Females do not avoid but prefer superparasitism in the parasitoid wasp Melittobia australica

Abstract

To reduce competition over limited resources, organisms often avoid resources already used by others. In parasitoid insects, females are generally expected to avoid parasitizing hosts already parasitized by other females, called superparasitism, and this prediction has been broadly supported across numerous species. However, in the parasitoid wasp Melittobia australica, multiple females were observed ovipositing on the same host despite the presence of nearby unparasitized hosts in the field. In this study, we first analysed field data using randomization tests, and found that females strongly aggregated on particular hosts rather than distributing randomly across hosts. We then conducted laboratory experiments to examine (1) whether this aggregation reflects an active preference for superparasitism and (2) whether such behaviour could improve parasitism efficiency. A choice experiment, in which females were provided with a parasitized and an unparasitized host, showed that females preferred superparasitism when hosts were non-diapausing. This result indicates that females of M. australica not only engage in superparasitism, but actively prefer parasitized hosts. Another experiment, in which varying numbers of females were allowed to oviposit on a host, found that offspring number per female decreased with increasing female number on diapausing hosts, whereas it remained constant irrespective of female number on non-diapausing hosts. This host-dependent result suggests that multiple females can more effectively paralyze vigorous non-diapausing hosts, thereby offsetting the negative effects of offspring competition. As Melittobia females are likely to attack non-diapausing hosts in most cases in the field, including those in the present study, this benefit of parasitism by multiple females may help explain their preference for superparasitism, in contrast to many other parasitoid species.