Factors affecting parasitism by Microctonus aethiopoides (Hymenoptera : Braconidae) and parasitoid development in natural and novel host species

A laboratory study of aspects of parasitoid host acceptance, suitability an d physiological regulation in natural and novel host species was carried ou t to investigate the degree of variability encountered with different hosts and to determine the value of such observations in host range determinatio n. The parasitoid Microctonus aethiopoides Loan was exposed to a natural ho st, Sitona discoideus Gyllenhal (Coleoptera: Curculionidae) and three novel hosts, the New Zealand native Nicaeana cervina Broun, the introduced weed biological control agent Rhinocyllus conicus (Froehlich), and a congeneric pest species, Sitona lepidus Gyllenhal (all Coleoptera: Curculionidae). Per cent parasitism of these species was 54%, 43%, 39% and 0%, respectively. T he results indicated that for both S. discoideus and R. conicus more males than females were parasitized (69% cf. 45%, and 49% cf. 32% respectively) b ut host size was not a significant factor. Overall, superparasitism was rec orded in about 29% of parasitized weevils and there was evidence that host discrimination to avoid superparasitism occurred in the natural host. Conve rsely, superparasitism occurred more frequently than would be expected in N . cervina (42%) coupled with higher survival of larvae in superparasitized hosts in this species. The frequency distribution of attack of R. conicus b y M. aethiopoides was not different from random. Parasitoid development was more rapid in the natural host, S. discoideus, and parasitoid size was pos itively correlated with host size. There was a strong positive relationship between parasitoid larval survival and the presence of teratocytes in all hosts. Host fecundity and fertility were reduced by parasitism for most spe cies, and in some cases, by exposure to parasitoids in the absence of detec table parasitism. It was concluded that laboratory observations can provide useful information on the compatibility between host and parasitoid which can complement traditional host range tests to predict field host range.