BEHAVIORAL ECOLOGY - ORIGINAL PAPER Size-mediated adaptive foraging: a host-selection strategy for insect parasitoids Lee Mason Henry Æ Brian O. Ma Æ Bernard D. Roitberg Received: 7 October 2008 / Accepted: 22 April 2009 / Published online: 6 June 2009 Ó Springer-Verlag 2009 Abstract 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 para- sitoids 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 parame- terized 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 para- sitoids 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. Keywords Acrythosiphon pisum  Aphidius ervi  Correlated phenotypes  Optimal host selection Introduction The importance of individual behaviour for population and community level processes has been demonstrated in a number of studies (e.g. Werner et al. 1983; Beckerman et al. 1997; Luttbeg and Schmitz 2000; Peacor and Werner 2001). An extensive body of theory has been developed that extends individual-level behaviour into a community ecology framework (Dill 1987; Fryxell and Lundberg 1994; Ma et al. 2003; Persson and De Roos 2003). In host– parasitoid systems, a parasitoid’s choice of habitat, host species or host size may inﬂuence parasitoid–host dynamics (Mangel and Roitberg 1992), which in turn can produce cascading effects on populations that shape the structure of communities (Petchey et al.2008). However, much of the work on the impact of individual behaviour on population dynamics has centered on between-generation processes (Bolker et al. 2003), and this is especially true in host–parasitoid systems [see Mangel and Roitberg (1992) for alternative approaches]. Thus, features of biological systems that are within-generation processes, such as adaptive host selection, have largely been ignored. We argue that determining the dynamics of within-generation Communicated by Jay Rosenheim. L. M. Henry and B. O. Ma contributed equally in the development of this manuscript. L. M. Henry (&)  B. O. Ma  B. D. Roitberg Department of Biological Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada e-mail: lhenry@sfu.ca 123 Oecologia (2009) 161:433–445 DOI 10.1007/s00442-009-1381-2