biodiversity, but it also exacerbates the harmful effects of other human‐mediated threats. The associated consequences are potentially severe, particularly in terms of threats to species preservation, as well as in the preservation of an array of ecosystem services provided by biodiversity. Among the most affected groups of animals are insects—central components of many ecosystems—for which climate change has pervasive effects from individuals to communities. In this contribution to the scientists' warning series, we summarize the effect of the gradual global surface temperature increase on insects, in terms of physiology, behavior, phenology, distribution, and species interactions, as well as the effect of increased frequency and duration of extreme events such as hot and cold spells, fires, droughts, and floods on these parameters. We warn that, if no action is taken to better understand and reduce the action of climate change on insects, we will drastically reduce our ability to build a sustainable future based on healthy, functional ecosystems. We discuss perspectives on relevant ways to conserve insects in the face of climate change, and we offer several key recommendations on management approaches that can be adopted, on policies that should be pursued, and on the involvement of the general public in the protection effort.
The galls induced by cynipids in host oaks have been proposed as biodiversity hotspots. They constitute support for the development of arthropod communities of great diversity and functional complexity that find in them nutrition and protection from external abiotic and biotic elements which change according to environmental gradients. We characterize the abundance, richness, and diversity of gall-inducing cynipids associated with the galls of the host oak Quercus rugosa, through an elevation gradient. This study was conducted on 140 oak trees belonging to seven populations (20/site) in central Mexico. We identified 39 gall-inducing cynipid species (Cinipini), three inquiline species of galls (Synergini), and seven parasitoids of gall-inducing cynipid (Chalcidoidea), and the secondary fauna was characterized by eleven arthropod orders. We detected a positive and significant effect exerted by the altitudinal gradient on abundance, species richness, and diversity of gall-inducing cynipids, parasitoids, inquilines, and secondary fauna (order richness level). We documented positive and significant relationships between the gall-inducing cynipids and their parasitoids, inquilines, and secondary fauna in terms of species richness, diversity, and abundance of individuals. Arthropod community composition associated with gall-inducing cynipids differed among localities. This study suggests that gall-inducing cynipids in Q. rugosa act as ecosystem engineers that modify the host plant to offer a new habitat to other arthropods. Also, our results supported that Cynipid gall acts to protect the galls-associated arthropod community from unfavorable abiotic conditions throughout the altitudinal gradient.
Simple Summary
Augmentative biological control relies on the more or less frequent/abundant releases of biological control agents (BCAs) that have to be adapted to their short-term local environment including (micro-)climatic conditions. Thermal biology of BCAs is thus a key component for their success. The extent to which thermal tolerance indices may be relevant predictors of the field efficiency is however still poorly documented. Within this frame, we investigated the intraspecific variability for the ability to move at low temperatures in the minute wasp, Trichogramma cacoeciae. We collected, molecularly characterized, and compared for their thermal tolerance indices numerous strains originating from three contrasting geographic areas. Our findings evidenced both a geographic differentiation between strains for one of the thermal tolerance indices and a positive correlation between two of them, demonstrating the existence of an intraspecific variability.
Abstract
Temperature is a main driver of the ecology and evolution of ectotherms. In particular, the ability to move at sub-lethal low temperatures can be described through three thermal tolerance indices—critical thermal minimum (CTmin), chill coma temperature (CCT), and activity recovery (AR). Although these indices have proven relevant for inter-specific comparisons, little is known about their intraspecific variability as well as possible genetic correlations between them. We thus investigated these two topics (intraspecific variability and genetic correlations between thermal tolerance indices) using the minute wasp, Trichogramma cacoeciae. Strains from T. cacoeciae were sampled across three geographic regions in France—two bioclimatic zones along a sharp altitudinal cline in a Mediterranean context (meso-Mediterranean at low elevations and supra-Mediterranean at higher elevations) and a more northwestern area characterized by continental or mountainous climates. Our results evidenced a significant effect of both the longitude and the severity of the cold during winter months on CCT. Results were however counter-intuitive since the strains from the two bioclimatic zones characterized by more severe winters (northwestern area and supra-Mediterranean) exhibited opposite patterns. In addition, a strong positive correlation was observed between CCT and CTmin. Neither strain differentiation nor the covariations between traits seem to be linked with the molecular diversity observed on the part of the mitochondrial marker COI.
This article is the third and last of a series of models developed to investigate the impact of climate on the spatiotemporal biology of parasitoids. After two earlier papers investigating Tranosema rostrale and Meteorus trachynotus, this last article concerns the tachinid fly Actia interrupta (Diptera: Tachinidae). An individual-based model of the seasonal biology of A. interrupta was developed to determine the impact of climate on its interactions with two of its hosts, the spruce budworm Choristoneura fumiferana (Lepidoptera: Tortricidae) and the obliquebanded leafroller C. rosaceana in eastern North America. The model is based on the developmental responses of ‘the parasitoid’s successive life stages and the ovipositional response of adult females to temperature. It was found that the number of generations this parasitoid undergoes each year varies geographically from two to four, and that its potential growth rate, as dictated by synchrony with larvae of its overwintering host C. rosaceana, is highly patterned geographically and topographically as a result of phenological matching with larvae of obliquebanded leafroller entering diapause in late summer.
Understanding how resource diversification affects ecological interactions, food web structure and ecosystem functioning is essential in both fundamental and applied ecology. While plant diversification strategies (either in‐field or around‐field) are often proposed in agricultural landscapes as practices to improve the biological control of herbivores by natural enemies, results remain variable and unsure.
Here, we studied the effect of an in‐field diversification practice (the intercropping of leguminous crops within cereal fields, an increasingly common practice but with inconsistent results on biological control) on cereal aphid control and the structure of a cereal aphid–parasitoid–hyperparasitoid food web for 2 years.
We report that aphid control was not increased in mixed fields, nor was cereal parasitoid diversity and food web complexity. Nevertheless, the provision of alternative hosts in mixed fields led to a functional community composition shift, favouring generalist parasitoid species over specialist ones.
Moreover, we observed a higher hyperparasitism rate in mixed fields, suggesting that secondary parasitoids were favoured by alternative resources, which may have disrupted aphid control by primary parasitoids.
Synthesis and applications. This study demonstrates that parasitoid community composition shift and increased top‐down control by the fourth trophic level can impact parasitoid efficiency to control herbivores. These results highlight the necessity to study fine‐scale mechanisms within food webs to be able to set up efficient methods to support biodiversity and associated ecosystem services in agricultural landscapes.
1. Protection against desiccation and chemical communication are two fundamental functions of cuticular hydrocarbons (CHCs) in insects. In the parasitoid jewel wasp Nasonia vitripennis (Walker), characterised by a cosmopolitan distribution through largely different environments, CHCs function as universally recognised female sex pheromones. However, CHC uniformity as basis for sexual recognition may conflict with the desiccation protection function, expected to display considerable flexibility through adaptation to different environmental conditions.
2. We compared male and female CHC profiles of N. vitripennis across a wide latitudinal gradient in Europe and correlated their CHC variation with climatic factors associated with desiccation. Additionally, we tested male mate discrimination behaviour between populations to detect potential variations in female sexual attractiveness.
3. Results did not conform to the general expectation that longer, straight-chain CHCs occur in higher proportions in warmer and drier climates. Instead, unexpected environmental correlations of intermediate chain-length CHCs (C31) were found exclusively in females, potentially reflecting the different life histories of the sexes in N. vitripennis.
4. Furthermore, we found no indication of population-specific male mate preference, confirming the stability of female sexual attractiveness, likely conveyed through their CHC profiles. C31 mono- and C33 di-methyl-branched alkanes were consistently and most strongly associated with sexual dimorphism, suggesting their potential role in encoding the female-specific sexual signalling function.
