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Gregarious development in alysiine parasitoids evolved through a reduction in larval aggression

Population genetic models have suggested that siblicide between the larvae of parasitoid wasps, once gained, can be lost only under stringent conditions, making transitions from solitary to gregarious development rare. However, 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 Aphaereta genevensis and A. pallipes in the laboratory, on the host Drosophila virilis. Aphaereta genevensis usually lays a single egg in each host, and only a single wasp usually develops successfully even when several eggs are laid. However, A. pallipes often lays more than one egg in each host, and several offspring often complete development. Dissections of superparasitized hosts showed that this difference is accompanied by differences in larval behaviour: 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. Aphaereta genevensis shows some features that may have helped select for reduction in larval aggression in the subfamily: a longer development time, multiple egg clutches and incomplete brood reduction. Aphaereta spp. show great promise as model systems for studying the evolution of siblicide. Copyright 1999 The Association for the Study of Animal Behaviour.