Interactions between the Fungal Entomopathogen Zoophthora radicans Brefeld (Entomophthorales) and Two Hymenopteran Parasitoids Attacking the Diamondback Moth, Plutella xylostella L

Abstract

Treatment of the hymenopteran parasitoids Diadegma semiclausum and Cotesia plutellae with a strain of the entomopathogenic fungus Zoophthora radicans, which was initially isolated from Plutella xylostella, showed that D. semiclausum was susceptible to the pathogen but that C. plutellae was not. In leaf shower bioassays, the susceptibility of D. semiclausum adults to Z. radicans was 70- and 133-fold less than the susceptibility of P. xylostella larvae and adults, respectively. When adult D. semiclausum were held in petri dishes with P. xylostella cadavers producing Z. radicans conidia the subsequent level of fungal infection in the parasitoids (20.3% became infected) was independent of the number of cadavers to which they were exposed. Female parasitoids were not shown to vector conidia of Z. radicans directly from sporulating cadavers to the susceptible P. xylostella larvae upon which they foraged. However, the presence of a foraging D. semiclausum female enhanced the level of Z. radicans infection in P. xylostella larvae feeding in proximity to a source of Z. radicans inoculum. Video analysis of the movement of P. xylostella larvae in the presence and absence of the parasitoids indicated that the increased levels of fungal infection in the presence of foraging D. semiclausum could be accounted for by the greater movement of larvae (in terms of both the total distance moved and the number of new "infective units" visited) when disturbed by this parasitoid. Although C. plutellae foraging also increased larval movement when compared with larvae not foraged upon, the increase was not sufficient to exceed a presumed threshold level required to enhance the fungal infection level of the larvae. The results are discussed in terms of the likely interactions between Z. radicans and field populations of D. semiclausum.