5. Our study sheds light on how both adaptive flexibility and conserved sexual attractiveness can potentially be integrated and encoded in CHC profiles of N. vitripennis females across a wide distribution range in Europe.
Climate change has the potential to change the distribution of pests globally and their resistance to pesticides, thereby threatening global food security in the 21st century. However, predicting where these changes occur and how they will influence current pest control efforts is a challenge. Using experimentally parameterised and field-tested models, we show that climate change over the past 50 years increased the overwintering range of a global agricultural insect pest, the diamondback moth ( Plutella xylostella ), by ~2.4 million km ² worldwide. Our analysis of global data sets revealed that pesticide resistance levels are linked to the species’ overwintering range: mean pesticide resistance was 158 times higher in overwintering sites compared to sites with only seasonal occurrence. By facilitating local persistence all year round, climate change can promote and expand pesticide resistance of this destructive species globally. These ecological and evolutionary changes would severely impede effectiveness of current pest control efforts and potentially cause large economic losses.
Identifying the interactions of functional, biotic, and abiotic factors that define plant–insect communities has long been a goal of community ecologists. Metabolomics approaches facilitate a broader understanding of how phytochemistry mediates the functional interactions among ecological factors. Ceanothus velutinus communities are a relatively unstudied system for investigating chemically mediated interactions. Ceanothus are nitrogen-fixing, fire-adapted plants that establish early post-fire, and produce antimicrobial cyclic peptides, linear peptides, and flavonoids. This study takes a metabolomic approach to understanding how the diversity and variation of C. velutinus phytochemistry influences associated herbivore and parasitoid communities at multiple spatiotemporal scales. Herbivores and foliar samples were collected over three collection times at two sites on the east slope of the Sierra Nevada Mountain range. Foliar tissue was subjected to LC-MS metabolomic analysis, and several novel statistical analyses were applied to summarize, quantify, and annotate variation in the C. velutinus metabolome. We found that phytochemistry played an important role in plant–insect community structure across an elevational gradient. Flavonoids were found to mediate biotic and abiotic influences on herbivores and associated parasitoids, while foliar oligopeptides played a significant positive role in herbivore abundance, even more than abundance of host plants and leaf abundance. The importance of nutritional and defense chemistry in mediating ecological interactions in C. velutinus plant–herbivore communities was established, justifying larger scale studies of this plant system that incorporate other mediators of phytochemistry such as genetic and metageomic contributions.
The onset of an overwintering strategy to overcome cold temperatures of a species of ectotherms can include remaining active or entering diapause. This in turn will depend on the relative costs of each strategy and therefore, could differ among populations along a latitudinal gradient. Thus, expecting higher levels of diapause in the coldest conditions and a higher incidence of individuals remaining active in the warmest conditions. We assessed the insect responses to photoperiod and temperature, in five Chilean populations of the aphid parasitoid Aphidius platensis. We analysed the variation in winter temperature along the latitudinal gradient and, under controlled conditions, examining the potential effects of three constant conditions of photoperiod/temperature: 8:16LD (Light: Dark, hours) at 10°C, 10:14LD at 14°C, and 16:8LD at 20°C, on diapause levels. Finally, we measured cold tolerance (CTMin), developmental time, fresh body mass and size, fat and water content, and egg load for the emerging parasitoids. Our results showed no clear latitudinal gradient in temperature but differences among sites were clear. None of the five populations of A. platensis expressed diapause at any tested condition, suggesting that the environmental thresholds for diapause induction are perhaps not reached in this species under the studied latitudes. Insects from the coldest point in the gradient (Pinto) showed the lowest CTMin suggesting local adaptation. Moreover, physiological and life-history traits seem to adjust rapidly through developmental thermal acclimation, showing that plasticity is involved in the parasitoid’s responses to the temperature differences found among localities. Consequently, both local adaptation and phenotypic plasticity contribute to this species remaining active during the whole winter, being an effective strategy to diapause in relatively mild and stable thermal environments.
Gradients in elevation impose changes in environmental conditions, which in turn modulate species distribution and abundance as well as the interactions they maintain. Along the gradient, interacting species (e.g., predators, parasitoids) can respond to changes in different ways. This study aims to investigate how egg parasitism of a forest pest, the pine processionary moth (PPM), Thaumetopoea pityocampa, vary along an elevational gradient (190–2000 m.a.s.l.) in a mountain range of SE Spain, including areas of recent elevational expansion, for a seven years period (2008–2014). We used generalized linear mixed models to ascertain the effect of both elevation and the winter North Atlantic Oscillation (NAO) index (a proxy of interannual climatic conditions) on the rate of parasitism, and the occurrence probabilities of two parasitoid species: a PPM specialist and a generalist species. Since four pine species are stratified along the elevational gradient, we repeated all the analyses separately for lowlands (190–1300 m. a.s.l.) and uplands (1350–2000 m. a.s.l.). Results showed a decrease in both parasitism rate and probability of occurrence of the two main parasitoid species with elevation, although decline was more severe for the specialist species. The effect of elevation was more conspicuous and intense in uplands than in lowlands. Positive NAO winter values, associated with cold and dry winters, reduced the rate of parasitism and the probability of occurrence of the two main parasitoid species—but particularly for the generalist species—as elevation increases. In a context of climate warming, it is crucial to mitigate PPM elevational and latitudinal expansion. Increasing tree diversity at the PPM expansion areas may favor the establishment of parasitoids, which could contribute to synchronizing host– parasitoid interactions and minimize the risk of PPM outbreaks.
Dominance hierarchy, the dominance ranking of members in a community, is determined by the relative population fitness of coexisting species. Climate change can shift dominance hierarchies depending on how species respond to changing climate, but ecological mechanisms underlying such shifts remain largely unknown. Specifically, dominance hierarchy shifts under climate change have rarely been linked to eco‐evolutionary responses to thermal extremes, which we consider in this study.
We conducted an 8‐year field survey to link extreme high‐temperature events to the shift in dominance hierarchy of two worldwide cereal aphid species (Rhopalosiphum padi and Sitobion avenae), which may respond rapidly through evolutionary or plastic responses to thermal extremes due to their short generation duration, clonal structure and high thermal sensitivity. To further understand the mechanism involved in this change in species' dominance, we characterised aphid heat tolerance and demography of the two species sourced from relatively mild (Xinxiang) and hot (Wuhan) locations in a common garden design and compared the same measures in up‐ and down‐selected lines of the two aphid species.
We found that accumulation of extreme high‐temperature events (EHTs) affected per capita growth rate of S. avenae but not R. padi, driving a shift in annual relative dominance of these two common species in wheat fields. Furthermore, evidence from common garden tests and artificial thermal selection revealed that evolutionary and plastic changes in thermal tolerance under EHTs were species‐dependent; R. padi evolved higher heat tolerance without reducing fitness while S. avenae did not evolve heat tolerance but showed detrimental transgenerational plastic changes under thermal extremes. Rhopalosiphum padi may take over the dominant position of S. avenae in a warmer future, due to a higher evolutionary potential of heat tolerance and less detrimental plasticity following selection.
Field surveys and laboratory experiments together point to a shift in species dominance related to interspecific differences in species' evolutionary rates and transgenerational plasticity of thermal traits during selection events associated with extreme climatic conditions. This study highlights that eco‐evolutionary dynamics can contribute to community responses to natural thermal extremes.
A free Plain Language Summary can be found within the Supporting Information of this article.
