Final Report Summary - COEVOLCLIM (How temperature affects co-evolutionary interactions between partners: a study on physiological underlying mechanisms) Global warming often triggers the question if species response to a changing environment will enable them to persist. However species persistence is also dependent on species interactions. Relationships between tightly co-evolved species within communities are expected to be disrupted by thermal changes. An example of a co-evolutionary arms race is found in parasitoid insects, which develop in other arthropods to complete their larval development. The outcome of host-parasitoid interactions is known to depend on thermal regime but a full understanding of the effect of temperature requires insight into the underlying mechanisms. The proposed research has elucidated the effect of temperature on the interactions between the grain aphid host pest Sitobion avenae and its main specialist parasitoid, Aphidius rhopalosiphi in cereal crops of Western Europe. We examined the thermal responses of the outcome of aphid parasitoid interactions in the pre-oviposition phase, when behavioural strategies largely determine parasitoid oviposition success, and in the post-oviposition phase, when the parasitoid is developing inside the aphid, as immunological interactions determine aphid or parasitoid survival. We found that both behavioural and immunological interactions were disrupted by temperature but that warmer temperatures had a stronger effect on these interactions. Most importantly, we have identified the underlying physiological mechanisms of behavioural and immunological responses that determine the outcome of the interaction. We demonstrated that differential independent physiological (e.g. metabolic rate) and behavioral responses (e.g. parasitoid attack rate and aphid defenses rates) between parasitoids and aphids could explain the disruption of their behavioral interactions at warmer temperatures. Behavioural interactions are also disrupted at low temperatures but this was through a reduction of the activity of both partners. Regarding immune interactions, there was an interaction effect of temperature and aphid resistance on parasitism efficiency: aphids differed in their resistance to parasitism (e.g. variation in the abundance of their protective symbiotic bacteria) but this divergence varied according to thermal conditions resulting in differential parasitism success. The combination of mechanistic and ecological approaches provided understanding of the impact of temperature on interacting species, and some insights of climate change consequences on host-parasitoid dynamics, that can be extrapolated to other systems, especially those consisting of co-evolved species and ectotherm organisms (e.g. lizards, fishes, amphibians, arthropods, etc.), for which ambient temperature matches their internal ones. This project also had an applied socio-economic interest, as understanding the effect of temperature on host-parasitoid interactions provides powerful tools for bio-control companies to improve pest regulation by natural enemies in greenhouses. KEYWORDS: Temperature, co-evolution, behavior, immunology, mechanisms
