Influence of plant resistance at the third trophic level: interactions between parasitoids and entomopathogenic fungi of cereal aphids

Host-plant resistance can affect herbivorous insects and their natural enem ies such as parasitoids and entomopathogenic fungi. This tritrophic effect acts on interspecific interactions between the two groups of natural enemie s distantly related in phylogenetic terms. The intra- and extra-host aspect s of the interaction between the cereal aphid parasitoid Aphidius rhopalosi phi and the entomopathogenic fungus Erynia neoaphidis developing on the gra in aphid, Sitobion avenae, on resistant and susceptible wheat (Triticum aes tivum) cultivars, were studied. The competitive outcome of the intra-host i nteraction depended on the timing of parasitoid oviposition and fungal infe ction and was affected by wheat resistance. In particular, survival of the parasitoid was lower on the resistant wheat cultivar than the susceptible w heat cultivar, when the competitive outcome of the interaction was favourab le for either parasitoid or fungal development. Before and after this perio d the influence of plant resistance was not significant. Furthermore, the e xtra-host interaction was not affected by the wheat cultivar, although an i ncrease in fungal infection of S. avenae was observed when parasitoids fora ged in the experimental arena with sporulating aphid cadavers compared with foraging in the absence of sporulating cadavers. Our results showed that t he host plant may affect interspecific interactions between parasitoids and fungi and that these interactions depended on the timing of parasitoid ovi position and fungal infection.