The effect of multi-species host density on superparasitism and sex ratio in a gregarious parasitoid

摘要

The ecological factors that influence key life-history traits such as brood size and sex ratio are enormously important to the survival and population dynamics of insect species. The effects of host density on life-history characteristics of a parasitoid, Pteromalus apum, were examined in a field experiment conducted in the angstrom land Islands, Finland. This gregarious parasitoid preys on two co-occurring butterfly species, Melitaea cinxia and Melitaea athalia. The abundances of both butterfly species contribute to host density for the parasitoid. The goals of the study were to: (i) test sex allocation theory in a field setting by evaluating how host density and species affect parasitoid brood size and sex ratio; and (ii) understand how parasitoid foraging behaviour and co-occurrence of multiple host species relate to life-history traits of the parasitoid. Local host density and species were experimentally manipulated and natural parasitism was then allowed to occur. Larger brood sizes were found at low host density due to higher rates of superparasitism. Further, parasitoid brood size and sex ratio (proportion male progeny) were positively related, with a much stronger effect at low host density. These results illustrate that host density affects life-history and population-level traits of parasitoids. The fact that low host density was associated with high superparasitism (larger brood size) supports predictions based on the apparent commensal' indirect interaction between the host species. This work highlights the value of research that integrates interactions between species with the study of foraging behaviour and life-history traits.