Interactions between the Fungal EntomopathogenZoophthora radicansBrefeld (Entomophthorales) and Two Hymenopteran Parasitoids Attacking the Diamondback Moth,Plutella xylostellaL.

Abstract

Treatment of the hymenopteran parasitoidsDiadegma semiclausumandCotesia plutellaewith a strain of the entomopathogenic fungusZoophthora radicans,which was initially isolated fromPlutella xylostella,showed thatD. semiclausumwas susceptible to the pathogen but thatC. plutellaewas not. In leaf shower bioassays, the susceptibility ofD. semiclausumadults toZ. radicanswas 70- and 133-fold less than the susceptibility ofP. xylostellalarvae and adults, respectively. When adultD. semiclausumwere held in petri dishes withP. xylostellacadavers producingZ. radicansconidia 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 ofZ. radicansdirectly from sporulating cadavers to the susceptibleP. xylostellalarvae upon which they foraged. However, the presence of a foragingD. semiclausumfemale enhanced the level ofZ. radicansinfection inP. xylostellalarvae feeding in proximity to a source ofZ. radicansinoculum. Video analysis of the movement ofP. xylostellalarvae in the presence and absence of the parasitoids indicated that the increased levels of fungal infection in the presence of foragingD. semiclausumcould 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. AlthoughC. plutellaeforaging 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 betweenZ. radicansand field populations ofD. semiclausum.