Soil microbial communities can impact on plant fitness and insect interactions. Here we focus on arbuscular mycorrhizal (AM) fungi which can change host plant quality for insect herbivores altering plant nutritional quality and/or by priming effects that lead to enhanced inducible and constitutive plant defences. This produces two pathways by which AM fungi could influence herbivore-parasitoid interactions: First, plant quality effects (due to altered nutrition or direct defenses) on herbivores could cascade to higher trophic levels influencing natural enemy control of insect pests. Second, via their priming influence, AM fungi could alter indirect defences by altering volatiles used to attract parasitoids. Building on previous work, we addressed the relative importance of these two pathways in a series of experiments in which we manipulated the presence of a natural AM fungal community, Solanum species and genotype, potato aphid genotype, and parasitism of potato aphids. We measured aphid growth rates, and conducted parasitism assays with aphids in a neutral environment and involving host plants (and host plant volatiles).
Results/Conclusions
We found that AM fungi altered parasitism via both pathways: As suggested by the first pathway, AM fungi increased parasitism success of aphids when they were parasitized in a neutral environment, and the parasitoid received no cues from the host plant. However, as we would expect from the second pathway, parasitoid attraction to plants hosting AM fungi was much greater than attraction to plants that did not host AM fungi. However, in both cases, the influence of these pathways was dependent upon Solanum species and genotype. As a result, AM fungi influenced parasitism via multiple mechanisms, but this influence was dependent on host plant. The redundancy in parasitoid influence may allow AM fungi to alter parasitism regardless of individual host plant responses to AM fungi.