Patch exploitation strategies of parasitic wasps under intraspecific competition

Abstract

The optimal residence time of a forager exploiting a resource patch in the absence of competitors has been much studied since the development of the marginal value theorem. However, several foragers are frequently observed exploiting the same patch simultaneously, and patch residence time has been surprisingly little studied in such competitive situations. The few theoretical models developed on this topic predict that foragers should engage in a war of attrition and stay in the patch longer than when foraging alone. We tested this prediction in Pachycrepoideus vindemmiae (Hymenoptera: Pteromalidae), a solitary parasitoid species in which females are known to defend the hosts they are exploiting via intraspecific fighting. By measuring the effect of direct (i.e., presence of conspecifics) and indirect (i.e., presence of already-parasitized hosts) competition on patch exploitation strategies, we revealed an apparent polymorphism of strategies. Indeed, in competitive situations, some members of the population tended to retreat almost immediately from the patch whereas other members tended to remain, further exploit the patch, and be more involved in interactions with conspecifics. The proportion of “retreaters” strongly depended on competition intensity and prior experience of competition. The forager's physiological state (age) also affects patch exploitation strategy and the intensity of interactions between competing females. Our results highlight the necessity for further theoretical studies that consider cases where contests between foragers are costly and take into account prior experience of competition and the forager's physiological state.