Summary1. Ectotherm organisms are known to be plastic in their responses to temperature. Individuals developing at low temperatures usually achieve a larger body size through a lower growth rate ('Temperature Size Rule', TSR) and, when compared at the adult stage in a common environment, have a higher metabolic rate than those developing at higher temperatures ('Thermal Compensation Hypothesis', TCH). 2. For such plastic individuals, increased metabolic rate may incur a switch in the energetic allocation between competing functions such as maintenance and reproduction. Consequently, this differential allocation is expected to affect the animals' foraging behaviour. 3. This study investigates the effect of developmental temperatures and temperature experienced as adults on metabolic rate, life-history traits and foraging behaviour of females of the parasitic wasp Aphidius rhopalosiphi. For this purpose, individuals were reared at 10, 15, 20 and 25°C. At emergence, we checked if their traits followed the TSR. We then placed all females in a common environment (20°C), measured their metabolic rate and examined if a switch of energetic allocation occurred between longevity and ⁄ or egg load. Finally, we examined the physiological consequences of temperature on their host patch exploitation behaviour. 4. First, our results showed that traits at emergence followed the TSR. Second, metabolic rates of individuals developing at low temperatures were higher, following the TCH and leading to a reduction in longevity but an increase in oviposition rate. Finally, we showed that patch exploitation behaviours conformed to the predictions of state-dependent foraging models and that hosts which were not exposed to different temperatures, modified their behavioural defences. 5. To our knowledge, this is the first study to demonstrate adaptive thermal developmental plasticity in metabolic rates and its consequences on thermal plasticity of life-history traits and foraging behaviour in ectotherms.

The rise of temperatures may enable species to increase their activities during winter periods and to occupy new areas. In winter, resource density is low for most species and an increased number of active consumers during this season may produce heightened competitive pressure. In Western France, the aphid parasitoid species Aphidius avenae Haliday has been known to adopt a winter diapausing strategy adjacent to newly sown cereal crops, until recent reports of active winter populations in cereal crops. We investigate how the addition of this species to the winter guild of parasitoids may change the structure of the aphid-parasitoid food web and the host-exploitation strategies of previously occurring parasitoids. We showed that in winter, Aphidius avenae was mostly associated with two aphid species, Sitobion avenae Fabricius and Metopolophium dirhodum Walker, while the generalist species Aphidius rhopalosiphi was restricted to the aphid species Rhopalosiphum padi L. in the presence of Aphidius avenae. Due to this new competition, winter food webs present a higher degree of compartmentalization and lower proportional similarity index values than spring ones. Parasitoid and aphid abundances responded significantly to changes in daily high temperatures, suggesting that the host-parasitoid community structure can be partly predicted by climate. This study demonstrates how a change in the winter strategy of one species of a guild can modify complex interspecific relationships in host-parasitoid systems.