Larval competition and brood sex ratios in the gregarious parasitoid Pachysomoides stupidus

Abstract

The haplodiploid genetics of the Hymenoptera, by which males and females develop from unfertilised and fertilised eggs, respectively, give rise to certain asymmetrical coefficients of relatedness among family members^1–3. These asymmetries, and predictions concerning the sex ratio that are based on them^2,4,5, make the Hymenoptera ideal for the study of social behaviour. Work has focused on the more highly social groups of ants, bees and wasps^6–10. Although there has been considerable work on the parasitoid Hymenoptera with respect to sex ratio theory^11–15, no study has examined the social interactions of gregarious parasitoids in the light of inclusive fitness theory¹. I discuss here conflict among family members of a parasitoid wasp over the allocation of host resources, and argue that males are the more altruistic/less selfish sex. This provides a further explanation for hymenopteran males being generally smaller than females and for investment ratios being biased towards females^4,16. Conflict is predicted between mothers and their daughters over the allocation of resources to the sexes. The observed brood sex ratios are interpreted as a means by which egg-layers equalise investment in the sexes¹⁷.