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Parasitoid life history evolution and climate change

Final Activity Report Summary - COMPAREVOL (Parasitoid life history evolution and climate change)

We studied how climate change affects insects, using parasitoids as model. We addressed phenotypic and long-term evolutionary responses in life history traits and behaviour and studied effects of temperature on these traits. We developed theory to predict the evolution of insect life history and reproductive behaviour when temperature increases. Selection may be directly through effects of changes in abiotic environment, or indirectly through changes in biotic environment. Changes in biotic environment may include the spatial and temporal distribution of resources, in host resistance, or in the community, due to climate-induced invasion of competitors. We have obtained convincing evidence that climate is a source of selection on these traits. Female parasitoids in dry and hot desert conditions were large and developed rapidly - in contrast to the trade-off between size and development time usually observed - whereas in humid climates, females were smaller and invested more in early reproduction. We predicted that host distribution, which depends on climate, would explain patterns better than climate itself. Hosts are more abundant during a longer season in humid regions, allowing parasitoids to spend less energy and time in host finding compared to wasps from desert populations. This results in selection for early reproduction in humid areas, traded off against smaller body size and shorter longevity. In hot and dry climates, larger body size and smaller initial egg load allow dispersal and facilitate host finding, while a short development time allows better tracking of host populations. In addition relatively high water content makes them more resistant against desiccation compared to individuals from humid areas.

We also studied the effect of climate on communities and addressed three questions: how species partition the same resource, how temperature affects constraints and trade-offs between behavioural, physiological, morphological and life history traits, if plasticity and thermal tolerance are under selection by temperature. The Aphidius studied differ in host species attacked and habitats they explore, but overlap in their use of wheat aphids. On wheat, they partition S.avenae hosts, because differences in behaviour result in species exploiting a different part of the host population. Moreover, their phenologies do not totally overlap. Other experiments showed that temperature during development affects key life history traits in adults: initial egg load, adult lifespan, metabolic rate and activity responded plastically to developmental temperature. Individuals with higher basal metabolic rates had a higher fecundity at the expense of a reduced longevity. A high metabolic rate may be an adaptation to cold climates allowing them to sustain physiological processes at low temperatures. Each Aphidius species had thermal tolerances matching their seasonal activity and differed in tolerance and resistance to extreme temperatures.

We also studied the effect of an invading competitor, which is expanding to the north due to climate change. We studied the impact of this invader on life history traits of two parasitoids (Asobara tabida and Leptopilina heterotoma). We predicted that A.tabida from south would have a higher fecundity, ovigeny-index and lower fat reserves at eclosion and shorter lifespan than populations from the north, and this was confirmed by our data. A. tabida from the south also has a better capacity (=lower wing loading) for flight over short distances, in association with a lower body mass and shorter lifespan compared to populations from the north. Differences for the other species, L. heterotoma, are smaller but in the same direction. We hypothesise that this is due to a smaller dependence on frugivorous hosts, as this species also exploits microhabitats other than fruits. As they are active much earlier in spring than L. boulardi and A. tabida, they will experience less competition from L. boulardi.