Clutch size in an obligately siblicidal parasitoid wasp

Insect parasitoids can be grouped by their pattern of brood production: 'gregarious' parasitoids can produce multiple offspring per host, but 'solitary' parasitoids can produce only a single offspring per host because their larval stages engage in lethal contest competition to monopolize the host resource. Theoretical and empirical studies have largely overlooked the possibility that solitary parasitoids may lay clutches of more than a single egg, leading to obligate siblicide. When this possibility has been recognized at all, it has been treated under the rubric of self-superparasitism (in which a parasitoid deposits eggs on the same host during two or more distinct host encounters). It is argued here, however, that multiple-egg clutches and self-superparasitism are fundamentally different behaviour patterns, entailing different potential costs and benefits. These differences are linked to variation in age structure within broods (self-superparasitism can produce mixed-age offspring within broods) and host quality assessment problems that are specific to self-superparasitism (during a second encounter, the parasitoid may not be able to assess its prior oviposition behaviour on the host). Clutch size was investigated in a solitary parasitoid wasp, Comperiella bifasciata (Howard) (Hymenoptera: Encyrtidae), that commonly produces multiple-egg clutches. Clutch size varied in response to aspects of the environment that were likely to shape the costs and benefits of multiple-egg clutches: larger clutches were produced when the parasitoid (1) had no contact with high-quality hosts and (2) experienced a high density of conspecific female parasitoids. Two types of costs are associated with depositing multiple-egg clutches: the opportunity cost of the extra egg(s) and the direct cost of the time required to deposit them. Multiple-egg clutches may represent a conditional strategy that increases reproductive success. A review of the literature shows that multiple-egg clutches are a widespread feature of the reproductive behaviour of solitary parasitoids. (C) 1996 The Association for the Study of Animal Behaviour