Increased tree mortality has become a widespread phenomenon and is largely attributed to climate change. Little field research has addressed the complex interactions between trees, herbivores, and their natural enemies as affected by temperature. We recorded the densities of bark insects and their natural enemies emerging from felled trees in Scots pine forests at 17 study sites along 6 elevation gradients encompassing different temperature ranges in 3 regions in Switzerland and Italy. We additionally measured tree resin defense at different elevations. The density of aggressive bark beetles decreased with increasing temperatures while that of non-aggressive species did not respond to temperature. Contrasting patterns were also found for natural enemies, with the densities of most predatory taxa decreasing with increasing temperature whereas densities of parasitoids increased. Consequently, bark beetle mortality by predators decreased and that by parasitoids increased with temperature. Exudation of resin increased with temperature. As the number of resin ducts did not change with temperature, this is assumed a physical effect of reduced viscosity. Despite lower densities of aggressive bark beetles and improved tree resin flow under higher temperatures, the currently experienced drought-induced reduction in tree vigor is likely to increase tree mortality under the ongoing climate warming.
Simple Summary
The Asian cynipid gall wasp (ACGW) “Dryocosmus kuriphilus” has become widespread in Europe. In all invaded areas, it is parasitized by native parasitoids associated with oak galls, for which the ACGW represents a non-saturated adaptation space. Considering the increase in the frequency of extreme climatic events over the last twenty years (e.g., low temperatures during the vegetative period of the chestnut tree), this study aimed to elucidate the effects of cold stress on both ACGW biology and parasitism by native and introduced parasitoids. The ACGW–parasitoid system represents an ideal subject in which to evaluate the effect of sudden cold stress events due to the wasps’ biological characteristics, which include the ability to complete development even in galls detached from plants. We show that parasitism on and the mortality of ACGWs in three chestnut fields were affected by a cold treatment. Our results reveal species-specific differences in the abundance and performance of parasitoids associated with the ACGW in response to cold stress. For example, the frequency of Eupelmus spp. and Mesopolobus tibialis doubled as a result of the cold treatment in all three chestnut fields in both study years. Therefore, the plasticity in response to short-term temperature variation is associated with individual fitness in some parasitoid species.
Abstract
Temperature variation affects interactions involving plants, herbivores, and parasitoids, causing a mismatch between their phenological cycles. In the context of climate change, climatic factors can undergo profound and sudden changes, such as sudden hot or cold snaps. Herein, we show that the number of episodes of short but sustained low temperatures has increased, mainly during May, over the last two decades. We subjected galls induced by the Asian chestnut gall wasp (ACGW) Dryocosmus kuriphilus to cold stress to assess whether and, if so, how it affected the pest and its parasitoids. Over the course of two years, we measured seasonal parasitism, parasitism rates, the relative abundance of each parasitoid species, and ACGW mortality. We found that the cold treatment affected both the pest and the parasitoids, resulting in a reduction in the emergence of ACGWs and differing ratios of species within the parasitoid community. The most striking example was the change in the relative frequency of three species of Eupelmus spp. and Mesopolobus tibialis, which doubled in cold-stressed galls in all chestnut fields. The effects of temperature on the development of the host and the direct effects of cold temperatures on the surface of galls (in terms of the humidity or hardness of the galls) warrant further research in this direction.
Organisms are exposed to temperatures that vary, for example on diurnal and seasonal time scales. Thus, the ability to behaviorally and/or physiologically respond to variation in temperatures is a fundamental requirement for long-term persistence. Studies on thermal biology in ectotherms are typically performed under constant laboratory conditions, which differ markedly from the variation in temperature across time and space in nature. Here, we investigate evolutionary adaptation and environmentally induced plastic responses of Drosophila simulans to no fluctuations (constant), predictable fluctuations or unpredictable fluctuations in temperature. We whole-genome sequenced populations exposed to 20 generations of experimental evolution under the three thermal regimes and examined the proteome after short-term exposure to the same three regimes. We find that unpredictable fluctuations cause the strongest response at both genome and proteome levels. The loci showing evolutionary responses were generally unique to each thermal regime, but a minor overlap suggests either common laboratory adaptation or that some loci were involved in the adaptation to multiple thermal regimes. The evolutionary response, i.e., loci under selection, did not coincide with induced responses of the proteome. Thus, genes under selection in fluctuating thermal environments are distinct from genes important for the adaptive plastic response observed within a generation. This information is key to obtain a better understanding and prediction of the effects of future increases in both mean and variability of temperatures.
The latitudinal biotic interaction hypothesis (LBIH) predicts that the strength of various biotic interactions decreases from low to high latitudes. Inconsistency between studies testing this hypothesis may result from variations among different types of interactions and among study systems. Therefore, exploration of multiple interactions within one system would help to disentangle latitudinal patterns across individual interactions and to evaluate latitudinal changes in the overall impact of enemies on prey.
We tested the prediction based on the LBIH that the pressure of natural enemies on herbivorous insects decreases with increase in latitude across the boreal forest zone. We also asked whether the impacts of major groups of these enemies exhibit similar latitudinal patterns and whether these patterns are consistent across study years.
In 10 forest sites located from 60°N to 69°N in Northern Europe, each summer, from 2016 to 2019, we measured (a) mortality of three groups of leafmining insects caused by birds, ants, parasitoids and unknown factors, (b) bird attacks on caterpillar‐shaped plasticine models and (c) birch foliar damage caused by defoliators and leafminers.
Latitudinal patterns in both insect herbivory on birch and top‐down pressure on herbivorous insects varied considerably and inconsistently among the four study years, so that only some of the year‐specific correlations with latitude were statistically significant. Nevertheless, meta‐analysis combining correlations across years, preys and enemies revealed general decreases in predation by birds (on both natural and model prey) and ants, but an increase in parasitism rates, from low to high latitudes.
We found that the direction of latitudinal changes in the strength of biotic interactions was interaction‐specific: predation and herbivory supported LBIH, whereas parasitism exhibited an opposite trend. Consequently, the overall impact of natural enemies on herbivorous insects did not change with latitude and was therefore an unlikely reason for the poleward decrease in herbivory observed in our gradient. Considerable among‐year variation in the strength of the latitudinal patterns in all the studied interactions suggests that this variation is a widespread phenomenon.
In situ adaptation to climate change will be critical for the persistence of many ectotherm species due to their relative lack of dispersal capacity. Climate change is causing increases in both the mean and the variance of environmental temperature, each of which may act as agents of selection on different traits. Importantly, these traits may not be heritable or have the capacity to evolve independently from one another. When genetic constraints prevent the “baseline” values of thermal performance traits from evolving rapidly, phenotypic plasticity driven by gene expression might become critical. We review the literature for evidence that thermal performance traits in ectotherms are heritable and have genetic architectures that permit their unconstrained evolution. Next, we examine the relationship between gene expression and both the magnitude and duration of thermal stress. Finally, we identify genes that are likely to be important for adaptation to a changing climate and determine whether they show patterns consistent with thermal adaptation. Although few studies have measured narrow-sense heritabilities of thermal performance traits, current evidence suggests that the end points of thermal reaction norms (tolerance limits) are moderately heritable and have the potential to evolve rapidly. By contrast, performance at intermediate temperatures has substantially lower evolutionary potential. Moreover, evolution in many species appears to be constrained by genetic correlations such that populations can adapt to either increases in mean temperature or temperature variability, but not both. Finally, many species have the capacity for plastic expression of the transcriptome in response to temperature shifts, with the number of differentially expressed genes increasing with the magnitude, but not the duration, of thermal stress. We use these observations to develop a conceptual model that describes the likely trajectory of genome evolution in response to changes in environmental temperature. Our results indicate that extreme weather events, rather than gradual increases in mean temperature, are more likely to drive genetic and phenotypic change in wild ectotherms.
