Ovicide in the whitefly parasitoid, Encarsia formosa

The oviposition decisions made by insect parasitoids when encountering host s of variable quality have been the subject of extensive theoretical and ex perimental investigation. For parasitoids that lay their eggs inside the ho st, the possible outcomes of encounters with parasitized hosts have been as sumed to include only oviposition (superparasitism), rejection, or in some cases feeding on host haemolymph. We document another outcome in Encarsia f ormosa Gahan (Hymenoptera: Aphelinidae), a species that has been a model sy stem for the study of oviposition behaviour. In E. formosa, females may kil l eggs previously laid within the host by jabbing them with their oviposito r before ovipositing themselves. (1) Our observations indicated that jabbed eggs were indeed killed. (2) In experimental arenas in the laboratory, ovi cide occurred in the majority of encounters with parasitized hosts and at h ighest frequency in encounters resulting in oviposition. (3) There was no s ignificant difference in the handling time associated with oviposition+ovic ide in parasitized hosts in comparison with oviposition alone, suggesting t hat there is no time cost to ovicide. (4) Ovicide did not appear to be inci dental to normal probing within a host. Radial analysis of the direction of ovipositor movement with respect to the centre of the previously laid egg within the host showed that females engaged in ovicidal bouts probed most o ften in the direction of the egg. This is the first well-documented study o f ovicide in an endoparasitoid. We suggest ovicide may be under-reported in other endoparasitoid species due to the difficulty of observing it. (C) 20 00 The Association for the Study of Animal Behaviour.