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Key words. Hymenoptera, Braconidae, Aphidius colemani, Hemiptera, Aphis gossypii, Myzus persicae, aphid, body size, host selection
Abstract. Body size is a main fitness component of insect parasitoids. We assessed the potential influence of maternal size of the parasitoid wasp Aphidius colemani Viereck (Hymenoptera: Braconidae: Aphidiinae) on its ability to parasitize the different instars of Aphis gossypii Glover (Hemiptera: Aphididae) on eggplant and cucumber. In the experiments "small" vs "large" parasitoid females were used. Females oviposited in all instars but more of the smaller hosts were parasitized. Host selection was affected by female size and the larger hosts were more frequently mummified by the large than the small females. Thus, parasitoid female size influenced host selection. This could affect the potential of the parasitoid to exploit populations of aphids that differ in their size structure. The importance of these results in terms of the ecological adaptations of the parasitoid and their implication for biological control are discussed.
INTRODUCTION
Changes in the behaviour of optimal foragers results in an increase in profitable gain when exploiting resources of variable quality. These adaptations are difficult to investigate as resource quality and quantity may vary in space and time. Nevertheless, specific classes of organisms such as parasitoids are ideal organisms for testing hypotheses regarding the fitness consequences of reproductive behaviour. The key feature is that their development is entirely dependent on the resources provided by a single host. Based on their development and host usage strategies parasitoids have been divided into two groups: koinobionts and idiobionts (Askew & Shaw, 1986). Koinobiont parasitoids allow further development of the host after parasitism, whereas idiobionts parasitize nongrowing hosts (eggs or pupae) and /or arrest the development of hosts by paralysis or killing it during oviposition.
Foraging theory predicts that optimal host selection aims to maximize the profits for the next parasitoid generation (Hubbard & Cook, 1978; Pyke, 1984). Taking into account that the host represents a finite food resource, acceptable host individuals should satisfy the minimum resource threshold for the parasitoid larva's growth (Vet et al., 1994; Harvey et el., 1995; Mackauer et al., 1996). In fact, the finite resources available during parasitoid development result in a close association between host size and offspring size (Harvey, 2005). Therefore, fitness of the offspring...