The biology of parasitoids in natural ecosystems remains very poorly studied, though they are key species for their functioning. Here we focused on Phobocampe confusa, a Nymphalini specialist, responsible for high mortality rates in charismatic butterfly species in Europe (genus Aglais). We studied its ecology and genetic structure in connection with those of its host butterflies in Sweden. To this aim, we gathered data from 428 P. confusa individuals reared from 6094 butterfly larvae (of A. urticae, A. io, and in two occasions of Araschnia levana) collected over two years (2017 and 2018) and across 19 sites distributed along a 500 km latitudinal gradient. We found that P. confusa is widely distributed along the latitudinal gradient. Its distribution seems constrained over time by the phenology of its hosts. The large variation in climatic conditions between sampling years explains the decrease in phenological overlap between P. confusa and its hosts in 2018 and the 33.5% decrease in the number of butterfly larvae infected. At least in this study, P. confusa seems to favour A. urticae as host. While it parasitized nests of A. urticae and A. io equally, the proportion of larvae parasitized is significantly higher for A. urticae. At the landscape scale, P. confusa is almost exclusively found in vegetated open land and near deciduous forests, whereas artificial habitats are negatively correlated with the likelihood of a nest to be parasitized. The genetic analyses on 89 adult P. confusa and 87 adult A. urticae using CO1 and AFLP markers reveal a low genetic diversity in P. confusa and a lack of genetic structure in both species, at the scale of our sampling. Further genetic studies using high-resolution genomics tools will be required to better understand the population genetic structure of P. confusa, its biotic interactions with its hosts, and ultimately the stability and the functioning of natural ecosystems.
The parasitoid lifestyle represents one of the most diversified life history strategies on earth. There are however very few studies on the variables associated with intraspecific diversity of parasitoid insects, especially regarding the relationship with spatial, biotic and abiotic ecological factors. Cotesia sesamiae is a Sub-Saharan stenophagous parasitic wasp that parasitizes several African stemborer species with variable developmental success. The different host-specialized populations are infected with different strains of Wolbachia, an endosymbiotic bacterium widespread in arthropods that is known for impacting life history traits, notably reproduction, and consequently species distribution. In this study, first we analyzed the genetic structure of C. sesamiae across Sub-Saharan Africa, using 8 microsatellite markers. We identified five major population clusters across Sub-Saharan Africa, which probably originated in the East African Rift region and expanded throughout Africa in relation to host genus and abiotic factors, such as Köppen-Geiger climate classification. Using laboratory lines, we estimated the incompatibility between the different strains of Wolbachia infecting C. sesamiae. We observed that incompatibility between Wolbachia strains was asymmetric, expressed in one direction only. Based on these results, we assessed the relationships between the direction of gene flow and Wolbachia infections in the genetic clusters. We found that host specialization was more influential on genetic structure than Wolbachia-induced reproductive incompatibility, which in turn was more influential than geography and current climatic conditions. These results are discussed in the context of African biogeography, and co-evolution between Wolbachia, virus parasitoid and host, in the perspective of improving biological control efficiency through a better knowledge of biological control agents' evolutionary ecology.
Linking variation in species' traits to large-scale environmental gradients can lend insight into the evolutionary processes that have shaped functional diversity and future responses to environmental change. Here, we ask how heat and cold tolerance vary as a function of latitude, elevation and climate extremes, using an extensive global dataset of ectotherm and endotherm thermal tolerance limits, while accounting for methodological variation in acclimation temperature, ramping rate and duration of exposure among studies. We show that previously reported relationships between thermal limits and latitude in ectotherms are robust to variation in methods. Heat tolerance of terrestrial ectotherms declined marginally towards higher latitudes and did not vary with elevation, whereas heat tolerance of freshwater and marine ectotherms declined more steeply with latitude. By contrast, cold tolerance limits declined steeply with latitude in marine, intertidal, freshwater and terrestrial ectotherms, and towards higher elevations on land. In all realms, both upper and lower thermal tolerance limits increased with extreme daily temperature, suggesting that different experienced climate extremes across realms explain the patterns, as predicted under the Climate Extremes Hypothesis. Statistically accounting for methodological variation in acclimation temperature, ramping rate and exposure duration improved model fits, and increased slopes with extreme ambient temperature. Our results suggest that fundamentally different patterns of thermal limits found among the earth's realms may be largely explained by differences in episodic thermal extremes among realms, updating global macrophysiological ‘rules’.
This article is part of the theme issue ‘Physiological diversity, biodiversity patterns and global climate change: testing key hypotheses involving temperature and oxygen’.
Biological control (biocontrol) is a safe, sustainable approach that takes advantage of natural enemies such as predators, parasitic insects or pathogens to manage pests in agroecosystems. Parasitoid wasps, a very large evolutionary group of hymenopteran insects, are well-known biological control agents for arthropod pests in agricultural and forest ecosystems. Here, we summarize the recent progress on the application of parasitoid wasps in biocontrol in China for the last five years. These include species diversity of parasitoid wasps, identification of dominant parasitoid wasps associated with insect pests and biocontrol practices (three types of biological control, i.e., classical, augmentative and conservation biological control) in several Chinese agroecosystems. We then treat different mass-rearing and release technologies and the commercialization of several parasitoid wasp species. We also summarize other work that may have a potential use in biocontrol, including the effect of plant volatiles on parasitoids and recent advance in the molecular mechanisms underlying the host regulation by parasitoid wasps. Future research area and applied perspectives are also discussed, noting that advances in biocontrol technologies in Chinese agriculture informs research at the global level.
Elevational gradients are characterized by strong abiotic variation within small geographical distances and provide a powerful tool to evaluate community response to variation in climatic and other environmental factors. We explored how temperature and habitat diversity shape the diversity of holometabolous predator and parasitoid insects along temperate elevational gradients in the European Alps. We surveyed insect communities along 12 elevational transects that were selected to separate effects of temperature from those of habitat diversity. Pitfall traps and pan traps were placed every 100 m of elevation increment along the transects ranging from 120 to 2200 m a.s.l. Sampling took place once a month from June to September 2015. Four groups characterized by having at least one life stage behaving as predator or parasitoid were examined: tachinids (Diptera), hoverflies (Diptera), sphecids (Hymenoptera) and ground beetles (Coleoptera). Species richness and evenness changed with elevation, but the shape and direction of the elevation–diversity patterns varied between groups. The effect of temperature on species richness was positive for all groups except for hoverflies. Habitat diversity did not affect species richness, while it modulated the evenness of most groups. Often, elevational patterns of species richness and evenness were contrasting. Our study indicates that natural enemies characterized by diverse ecological requirements can be differentially affected by temperature and habitat diversity across the same elevational gradients. As climate warming is predicted to increase mean annual temperatures and exacerbate weather variability, it is also expected to strongly influence natural enemies and their ability to regulate herbivore populations.
Temperature is both a selective pressure and a modulator of the diapause expression in insects from temperate regions. Thus, with climate warming, an alteration of the response to seasonal changes is expected, either through genetic adaptations to novel climatic conditions or phenotypic plasticity. Since the 1980s in western France, the winter guild of aphid parasitoids (Hymenoptera: Braconidae) in cereal fields has been made up of two species: Aphidius rhopalosiphi and Aphidius matricariae. The recent activity of two other species, Aphidius avenae and Aphidius ervi, during the winter months suggests that a modification of aphid parasitoid overwintering strategies has taken place within the guild. In this study, we first performed a field survey in the winter of 2014/15 to assess levels of parasitoid diapause incidence in agrosystems. Then, we compared the capacity of the four parasitoid species to enter winter diapause under nine different photoperiods and temperature conditions in the laboratory. As predicted, historically winter-active species (A. rhopalosiphi and A. matricariae) never entered diapause, whereas the species more recently active during winter (A. avenae and A. ervi) did enter diapause but at a low proportion (maximum of 13.4 and 11.2%, respectively). These results suggest rapid shifts over the last three decades in the overwintering strategies of aphid parasitoids in Western France, probably due to climate warming. This implies that diapause can be replaced by active adult overwintering, with potential consequences for species interactions, insect community composition, ecosystem functioning, and natural pest control.
