Water availability alters the tri-trophic consequences of a plant-fungal symbiosis

Plant-microbe protection symbioses occur when a symbiont defends its host against enemies (e.g., insect herbivores); these interactions can have important influences on arthropod abundance and composition. Understanding factors that generate context-dependency in protection symbioses will improve predictions on when and where symbionts are most likely to affect the ecology and evolution of host species and their associated communities. Of particular relevance are changes in abiotic contexts that are projected to accompany global warming. For example, increased drought stress can enhance the benefits of fungal symbiosis in plants, which may have multi-trophic consequences for plant-associated arthropods. Here, we tracked colonization of fungal endophyte-symbiotic and aposymbiotic Poa autumnalis (autumn bluegrass) by Rhopalosiphum padi (bird-cherry-oat aphids) and their parasitoids (Aphelinus sp.) following manipulations of soil water levels. Endophyte symbiosis significantly reduced plant colonization by aphids. Under low water, symbiotic plants also supported a significantly higher proportion of aphids that were parasitized by Aphelinus and had higher above-ground biomass than aposymbiotic plants, but these endophyte-mediated effects disappeared under high water. Thus, the multi-trophic consequences of plant-endophyte symbiosis were contingent on the abiotic context, suggesting the potential for complex responses in the arthropod community under future climate shifts.