How do soil microbes influence plant attraction of insect herbivores and/or parasitoids of herbivores?

Plants are exposed to multiple stressors simultaneously, but can receive help deterring biotic stressors from belowground mutualists such as arbuscular mycorrhizal fungi. Previous research has identified that this “help” occurs primarily via enhanced direct defences against chewing herbivores. However, our preliminary data had suggested that AM fungi promote indirect defenses against sucking herbivores like aphids. Here we tested this premise in tomato. We exposed plants to colonization by AM fungi or not, and herbivory by potato aphids or not. We then measured plant biomass as well as changes in volatile organic chemistry over time, and attraction to plants within our treatments by parasitoids in wind tunnel trials. While analyses are ongoing, we found an influence of AM fungi on plant biomass and a trend toward greater attraction of parasitoids to plants hosting AM fungi. Surprisingly we found no impact of aphid herbivores on the attraction of their parasitoids, and no interaction between AM fungi and aphid herbivory on parasitoid attraction. This suggests that AM fungi do, as we hypothesized, promote indirect defences of plants.