As biological invasions, intentional introductions often result in a loss of genetic diversity in the new founder populations. In classical biological control programs, natural enemies introduced into novel environments are likely to suffer from population bottlenecks. Unlike invasive populations, individuals for biological control are typically kept in quarantine during several generations before being released in the field. This procedure reduces further the effective population size of the introduced populations, which thus increases the effects of inbreeding and genetic drift, resulting in a greater loss of genetic diversity. This study addresses the genetic consequences of the introduction of the parasitoid wasp Aphidius ervi, a successful biocontrol agent of important aphid target-pests in Chile. This was assessed by examining the genetic diversity and differentiation at nuclear and mitochondrial genetic markers in terms of (1) the magnitude of the genetic diversity loss after 38 years of the introduction of A. ervi, (2) the current level of genetic differentiation between Chilean introduced populations and putative native populations from France, and (3) the genetic relationships and magnitude of the genetic diversity loss between introduced populations of A. ervi in Chile compared to those introduced in North America. The results provide evidence that parasitoid populations suffered the effects of a moderate genetic bottleneck during the introduction, showing further a strong geographical genetic differentiation between populations in the natal and novel environments. In addition mtDNA sequences analysis showed evidence of a single main event of introduction in Chile, unlike the North American situation, where there is evidence for multiple introductions. The significance of the loss of genetic diversity during introductions related to the success of parasitoids as biocontrol agents in classical biological control programs is discussed.
Susceptibility to global warming relies on how thermal tolerances respond to increasing temperatures through plasticity or evolution. Climatic adaptation can be assessed examining the geographic variation in thermal-related traits. We studied latitudinal patterns in heat tolerance in Drosophila subobscura reared at two temperatures. We used four static stressful temperatures to estimate the thermal death time curves (TDT), and two ramping assays with fast and slow heating rates. TDT curves allow estimating the critical thermal maximum CTmax , by extrapolating to the temperature that would knockdown the flies almost 'instantaneously', and the thermal sensitivity to increasing stressful temperatures. We found a positive latitudinal cline for CTmax , but no clinal pattern for knockdown temperatures estimated from the ramping assays. Although high-latitude populations were more tolerant to an acute heat stress, they were also more sensitive to prolonged exposure to less stressful temperatures, supporting a trade-off between acute and chronic heat tolerances. Conversely, developmental plasticity did not affect CTmax but increased the tolerance to chronic heat exposition. The patterns observed from the TDT curves help to understand why the relationship between heat tolerance and latitude depends on the methodology used and, therefore, these curves provide a more complete and reliable measurement of heat tolerance. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
Understanding the stability of communities is fundamental in theoretical and applied ecology. Organisms atop trophic chains are particularly sensitive to disturbance, especially when they are dependent on a specific trophic resource subject to strong fluctuations in density and quality, which is the case of parasitoids. We investigated the (1) variability in spatiotemporal relative abundance patterns of a cereal aphid parasitoid community, determining at what scales such patterns vary in agrosystems. We also investigated whether (2) parasitoid relative abundances are strongly influenced by host relative abundances and if (3) different host species are exploited at distinct rates. Aphid parasitoid populations were monitored in three remote agricultural regions in France between 2010 and 2012. Five parasitoid species (Hymenoptera: Braconidae: Aphidiinae) and three aphid species (Hemiptera: Aphididae) were identified in each of those three regions. Fields sampled in one region in a single year exhibited similar relative abundance patterns, with aphid parasitoid communities varying across regions and years. All parasitoid species were able to exploit each monitored host species. Metopolophium dirhodum Wlk was consistently a more frequent species among parasitized aphids than among living aphids, indicating that this aphid species was exploited at a higher rate than the other two aphid species found (Sitobion avenae F and Rhopalosiphum padi L). Those findings suggest the cereal aphid-parasitoid network is not strictly determined by the intrinsic permanent environmental characteristics but partially varies from one year to another. The similarity in abundance patterns in different fields of a region in a given year suggests the existence of a mechanism allowing a rapid synchronisation of the relative abundance patterns at an intraregional scale. This phenomenon could be useful in predicting host-parasitoid communities and bear important consequences for the ecosystem service provided by parasitoids.
Energy allocation is determined by resource availability and trade-offs among traits, and so organisms have to give some traits priority over others to maximize their fitness according to their environment. In this study, we investigated the geographic variations in life history traits and potential trade-offs in populations of the parasitoid Leptopilina heterotoma (Hymenoptera: Figitidae) originating from the north and the south of the Rhône-Saône valley (over a gradient of 300 km, South-East France). We measured a set of traits related to reproduction, maintenance, and mobility using several estimators of each of these main functions determined at different times. We did not find any clear differences between populations from contrasting areas, whereas the southern populations, which were all assumed to be exposed to similar environmental conditions, displayed contrasting patterns of energy allocation. Thus, the most likely explanation seems to be that the evolution of the life history of L. heterotoma is probably shaped by local selective pressures, such as microclimate, microhabitats, or intensity of competition, rather than by regional ecological conditions. Using our study as an example, we discuss the interest of considering several traits and using different ways of measuring them, concluding that multiple measurements should be performed in future studies to ensure the robustness of the results.
Abstract Bergmann's rule states that individuals of a species/clade at higher altitudes or latitudes will be larger than those at lower ones. A systemic review of the known literature on inter- and intraspecific variation in insect size along latitudinal or altitudinal clines was done to see how often such clines appeared and whether they reflected classwide, species-specific, or experimentally biased tendencies. Nearly even numbers of studies showed Bergmann clines and converse-Bergmann clines, where insects get smaller as latitude/altitude increases. In fact, the majority of studies suggested no clines at all. Small ranges may have obscured certain clines, while giant ranges may have introduced artifacts. Researchers examining interspecific patterns found clines less frequently than those examining intraspecific patterns because of variation among species within the clades, which renders interspecific studies unhelpful. Bergmann's rule does not apply to hexapods with nearly the same consistency as it does to endothermic vertebrates. The validity of Bergmann's rule for any group and range of insects is highly idiosyncratic and partially depends on the study design. I conclude that studies of Bergmann's rule should focus within species and look at widespread but contiguous populations to account for all sources of variation while minimizing error.
Climate change is ten times faster now than in the last global warming event, 56 million years ago, with temperature and extreme weather dramatically increasing due to human activity. This rapid changes in climate affect all levels of biodiversity. However, despite their high global biodiversity, only 3 percent of global climate change literature is based on invertebrates. Evidence from the fossil record has revealed low extinction rates for insect families in past catastrophic global events and omic sciences have allowed a much deeper understanding of the insect physiological and phenological responses to heat and their genetic basis. Insects acclimate and adapt to climate change, but several fail and suffer important reductions in population sizes besides local and global extinctions. The challenge is mainly driven by climate change potentiating the negative effects of other stressful conditions such as deforestation and pollution. Some species, notably pests and vectors, benefit from current climate change and either expand their distribution ranges or invade new environments. Moreover, biotic interactions involving insects and other organisms are threatened by climate change, generating cascading effects and affecting ecosystem functions and human wellbeing. Refugia from climate change across continents are fundamental for insects to withstand climate change and are of priority protection. This book provides key reflections regarding what we know and ignore about insect physiology, evolution, ecology and conservation and concludes by identifying fundamental aspects that still limit our understanding of how insects respond to climate change.
