The biology, egg and larvae of Acaenitus dubitator (Panzer) (Hymenoptera, Ichneumonidae: Acaenitinae)

Abstract

• Acaenitus dubitator (Panzer) is found to be a koinobiont endoparasitoid of the larva of an endophytic beetle, Cleonis piger (Scopoli) (Curculionidae), in Britain, suggesting a similar mode of development for the ichneumonid subfamily Acaenitinae as a whole. The parasitoid can overwinter in its cocoon in one of two ways. Individuals overwintering as essentially unaltered mature larvae do not become adult the following summer, while those that overwinter as morphologically distinct prepupae are committed to pupate and become adult immediately afterwards. The change from mature larva to prepupa takes place in late summer, soon after the time of cocoon formation, but a proportion of mature larvae lie over in the first year, and perhaps subsequently. This appears to be an adaptation to life in a particularly harsh and uncertain environment. The egg, prepupa, and first, second and final instar larvae are described and figured. Previous interpretations of the cephalic sclerites of final instar acaenitines are revised.