Ecological Determinants of Sex Allocation in an Autoparasitoid Wasp

Abstract

Autoparasitoids are parasitic wasps that lay female eggs in homopteran nymphs (primary hosts) and male eggs in immature parasitoids of their own or different species (secondary hosts). This unique life history is restricted to some members of the family Aphelinidae. Theory suggests that autoparasitoids whose reproductive success is strictly limited by egg supply should produce equal numbers of sons and daughters, while the sex ratio of wasps that are strictly host-limited should exactly reflect the relative abundance of primary and secondary hosts. In this paper, models are developed to predict the reproductive strategy of wasps that are partially host- and egg-limited. Behaviour is influenced by the relative frequencies of host types and by a single parameter combination that describes the extent of host limitation. In a laboratory experiment, host density and the relative abundance of the two host types were varied and autoparasitoid sex ratio measured. Primary and secondary hosts were either intermingled or placed on separate leaves. When hosts were abundant (and the parasitoid relatively egg-limited), sex ratios were near equality. When hosts were at lower densities (and the parasitoid relatively host-limited), sex ratios were biased towards the sex that developed in the more common host type. These patterns provide strong qualitative support for the predictions of theory. More male-biased sex ratios were found in replicates where the two host types were intermingled on the same leaves. In these replicates, primary hosts were used more often as sources of food than as oviposition sites.