Climate change facilitates biological invasions globally. Predicting potential distribution shifts of invasive crop pests under climate change is essential for global food security in the context of ongoing world population increase. However, existing predictions often omit the capacity of crop pests to mitigate the impacts of climate change by using microclimates, as well as through thermoregulation, life history variation and evolutionary responses. Microclimates provide refugia buffering climate extremes. Thermoregulation and life history variation can reduce the effects of diurnal and seasonal temperature variability. Evolutionary responses allow insects to adapt to long-term climate change. Neglecting these ecological processes may lead to overestimations in the negative impacts of climate change on invasive pests whereas in turn cause underestimations in their range expansions. To improve model predictions, we need to incorporate the fine-scale microclimates experienced by invasive crop pests and the mitigation responses of insects to climate change into species distribution models.
Many organisms escape from lethal climatological conditions by entering a resistant resting stage called diapause, which needs to be optimally timed with seasonal change. As climate change exerts selection pressure on phenology, the evolution of mean diapause timing, but also of phenotypic plasticity and bet‐hedging strategies is expected. The potential of the latter strategy as a means of coping with environmental unpredictability has received little attention in the climate change literature.
Populations should be adapted to spatial variation in local conditions; contemporary patterns of phenological strategies across a geographic range may hence provide information about their evolvability. We thus extracted 458 diapause reaction norms from 60 studies. First, we correlated mean diapause timing with mean winter onset. Then we partitioned the reaction norm variance into a temporal component (phenotypic plasticity) and among‐offspring variance (diversified bet‐hedging) and correlated this variance composition with variability of winter onset. Mean diapause timing correlated reasonably well with mean winter onset, except for populations at high latitudes, which apparently failed to track early onsets. Variance among offspring was, however, limited and correlated only weakly with environmental variability, indicating little scope for bet‐hedging. The apparent lack of phenological bet‐hedging strategies may pose a risk in a less predictable climate, but we also highlight the need for more data on alternative strategies.
The Earth's climate is changing at a rapid pace. To survive in increasingly fluctuating and unpredictable environments, species can either migrate or evolve through rapid local adaptation, plasticity and/or bet-hedging. For small ectotherm insects, like parasitoids and their hosts, phenotypic plasticity and bet-hedging could be critical strategies for population and species persistence in response to immediate, intense and unpredictable temperature changes. Here, we focus on studies evaluating phenotypic responses to variable predictable thermal conditions (for which phenotypic plasticity is favoured) and unpredictable thermal environments (for which bet-hedging is favoured), both within and between host and parasitoid generations. We then address the effects of fluctuating temperatures on host–parasitoid interactions, potential cascading effects on the food web, as well as biological control services. We conclude our review by proposing a road map for designing experiments to assess if plasticity and bet-hedging can be adaptive strategies, and to disentangle how fluctuating temperatures can affect the evolution of these two strategies in parasitoids and their hosts.
Global change includes a substantial increase in the frequency and intensity of extreme high temperatures (EHTs), which influence insects at almost all levels. The number of studies showing the ecological importance of EHTs has risen in recent years, but the knowledge is rather dispersed in the contemporary literature. In this article, we review the biological and ecological effects of EHTs actually experienced in the field, i.e., when coupled to fluctuating thermal regimes. First, we characterize EHTs in the field. Then, we summarize the impacts of EHTs on insects at various levels and the processes allowing insects to buffer EHTs. Finally, we argue that the mechanisms leading to positive or negative impacts of EHTs on insects can only be resolved from integrative approaches considering natural thermal regimes. Thermal extremes, perhaps more than the gradual increase in mean temperature, drive insect responses to climate change, with crucial impacts on pest management and biodiversity conservation.
Expected final online publication date for the Annual Review of Entomology, Volume 66 is January 11, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
• Insect parasitoids may be an exception to the typical biogeographic pattern of increasing species richness at lower latitudes exhibited by most taxa. Evidence for this ‘anomalous’ latitudinal gradient has been derived from observations of hymenopteran parasitoids and it has been argued that other parasitoid groups should show a similar pattern of diversity. Several mechanisms have been proposed to explain this disparity, most notably the nasty host and resource fragmentation hypotheses.
• We review and evaluate these hypotheses with respect to tachinid flies (Diptera: Tachinidae), and bring to the argument evidence from eight trapping surveys from temperate and tropical regions in the Americas including the United States, Costa Rica, and Ecuador. We find no evidence that tachinid fly diversity is lower in the tropics than in the temperate region. Our results, along with other lines of evidence, rather suggest that New World Tachinidae likely conform to the same negative relationship between latitude and richness as their largely phytophagous host taxa.
• We discuss geographic patterns of tachinid diversity in relation to ecological and evolutionary processes, and why they may differ from their hymenopteran parasitoid counterparts. Parasitoid taxa appear to vary strongly in their diversity responses to latitude and we concur with previous researchers that more survey data are necessary to reach strong conclusions about parasitoid latitudinal diversity patterns.
1. Eastern tent caterpillars (Malacosoma americanum) are broadly distributed within North America, with populations spanning a wide range of environmental conditions. Their egg masses are consistently attacked by a variety of wasps in the superfamily Chalcidoidea. We performed a reciprocal transplant‐type experiment to assess the performance of three populations spanning 15° of latitude when subjected to temperature regimes resembling southern or northern conditions.
2. Exposure to warm temperatures and short overwintering periods (southern conditions) resulted in the increased survival of both caterpillars and parasitoids from all populations. By contrast, the ability of caterpillars to withstand starvation was maximised when exposed to conditions similar to their native region.
3. Caterpillar and wasp phenology differed among populations even when exposed to the same temperature regime. Individuals exposed to novel conditions hatched 2–6 weeks later than those experiencing native conditions. Under typical conditions, the relative phenology of wasps and their hosts exhibited a latitudinal gradient consistent with growing season length, with southern, central, and northern wasps, emerging 50, 45, and 36 days, respectively, after their hosts.
3. We identified four genera of primary parasitoids, which emerged within a narrow 2‐week span, and one hyperparasitoid, which emerged in distinct pulses over an approximately 5‐week span, possibly indicating the presence of a second generation.
4. Caterpillars and wasps exhibited distinct phenological responses according to population of origin, indicating that not only pre‐hatching winter and spring conditions, but also historical factors, which may include local adaptation, maternal effects, and oviposition time, influence their phenological responses.
Many studies have investigated species diversity patterns across space and time, but few have explored patterns of coexistence of tightly interacting species. We documented species diversity patterns in a host-parasitoid system across broad geographic location and seasons. We calculated species diversity (H and eH ') and compared the relationship between community similarity and geographic distances of frugivorous Drosophila host (Diptera: Drosophilidae) and Leptopilina parasitoid (Hymenoptera: Figitidae) communities across Eastern North America, from New Hampshire to Florida, at two time points during the breeding season. We also analyzed the influence of environmental factors on species assemblages via constrained correspondence analysis and lastly calculated cluster dendrograms to identify potential host-parasitoid interactions. We found that the composition of Drosophila-Leptopilina communities varied significantly with latitude. Interestingly, diversity increased with increasing latitude, a trend counter to latitudinal patterns of diversity observed in many other taxa. We also found seasonal effects of monthly temperature range and precipitation on host biodiversity patterns across geographic locations. Cluster dendrograms nominated potential parasitoid-hosts and competitive interactions to be validated in the future studies. The present study fills an important gap of knowledge in North American Drosophila-Leptopilina species diversity patterns and lays the groundwork for future ecological and evolutionary studies in this system.
