Foraging egg parasitoids, Trissolcus vassilievi (Hymenoptera: Platygastridae), respond to host density and conspecific competitors in a patchy laboratory environment.

Abstract

The egg parasitoid Trissolcus vassilievi (Mayr) is a key natural enemy of sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae). This laboratory study examined how the aggregation of parasitoids varied in response to host density and numbers of foraging conspecifics. Five host densities (1, 2, 4, 8, and 15 clutches of E. integriceps eggs), were offered simultaneously to 1, 2, 4, 8, or 16 female wasps in a standardized arena (30 cm diameter) and patch residence times were recorded. Patch residence time was strongly correlated with parasitism, and increased as the number of host clutches increased up to eight, but declined when more clutches were offered. Wasps displayed low mobility and tended to remain in the patch initially encountered, even when it contained few egg masses. At higher wasp densities, patches were occupied more quickly, host exploitation began sooner, and per capita handling time was reduced. However, total patch residence times were similar across all densities. Thus, females responded to conspecifics with increased movement, which increased dispersal and resulted in a more homogeneous distribution of wasps among patches. Pseudo-interference, resulting from wasps remaining in some patches and neglecting others, had greater impact on final levels of host exploitation than did actual interference (patch abandonment following conspecific encounters). These phenomena resulted in decreased parasitoid searching rates and a 2-fold increase in host survival at higher wasp densities.