Abstract

Laboratory experiments were performed to evaluate at the population level the influence of host-plant resistance on interspecific interactions of the cereal aphid Sitobion avenue with two phylogenetically distant species of natural enemies, the parasitoid Aphidius rhopalosiphi and the entomopathogenic fungus Pandora neoaphidis. Indirect effects of wheat resistance on interspecific interactions between parasitoid and fungus were caused by interaction modifications, i.e., changes in per capita interspecific effects, as evidenced by significant statistical interactions between the effects of the fungus and wheat resistance on per capita population growth rate (PCPGR) of the parasitoid. Reduction by the fungus of parasitoid PCPGR was lower on the susceptible wheat than on the resistant wheat; this was interpreted as the population level expression of the asymmetric competitive effect of the fungus on parasitoid development. As expected from the asymmetric interaction between parasitoid and fungus, no indirect effect of the parasitoid on the fungus was found, as evidenced by no significant statistical interactions between wheat cultivar and parasitoid. However, increased fungus PCPGR was found in the presence of the parasitoids regardless of wheat cultivar; this may also be explained by increased fungal infection during parasitoid foraging. Path analysis, a statistical technique to discern the relative contributions of direct and indirect effects, also detected indirect effects between fungus and parasitoid.