Host finding and selection in four species of chalcid parasites

Abstract

A comparative study of four species of Chalcid (Pteromalidae) parasites: Muscidifurax zaraptor, Muscidifurax uniraptor, Pachycrepoideus vindemiae and Spalangia endius, was made. The research examined two main aspects: (1) Host finding. The olfactory response of the adult parasites to a series of specific odours in the olfactometer and their ability to locate host pupae under varying depths of sawdust was determined. (2) Host selection. Discrimination between pupae of different sizes, housefly strains, fly species, and parasitized and unparasitized pupae. The behavioural response of the parasites to a series of odour stimuli associated with host-habitat showed that S.endius and P.vindemiae responded strongly to odours and were more active in a high humidity environment. The Muscidifurax species were more sensitive to humidity than odours. They were more active in a drier environment and avoided a damp habitat. S.endius was capable of locating host pupae buried under varying depths of sawdust whereas the other three species appeared to be less effective searchers. The results of host selection tests demonstrated that all species except S.endius showed no preference for host size and host strains whereas S.endius preferred smaller host pupae. Different host size and strains had no effect on sex ratio nor survival rate of progeny. However parasites were affected by host species. They exercised oviposition restraint on Drosophila melanogaster pupae when offered a choice and only oviposited in D.melanogaster pupae in the absence of Musca domestica. M.zaraptor avoided D.melanogaster host pupae. P.vindemiae showed no preference between M.domestica and Calliphora quadrimaculata host pupae whereas the other three species selected M.domestica over C.quadrimaculata. This was especially marked in the case of M.uniraptor. Furthermore, all species except M.uniraptor attacked similar numbers of Lucilia sericata and M.domestica pupae. M.uniraptor selected M.domestica over L.sericata when given a choice between both hosts' pupae. Besides these, all species of parasites preferred to oviposit on unparasitized hosts and were capable of discriminating between parasitized and unparasitized hosts both intra- and inter-specifically. S.endius discriminated between parasitized and unparasitized host pupae with their ovipositor whereas the other species discriminated with their antennae after examining the pupae as well as with their ovipositors after inserting into the puparium. It seems that quantity and quality of food played an important role in host selection. Generally, parasites chose to oviposit on those hosts which were suitable for complete development of their progeny.