Most insect species are affected by Human Induced Rapid Environmental Changes (HIREC). Multiple responses to HIREC are observed in insects, such as modifications of their morphology, physiology, behavioural strategies or phenology. Most of the responses involve phenotypic plasticity rather than genetic evolution. Here, we review the involvement of behavioural plasticity in foraging, reproduction, habitat choice and dispersal; and how behavioural plasticity modifies social behaviour and inter-specific interactions. Although important, behavioural plasticity is rarely sufficient to cope with HIREC. An increasing number of studies find species to respond maladaptively or insufficiently to various anthropogenic disturbances, and less often is large degree of plasticity linked to success.
Body size affects rates of most biological and ecological processes, from individual performance to ecosystem function, and is fundamentally linked to organism fitness. Within species, size at maturity can vary systematically with environmental temperature in the laboratory and across seasons, as well as over latitudinal gradients. Recent meta‐analyses have revealed a close match in the magnitude and direction of these size gradients in various arthropod orders, suggesting that these size responses share common drivers.
As with increasing latitude, temperature also decreases with increasing altitude. Although the general direction of body size clines along altitudinal gradients has been examined previously, to our knowledge altitude–body size (A–S) clines have never been synthesised quantitatively, nor compared with temperature–size (T–S) responses measured under controlled laboratory conditions.
Here we quantitatively examine variation in intraspecific A–S clines among 121 insect species from 50 different global locations, representing 12 taxonomic orders. While some taxa were better represented in the literature than others, our analysis reveals extensive variation in the magnitude and direction of A–S clines. Following the assumption that temperature on average declines by 1°C per 150 m increase in altitude, order‐specific A–S clines in the field appear to deviate from laboratory T–S responses. Specifically, the magnitude of A–S clines and T–S responses are more closely matched in some taxonomic orders (e.g. Diptera) than others (e.g. Orthoptera). These findings contrast with the strong co‐variation observed between latitude‐size clines and T–S responses, and between laboratory and seasonal T–S responses.
The lack of clear size relationships with elevation, and hence temperature, is likely due to the counteracting effects of other major drivers with altitude, including season length and oxygen partial pressure. Switches in voltinism within species across altitude, and the dispersal of individuals across different elevations, may also obscure trends.
A plain language summary is available for this article.
Diapause variation is a key factor affecting the development and success of parasitoids employed as biological control agents. Trichogramma dendrolimi Matsumura is mass-reared for biocontrol purposes in several countries around the world. Here we investigated the rate of diapause induction in three populations of T. dendrolimi from different regions in China: Heilongjiang (44° 55' N, 128° 26' E; HLJ), Liaoning (40° 18' N, 123° 22' E; LN), and Jiangsu province (32° 30' N, 120° 09' E; JS). We measured the rates of diapause induction for populations from each region when exposed to temperatures of 8, 10, 12, and 14 °C, for periods ranging from 5 to 35 d. Results showed that both the temperature and exposure duration influenced diapause induction in the three populations of T. dendrolimi. The HLJ and LN populations showed the highest percentage of diapause under 12 and 10 °C, regardless of time period tested. After 20 d at 12 and 10 °C, 97.42% and 95.94% of individuals of HLJ and LN, respectively, entered diapause. After treatment for 20-35 d under 8 °C, or 15-25 d under 10 °C, the diapause rate of LN was higher than those of the other two populations. In contrast, the higher temperatures (12-14 °C) induced diapause rapidly and steadily in HLJ. The diapause rate of JS was low for all temperatures and time periods. After 30 d at 12 °C, the JS population reached its highest diapause rate (29.56%). Our results showed significant variance in diapause rates among geographically distinct populations of T. dendrolimi in response to various abiotic conditions.
Species and clades are characterized by their unique combinations, or suites, of life-history traits. In parasitoids, traits include a core group common to other organisms, and a parasitoid-specific group. These organize into several sets of mutually covarying traits which overlap a little, but not wholly, with other sets. Across parasitoid species, host size, clutch size, and body size tend to covary. Roughly independent of these is a dichotomy between idiobionts (host does not develop after parasitization), which tend to have fast development but slow adult life histories, and koinobionts (hosts develop after parasitization) with the opposite set of traits. Consistent links between the dichotomy and host characteristics remain elusive. A low ovigeny index (low allocation to early reproduction) is found in idiobionts, and is a predictor of some of the dichotomous set, but also more host feeding, egg resorption, solitary development, and larger bodies. Variation in fecundity, in taxonomically restricted studies, is predicted by the host stage attacked, but this is not reflected in taxonomically broad studies. The reasons behind trait co-variation are only partly understood. Analyses of evolutionary lability suggest that variation in development mode and body size tends to be clustered within higher taxonomic levels, with variation in other traits such as lifespan, fecundity, and egg size more evenly distributed across taxonomic levels. Thus, taxonomically constrained radiations of parasitoids tend to retain a particular suite of traits that revolve around fundamental shifts in hosts and their use that occur relatively rarely. Parasitoids illustrate how the fast-slow continuum can be much less extensive than in mammals, how the ecology of the host affects the life histories of parasitic organisms, how different taxa require different life-history theories, and how understanding resource allocation in early adult life can help explain life-history variation.
The structure of populations across landscapes influences the dynamics of their interactions with other species. Understanding the geographic structure of populations can thus shed light on the potential for interacting species to co-evolve. Host-parasitoid interactions are widespread in nature and also represent a significant force in the evolution of plant-insect interactions. However, there have been few comparisons of population structure between an insect host and its parasitoid. We used microsatellite markers to analyse the population genetic structure of Pleistodontes imperialis sp. 1, a fig-pollinating wasp of Port Jackson fig (Ficus rubiginosa), and its main parasitoid, Sycoscapter sp. A, in eastern Australia. Besides exploring this host-parasitoid system, our study also constitutes, to our knowledge, the first study of population structure in a nonpollinating fig wasp species. We collected matched samples of pollinators and parasitoids at several sites in two regions separated by up to 2000 km. We found that pollinators occupying the two regions represent distinct populations, but, in contrast, parasitoids formed a single population across the wide geographic range sampled. We observed genetic isolation by distance for each species, but found consistently lower FST and RST values between sites for parasitoids compared with pollinators. Previous studies have indicated that pollinators of monoecious figs can disperse over very long distances, and we provide the first genetic evidence that their parasitoids may disperse as far, if not farther. The contrasting geographic population structures of host and parasitoid highlight the potential for geographic mosaics in this important symbiotic system.
The selective past of populations is presumed to affect levels of phenotypic plasticity. Experimental evolution at constant temperatures is generally expected to lead to a decreased level of plasticity due to presumed costs associated with phenotypic plasticity when not needed. In this study we investigated the effect of experimental evolution in constant, predictable and unpredictable daily fluctuating temperature regimes on the levels of phenotype plasticity in several life history and stress resistance traits in Drosophila simulans. Contrary to the expectation, evolution in the different regimes did not affect the levels of plasticity in any of the traits investigated even though the populations from the different thermal regimes had evolved different stress resistance and fitness trait means. Though costs associated with phenotypic plasticity are known, our results suggest that the maintenance of phenotypic plasticity might come at low and negligible costs, and thus the potential of phenotypic plasticity to evolve in populations exposed to different environmental conditions might be limited. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
The frequency and magnitude of extreme events are predicted to increase under future climate change. Despite recent advancements, we still lack a detailed understanding of how changes in the frequency and amplitude of extreme climate events are linked to the temporal and spatial structure of natural communities. To answer this question we used a combination of laboratory experiments, field experiments, and analysis of multi-year field observations to reveal the effects of extreme high temperature events on the demographic rates and relative dominance of three co-occurrence aphid species which differ in their transmission efficiency of different agricultural pathogens. We then linked the geographical shift in their relative dominance to frequent extreme high temperatures through a meta-analysis. We found that both frequency and amplitude of extreme high temperatures altered demographic rates of species. However, these effects were species specific. Increasing the frequency and amplitude of extreme temperature events altered which species had the highest fitness. Importantly, this change in relative fitness of species was consistent with significant changes in the relative dominance of species in natural communities in a one year long field heating experiment and six year long field survey of natural populations. Finally, at a global spatial scale, we found the same relationship between relative abundance of species and frequency of extreme temperatures. Together, our results indicate that changes in frequency and amplitude of extreme high temperatures can alter the temporal and spatial structure of natural communities, and that these changes are driven by asymmetric effects of high temperatures on the demographic rates and fitness of species. They also highlight the importance of understanding how extreme events affect the life-history of species for predicting the impacts of climate change at the individual and community level, and emphasize the importance of using a broad range of approaches when studying climate change. This article is protected by copyright. All rights reserved.
