Rising temperature reduces divergence in resource use strategies in coexisting parasitoid species.

International audienceCoexistence of species sharing the same resources is often possible if species are phylogenetically divergent in resource acquisition and allocation traits, decreasing competition between them. Developmental and life-history traits related to resource use are influenced by environmental conditions such as temperature, but thermal trait responses may differ among species. An increase in ambient temperature may, therefore, affect trait divergence within a community, and potentially species coexistence. Parasitoids are interesting models to test this hypothesis, because multiple species commonly attack the same host, and employ divergent larval and adult host use strategies. In particular, development mode (arrested or continued host growth following parasitism) has been recognized as a major organiser of parasitoid life histories. Here, we used a comparative trait-based approach to determine thermal responses of development time, body mass, egg load, metabolic rate and energy use of the coexisting Drosophila parasitoids Asobara tabida, Leptopilina heterotoma, Trichopria drosophilae and Spalangia erythromera. We compared trait values between species and development modes, and calculated trait divergence in response to temperature, using functional diversity indices. Parasitoids differed in their thermal response for dry mass, metabolic rate and lipid use throughout adult life, but only teneral lipid reserves and egg load were affected by developmental mode. Species-specific trait responses to temperature were probably determined by their adaptations in resource use (e.g. lipogenesis or ectoparasitism). Overall, trait values of parasitoid species converged at the higher temperature. Our results suggest that local effects of warming could affect host resource partitioning by reducing trait diversity in communities

Scientists' warning on climate change and insects

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

Evolution des stratégies de reproduction de parasitoïdes de drosophiles en réponse au climat

In this thesis, we investigated (1) the relative role of climate and biotic factors varying with climate in the selection of local adaptations of drosophila parasitoids (2) how the phenotypic plasticity of life history traits has evolved in reponse to the environment and egg maturation strategies of these organisms. Contrarily to the primary consumers in which evolution of life histories is directly affected by temperature, biotic factors varying with climate, such as host distribution and interspecific competition may be the principal agent of natural selection in parasitoids. Host distribution may explain the very strong variations that we observed, such as the existence of proovigenic and synovigenic populations in a same species and populations able or not to synthesise lipids during adult life. This last intraspecific variation may have affected trade-offs between traits and the evolution of metabolic rate. Strong variations in the level of phenotypic plasticity were also observed. Variability of the environment of origin explained these variations whereas life histories of organisms did not. To predict evolution of parasitic wasps in response to the global warming, biotic factors and environmental variability depending on climate should thus be integrated, and not only climate.Durant cette thèse, nous avons cherché à (1) déterminer le rôle du climat et de facteurs biotiques associés à celui-ci dans la sélection d'adaptations locales chez des parasitoides de drosophiles (2) comprendre comment la plasticité phénotypique des traits d'histoire de vie a évolué en réponse à l'environnement et aux stratégies de maturation de ces organismes. Contrairement aux consommateurs primaires pour lesquels la température d'origine affecte directement l'évolution des histoires de vie, il semblerait que les facteurs biotiques dépendants du climat comme la distribution des hôtes et la compétition interspécifique soient le moteur principal de la sélection naturelle chez les parasitoides. La distribution des hôtes expliquerait les très fortes variations géographiques observées sur des échelles fines, comme l'existence de populations proovogéniques ou synovogéniques au sein d'une même espèce, ainsi que de populations capables ou non de lipogenèse à l'age adulte. Cette dernière variation aurait notamment affecté les compromis entre traits et l'évolution du taux de métabolisme. La force de la plasticité phénotypique présente également de fortes variations géographiques. Celles-ci peuvent être attribuées à la variabilité de l'environnement d'origine et non aux histoires de vie des organismes. Prédire l'évolution des parasitoides en réponse au réchauffement global nécessite donc d'intégrer non pas seulement un effet direct du climat, mais également les facteurs biotiques et la variabilité environnementale associés au climat

Evolution des stratégies de reproduction de parasitoïdes de drosophiles en réponse au climat

In this thesis, we investigated (1) the relative role of climate and biotic factors varying with climate in the selection of local adaptations of drosophila parasitoids (2) how the phenotypic plasticity of life history traits has evolved in reponse to the environment and egg maturation strategies of these organisms. Contrarily to the primary consumers in which evolution of life histories is directly affected by temperature, biotic factors varying with climate, such as host distribution and interspecific competition may be the principal agent of natural selection in parasitoids. Host distribution may explain the very strong variations that we observed, such as the existence of proovigenic and synovigenic populations in a same species and populations able or not to synthesise lipids during adult life. This last intraspecific variation may have affected trade-offs between traits and the evolution of metabolic rate. Strong variations in the level of phenotypic plasticity were also observed. Variability of the environment of origin explained these variations whereas life histories of organisms did not. To predict evolution of parasitic wasps in response to the global warming, biotic factors and environmental variability depending on climate should thus be integrated, and not only climate.Durant cette thèse, nous avons cherché à (1) déterminer le rôle du climat et de facteurs biotiques associés à celui-ci dans la sélection d'adaptations locales chez des parasitoides de drosophiles (2) comprendre comment la plasticité phénotypique des traits d'histoire de vie a évolué en réponse à l'environnement et aux stratégies de maturation de ces organismes. Contrairement aux consommateurs primaires pour lesquels la température d'origine affecte directement l'évolution des histoires de vie, il semblerait que les facteurs biotiques dépendants du climat comme la distribution des hôtes et la compétition interspécifique soient le moteur principal de la sélection naturelle chez les parasitoides. La distribution des hôtes expliquerait les très fortes variations géographiques observées sur des échelles fines, comme l'existence de populations proovogéniques ou synovogéniques au sein d'une même espèce, ainsi que de populations capables ou non de lipogenèse à l'age adulte. Cette dernière variation aurait notamment affecté les compromis entre traits et l'évolution du taux de métabolisme. La force de la plasticité phénotypique présente également de fortes variations géographiques. Celles-ci peuvent être attribuées à la variabilité de l'environnement d'origine et non aux histoires de vie des organismes. Prédire l'évolution des parasitoides en réponse au réchauffement global nécessite donc d'intégrer non pas seulement un effet direct du climat, mais également les facteurs biotiques et la variabilité environnementale associés au climat

