Lifetime gains of host-feeding in a synovigenic parasitic wasp D. GIRON, S. PINCEBOURDE andJ. CASAS Institut de Recherche sur la Biologie de l’Insecte (Centre Nationale de la Recherche Scientifique, Unite´ mixte de Recherche 6035), Faculte´ des Sciences, Universite´ de Tours, France Abstract. Understanding behavioural decisions relative to host use for feeding or reproduction by foraging parasitoids requires not only the study of metabolic pathways followed by nutrients, but also the quantification of lifetime fitness gains of each alternative behaviour. By using a combination of observational and manipulative approaches, the lifetime host-feeding gains are measured both in terms of fecundity and longevity in the parasitoid Eupelmus vuilletti. Host-feeding increases both egg production and longevity. The increase in fecundity is mainly determined by the amount of lipids obtained whereas the lifespan extension is mainly determined by carbohydrates. Proteins obtained through host-feeding have been implicated previously in egg production by parasitoids but protein intake has no effect on fecundity and longevity in E. vuilletti. The amount of nutrients gained through host-feeding and invested in eggs is variable and changes over the lifetime of the animal. Therefore, lifetime feeding gains are best under- stood through the construction of dynamical budgets running over the entire lifespan of an insect. Key words. Foraging ecology, host-feeding, life-history strategies, lifetime fitness gains, parasitoids, reproduction, survival. Introduction Dynamic state models have been used extensively as a powerful technique in evolutionary ecology to make detailed predictions about the optimal behavioural deci- sions of organisms such as parasitoid wasps (Chan & Godfray, 1993; Collier etal., 1994; Godfray, 1994; Heimpel etal., 1994, 1998; Collier, 1995; Roff, 2002). In turn, these behavioural models play an important role in predicting the population dynamics and stability of host–parasitoid inter- actions and the outcome of biological control efforts (Kidd & Jervis, 1989; Murdoch, 1990; Kidd & Jervis, 1991b; Krivan & Sirot, 1997). Parasitoids search the environment for hosts, usually other insects, in which to lay their eggs. However, reproduction is not the only way to use the host. Indeed, in many species, female parasitoids use the host for feeding, instead of ovipositing. Even though con- current host-feeding and oviposition are sometimes observed, it is usually assumed that host-feeding kills the host or reduces its quality as an oviposition site (Jervis & Kidd, 1986; Heimpel & Collier, 1996). Thus, on finding a host, a female parasitoid makes a series of behavioural decisions, such as whether or not to renounce the opportun- ity for current reproduction (oviposition) in favour of anticipated chances for future reproduction (host-feeding). These decisions are inevitably associated, at least in part, with physiological thresholds. Theoretical studies have shown that the optimal behavioural decision relies heavily on the nature and function of host-feeding (Jervis & Kidd, 1986; Mangel, 1989; Chan, 1991; Kidd & Jervis, 1991a; Houston etal., 1992; Chan & Godfray, 1993; Collier etal., 1994; Collier, 1995; Heimpel etal., 1998). Whether nutrients obtained from host feeding are employed for egg produc- tion, maintenance, or both, has consequences for the way in which parasitoids partition their time and energy between reproduction and feeding activities (Chan & Godfray, 1993; Rivero & Casas, 1999a). The incorporation of physiological realism into models of parasitoid behaviour and popula- tion dynamics has thus raised a series of new questions Correspondence: D. Giron, Entomology Department, University of Georgia, Athens GA 30602, U.S.A. Tel.: þ1 706 583 8238; fax: þ1 706 542 2279; e-mail: giron@bugs.ent.uga.edu Physiological Entomology (2004) 29, 436–442 436 # 2004 The Royal Entomological Society