1. Climate is an important source of selection on life histories, and local adaptations to climate have been described in several cline studies. Temperature is the main climatic factor that has been considered as an agent of selection, whereas other factors may vary with it, such as precipitation.
2. We compared life-history traits of five populations of Leptopilina boulardi, a Drosophila parasitoid, originating from contrasting climates. Referring to cline studies, we hypothesised shorter lifespan, earlier reproduction, and lower lipid content in populations from the hottest and driest areas if life histories have been selected in response to temperature and/or humidity.
3. Our results are opposite to these predictions. Females from humid and mild climates invested more in early reproduction and lived for fewer days than females from dry and hot areas, which were synovigenic (i.e. they matured additional eggs during adult life) and able to synthesise lipids during adult life.
4. We suggest that life histories are more adapted to host distribution than to climatic factors. Drosophila patches are more abundant in the humid area, allowing the parasitoids to spend less energy and time finding hosts. This may result in selection for early reproduction traded-off against longevity. In the hot and dry climate, females have to fly large distances to find host patches. Synovigeny, a long lifespan, lipogenesis, and high dispersal ability may be adaptive there. This is the first time that between-population differences in the ability to synthesise lipids have been described in parasitoids.
Living in seasonally changing environments requires adaptation to seasonal cycles. Many insects use the change in day length as a reliable cue for upcoming winter and respond to shortened photoperiod through diapause. In this study, we report the clinal variation in photoperiodic diapause induction in populations of the parasitoid wasp Nasonia vitripennis collected along a latitudinal gradient in Europe. In this species, diapause occurs in the larval stage and is maternally induced. Adult Nasonia females were exposed to different photoperiodic cycles and lifetime production of diapausing offspring was scored. Females switched to the production of diapausing offspring after exposure to a threshold number of photoperiodic cycles. A latitudinal cline was found in the proportion of diapausing offspring, the switch point for diapause induction measured as the maternal age at which the female starts to produce diapausing larvae, and the critical photoperiod for diapause induction. Populations at northern latitudes show an earlier switch point, higher proportions of diapausing individuals and longer critical photoperiods. Since the photoperiodic response was measured under the same laboratory conditions, the observed differences between populations most likely reflect genetic differences in sensitivity to photoperiodic cues, resulting from local adaptation to environmental cycles. The observed variability in diapause response combined with the availability of genomic tools for N. vitripennis represent a good opportunity to further investigate the genetic basis of this adaptive trait.
1. Many evolutionary models of parasitoid behaviour assume a positive correlation between size and fitness. In this paper we study the size–fitness relationship in the laboratory and in the field using females of the solitary parasitoid Asobara tabida (Hymenoptera: Braconidae).
2. In the laboratory, fecundity, fat reserves and longevity without food were positively correlated with size.
3. Release–recapture experiments in the field showed that dispersal diminishes fat reserves. Dispersal ability is size-dependent: larger females, with larger fat reserves, disperse over larger distances than smaller females.
4. The form of the relationship between size and fitness in the field was estimated in two ways: one based on a comparison of the size distribution of released and recaptured females; the other based on the egg load and fat reserves of wild-caught females. Both showed an accelerating increase of fitness with size.
5. The majority of females appeared to be time-limited. Therefore, the increase in fitness with size is predominantly due to a larger dispersal ability and not to a higher egg load.
Parasitoids, which mainly include taxa belonging to the Hymenoptera, play an important role in the maintenance of other arthropod populations, acting as regulators of host densities. However, the large‐scale patterns of these insects and the factors that shape them are still not well established. In this review we focus on several aspects of the biology and macroecological patterns of parasitoids that have received some attention in the past, including recent studies on island parasitoid faunas. Parasitoid latitudinal gradients have been widely analyzed, with most studies apparently showing the anomalous pattern of species richness decreasing towards the tropics, particularly in the Ichneumonidae. This pattern has been proposed to be related to host feeding niche and parasitoid attack strategy. However, it is quite possible that these latitudinal gradients might be artifacts of sampling or description biases. Island parasitoid faunas also show latitudinal gradients in species richness, with the proportion of generalist species increasing towards the tropics. Island communities are in fact biased towards generalist species, being highly influenced by the communities of the surrounding territories and by the existence of rainforests. We conclude by briefly discussing the need for the development of future studies on parasitoid large scale diversity patterns, and proposing new research lines.
Life history variation can be genetically based, or it may be due to environmental effects on the phenotype. In this paper we examine life history variation in relation to differences in habitat in the parasitoid Asobara tabida. Differences in the spatial distribution of host patches, the length of the season, host suitability, and competition between parasitoids all contribute to the selection for differences in life history between strains from northern and southern Europe.
Strains were compared with respect to the allocation between reproduction and survival in experimental environments that varied with regard to the availability of food or to the number of hosts offered per day. Upon emergence parasitoids originating from southern Europe have higher egg loads and smaller fat reserves than parasitoids originating from northern Europe. Parasitoids from both southern and northern Europe show plasticity in allocation: food shortage causes a decrease in egg production, and rich breeding opportunities an increase in egg production. This plasticity is greater in the northern strain. Fat reserves play a central role in the physiology of this allocation. Fat can be used for both reproduction and survival. There is no oosorption, so once fat is allocated to reproduction it can no longer be used for survival. Differences in plasticity in allocation can therefore be considered as differences in the timing of egg production.
The loss of a species from an ecological community can trigger a cascade of secondary extinctions. Here we investigate how the complexity (connectance) of model communities affects their response to species loss. Using dynamic analysis based on a global criterion of persistence (permanence) and topological analysis we investigate the extent of secondary extinctions following the loss of different kinds of species.
We show that complex communities are, on average, more resistant to species loss than simple communities: the number of secondary extinctions decreases with increasing connectance. However, complex communities are more vulnerable to loss of top predators than simple communities.
The loss of highly connected species (species with many links to other species) and species at low trophic levels triggers, on average, the largest number of secondary extinctions. The effect of the connectivity of a species is strongest in webs with low connectance.
Most secondary extinctions are due to direct bottom‐up effects: consumers go extinct when their resources are lost. Secondary extinctions due to trophic cascades and disruption of predator‐mediated coexistence also occur. Secondary extinctions due to disruption of predator‐mediated coexistence are more common in complex communities than in simple communities, while bottom‐up and top‐down extinction cascades are more common in simple communities.
Topological analysis of the response of communities to species loss always predicts a lower number of secondary extinctions than dynamic analysis, especially in food webs with high connectance.
Climate change 2022: impacts, adaptation and vulnerability
- IPCC