Size-mediated adaptive foraging: a host-selection strategy for insect parasitoids

Foraging models are useful tools for generating predictions on predator-prey interactions, such as habitat or diet choice. However, the majority of studies attempting to explain adaptive behaviour using optimality criteria have assumed that there is no trait (e.g. size) variation among individual consumers or their prey. Hymenopteran parasitoids that attack the free-living stages of their host are an ideal system for studying the influence of body size on host selection because of the wide range of adult parasitoid sizes coupled with the defensive capabilities of their hosts. We report here our application of an experimentally parameterized host selection model to investigate the influence of parasitoid body size on the range of acceptable host instar classes. Using a demographic model, we compared the efficiency of parasitoids using an optimal host selection strategy against parasitoids using an indiscriminate host selection strategy over a range of different parasitoid body sizes. Net fitness accrual of parasitoids and the impact of host instar selection on aphid recruitment were assessed on different stage-structured aphid populations. Our results demonstrate that optimal host selection allows larger parasitoids to utilize a wider range of hosts. However, smaller parasitoids receive the greatest benefits from selecting hosts optimally by utilizing a restricted range of small, poorly defended hosts when they are abundant. We argue that the correlation between flexible host selection behaviour and adult body size may be a general phenomenon that applies to the majority of hymenopteran parasitoids that attack free-living, well-defended hosts. The potential of within-generation behavioural interactions to impact between-generation dynamics in host-parasitoid populations are discussed.