Gregarious development in alysiine parasitoids evolved through a reductionin larval aggression

Population genetic models have suggested that siblicicle between the larvae of parasitoid wasps, once gained, can be lost only under stringent;conditi ons, making transitions from solitary to gregarious development rare. Howev er, phylogenetic studies suggest that gregarious development has evolved on numerous occasions, although the mechanisms are largely unknown. We report experiments, on two morphologically similar species of alysiine braconids, directed at an understanding of how gregarious development evolved in one subfamily. We compared the oviposition behaviour and development of Aphaere ta genevensis and A. pallipes in the laboratory, on the host Drosophila vir ilis. Aphaereta genevensis usually lays a single egg in each host, and only a single wasp usually develops successfully even when several eggs are lai d. However, A. pallipes often lays more than one egg in each host, and seve ral offspring often complete development. Dissections of superparasitized h osts showed that this difference is accompanied by differences in larval be haviour: first-instar A. genevensis use their sharp mandibles to kill other parasitoid eggs or larvae in the same host. First-instar A. pallipes also have sharp mandibles, but do not attack conspecific larvae, suggesting that siblicide might have been lost by a simple change in larval behaviour. Aph aereta genevensis shows some features that may have helped select for reduc tion in larval aggression in the subfamily: a longer development time, mult iple egg clutches and incomplete brood reduction. Aphaereta spp. show great promise as model systems for studying the evolution of siblicide. (C) 1999 The Association for the Study of Animal Behaviour.