Sex ratio variations with temperature in an egg parasitoid: behavioural adjustment and physiological constraint

International audienceSex allocation in haplodiploid arthropods is a central fitness-related decision that has received much attention in insect parasitoids. The effect of temperature on the reproductive strategy of female para-sitoids has rarely been addressed, despite evidence of its influence on other fitness-related traits. We explored mechanisms inducing the higher production of males typically observed at low and high temperature in parasitic wasps, considering that this pattern may result from both behavioural adjustment and physiological constraint. By observing the oviposition behaviour of an egg parasitoid, Tricho-gramma euproctidis, we were able to distinguish the sex ratio intended by the female from the secondary sex ratio, thereby discriminating between a change in behaviour and a physiological constraint on egg fertilization. More males emerged from eggs laid at low (þ45%) or high (þ80%) temperature than at medium temperature, but the underlying mechanisms differed between the two conditions. We observed a behavioural change in sex allocation at high temperature, suggesting that laying of sons may be advantageous at high temperature. At low temperature, the females' intended sex ratio was similar to that at medium temperature, but physiological constraints prevented egg fertilization during oviposition, resulting in more males emerging from eggs intended to be females. To our knowledge this is the first experimental evidence that temperature modulates both sex allocation and physiological constraints in egg fertilization in parasitoids

Ovigeny index increases with temperature in an aphid parasitoid: Is early reproduction better when it is hot?

International audienceStudying relative investment of resources towards early and delayed reproduction is central to understand life history evolution since these traits are generally negatively correlated and traded-off against several other fitness components. For this purpose, ovigeny index (OI), which is calculated as the fraction of the maximum potential lifetime egg complement that is mature upon female emergence, has been developed in insects. Despite the central role of temperature on life history evolution in ectotherms, its influence on ovigeny index has never been tested. Adaptive models imply that OI should increase with temperature because of changes in body size, but the same influence may be expected considering physiological effects of temperature on egg maturation rate or amount of energy available. We investigated in the aphid parasitoid Aphidius ervi the influence of temperature experienced by the immature and/or the adult (from 12 °C to 28 °C) on ovigeny index and oviposition behaviour. As predicted, OI increased between 16 and 28 °C, i.e. females were able to reproduce earlier as temperature increased but this was traded off against a lower delayed reproduction. The highest OI was however observed at 12°, probably because this temperature was too low for females to mature eggs. Females that developed at 20 °C and were transferred as adult at 24 °C and 28 °C had the highest ovigeny index and laid more eggs during the early oviposition period while those transferred at 16 °C laid more eggs at the end of their life. Our results suggest that ovigeny index is not only influenced by body size-i.e. the adaptive explanation-but also by adult egg maturation rate, lifespan or amount of energy available-i.e. a physiological and adaptive explanation

Temperature influences host instar selection in an aphid parasitoid: support for the relative fitness rule

International audienceThe relative fitness rule states that parasitoid females should adopt risk-prone reproductive behaviours when expecting low reproductive success. Temperature influences the reproductive success of insects by affecting their basal metabolic rate during development, their egg load at emergence, and their life expectancy as adults. Using an aphid-parasitoid model system, we investigated the influence of developmental and adult temperature on the risk-sensitive decision-making of females. We considered the use of a low-quality host nymphal instar by the aphid parasitoid Aphidius ervi to be a risk-prone behaviour. Immature females were reared at 12, 20 or 28 °C and had access to the four nymphal instars of the potato aphid Macrosiphum euphorbiae for oviposition at one of these temperatures. Host selection behaviour was continuously recorded during exploitation of an aphid patch. We observed that warm-developed females and parasitoids foraging at high temperature attacked low-quality hosts more frequently than females from other treatments. These results support the hypothesis that a decrease in expected parasitoid reproductive success resulted in risk-prone behaviours. To our knowledge, this is the first experimental evidence suggesting that temperature influences host stage selection and risk-sensitive making decision in parasitoids, and the present study is the first to support the relative fitness rule

Plasticité des traits de vie du parasitoïde de drosophiles Leptopilina boulardi: Sélection en réponse au climat ou à la distribution des ressources?

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Local adaptations of life histories of a parasitoid, Leptopilina boulardi: climate or host distribution as agents of selection?

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Local adaptations of life histories of a parasitoid, Leptopilina boulardi: climate or host distribution as agents of selection?

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