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When organisms coevolve, any change in one species can affect phenotypes and ecology of the other species. Upper trophic levels have to synchronize their life-cycle to both abiotic conditions and lower trophic level species phenotypic variations and phenology. The role such interactions play in ecosystems is central, but their mechanistic bases remain underexplored. We tested the effect of seasonal variation in host quality on parasitoid diapause induction by using viviparous and oviparous female morphs of the pea aphid Acyrthosiphon pisum. Aphidius ervi parasitoids from two populations of contrasted climatic origin (harsh vs. mild winter areas) were allowed to parasitize each morph in a split-brood design and were next reared under either fall-like or summer-like temperature-photoperiod conditions. We found that oviparous morphs, present before winter, are cues per se for diapause induction; parasitoids entered diapause at higher levels when developing in oviparous hosts (19.4 3.0%) than in viviparous ones (3.6 1.3%), under summer-like conditions (i.e., when oviparous aphids appear in the fields). This pattern was only observed in parasitoids from the harsh winter area, suggesting local adaptations to variation in host quality as an overwintering cue. Analyses of the relative proportion of forty-seven metabolites and lipid reserves in both aphid morphs produced under the same conditions suggest parasitoids response to be mainly influenced by chemical cues derived from the host, with higher proportion of polyols and sugars, and more fat reserves being found in oviparous morphs. Host physiological state thus varies across the seasons and represents one of the multiple environmental indicators of upcoming detrimental conditions, affecting parasitoid overwintering strategies and diapause. Our results underline coevolutionary processes between hosts and parasitoids in their area of origin, likely leading to phenological synchronization with the environment. We point out the importance of such bottom-up effects for ecosystem functioning and for the provision of ecosystem services such as biological control.

Physiological thermotolerance and behavioral thermoregulation are central to seasonal cold adaptation in ectothermic organisms. For species with enhanced mobility, behavioral responses may be of greater importance in the cold stress response. Employing the carabid beetles as a study organism, the current study compared physiological thermotolerance and behavioral thermoregulation in carabid species inhabiting cereal fields in different landscape contexts, from fine grain heterogeneous "complex" landscapes to homogenous "simple" landscapes. Physiological thermotolerance was determined via measurement of the CTmin and chill coma temperature. Behavioral responses to cold temperature exposure were determined employing a purpose built arena, and thoracic temperature measured to estimate the efficacy of the behavior as a form of behavioral thermoregulation. Results revealed an influence of landscape composition on the cold tolerance of carabid beetles, although species differed in their sensitivity to landscape intensification. A reduced effect of landscape on the thermotolerance of larger carabid beetles was observed, thought to be the consequence of greater mobility preventing local acclimation to microclimatic variation along the landscape intensification gradient. Investigation into behavioral thermoregulation of the 3 largest species revealed burrowing behavior to be the main behavioral response to cold stress, acting to significantly raise carabid body temperature. This finding highlights the importance of behavioral thermoregulation as a strategy to evade cold stress. The use of behavioral thermoregulation may negate the need to invest in physiological thermotolerance, further offering explanation for the lack of landscape effect on the physiological thermotolerance of larger carabids. [ABSTRACT FROM AUTHOR]