Relative influences of plant type and parasitoid initial density on host–parasitoid relationships in a tritrophic system

To explore the effects of bottom-up and top-down forces on the relationships between a host, Plutella xylostella (L.) (Lepidoptera, Plutellidae), and its parasitoid, Cotesia vestalis (Haliday) (Hymenoptera, Braconidae), a short-term field experiment was established as a factorial experiment using three different host plants (Brassica pekinensis cv. Yuki F1, Brassica oleracea var. capitata cv. Midorimaru F1 and B. oleracea var. botrytis cv. Snow Crown) in the presence of C. vestalis at two different levels (low and high initial release). The tritrophic interactions were monitored by census counts of live adults 20 days after parasitoid release. The mean numbers of P. xylostella and C. vestalis adults were compared using log-linear analysis of deviance. Also, differences in the levels of parasitism were analysed using logistic analysis of deviance. There was a significant effect of host plant type on the abundance of P. xylostella , the abundance of C. vestalis and the percentage parasitism of P. xylostella by C. vestalis . The mean number of P. xylostella adults per cage on common cabbage or cauliflower was significantly greater than that on Chinese cabbage. The mean number of C. vestalis adults and the proportion of hosts attacked by C. vestalis per cage were significantly greater on Chinese cabbage compared with common cabbage or cauliflower. Indeed, initial parasitoid release did not significantly affect the abundance of P. xylostella but there was a significant influence of initial parasitoid release on the abundance of C. vestalis and the levels of parasitism of P. xylostella by C. vestalis . The mean number of C. vestalis adults and the proportion of P. xylostella parasitised by C. vestalis per cage were greater in high level of parasitoid release compared with low level of parasitoid release. However, there were no significant interacting effect of the factors (plant type × parasitoid initial abundance) on the abundance of P. xylostella , the population size of C. vestalis and parasitism of P. xylostella by C. vestalis .