Inter-relationships between the polyembryonic parasitoid Litomastix maculata (Hymenoptera : Encyrtidae) and its host the green looper caterpillar Chrysodeixis eriosoma (Lepidoptera : Noctuidae)

Abstract

The relationship between the polyembryonic parasitoid Litomastix maculata Ishii and its host in New Zealand, the green looper caterpillar Chrysodeixis eriosoma (Doubleday), is examined. The stages in the polyembryonic development of the parasitoid within the host and the synchronization between parasitoid and host in the later stages of the host's life are described. A preliminary 15 month sampling programme provided information on the role of L. maculata in the parasitoid complex attacking the host and indicated the existence of a period of low parasitism by L. maculata in late winter/early spring. Two other findings of the study were that parasitism affected the head capsule width distribution of the host and that the proportion of L. maculata broods which contained both males and females was correlated with percentage parasitism. A second more intensive field study provided population density trends for both host and parasitoid which was used to assess the synchronization between them. The effect of parasitism on the growth rate, size and consumption of the host was examined. Parasitized larvae had more instars, grew bigger and ate more than unparasitized hosts. Parasitoid emergence was well synchronized with host emergence when parasitized and unparasitized individuals were reared at 20-25°C, but at 12°C and 15°C development of the parasitoid took significantly longer than development of the host. In spring and in colonizing situations, parasitoid density is low relative to the host and consequently few hosts are parasitized by more than one female. Most females lay only a single egg per oviposition so in spring and in new crops the majority of L. maculata broods contain individuals of only one sex. As parasitoid density relative to the host increases, more hosts are parasitized by more than one female and the proportion of broods containing both sexes increases. Mixed broods are strongly female biased so a high proportion of mixed broods is associated with a very low sex ratio (proportion males). Mixed broods are also more conducive to inbreeding, so Hamilton's (1967) hypothesis that female biased sex ratios are associated with high levels of inbreeding is confirmed.