Understanding factors that affect the context-dependency of species interactions has been identified as a critical research area in ecology. The context in which an interaction occurs can even influence whether it positively or negatively influences participants. In fact, where a symbiosis falls out on the continuum from mutualism to parasitism can depend on spatio-temporal variation in biotic and abiotic factors. Symbiotic foliar fungal endophytes produce alkaloids that can confer resistance to herbivores, increase drought tolerance, and enhance nutrient uptake in economically-important agronomic grasses. Although endophytes are very common in nature, relatively little is known about their effects on native, wild grass species, and a recent meta-analysis suggested that endophytes are only beneficial in agronomic settings. In a series of studies, we explored the context-dependency of endophyte-mediated benefits in the symbiosis between the wild grass Festuca subverticillata and the fungal endophyte Neotyphodium sp. We conducted a factorial design experiment manipulating endophyte infection and water availability to examine the benefits of symbiosis under drought and watered conditions. We also performed choice trials for five insect species and a greenhouse experiment with one species to assess how endophyte-mediated resistance to herbivory may depend on herbivore identity.

Results/Conclusions

In both experiments, we found evidence for context-dependent benefits of endophyte symbiosis. In the water availability experiment, we found that hosting endophyte reduced mortality in drought conditions. In the feeding trials, endophyte presence altered the preference of all five insect species tested. However, the magnitude and direction of preference depended on herbivore identity. Pterophylla camellifolia, Spodoptera frugiperda, and Rhopalosiphum padi preferred endophyte-disinfected plants and Encoptolophus costalis and Romalea guttata preferred endophyte-symbiotic plants. Despite reducing insect preference, the endophyte had no significant effect on S. frugiperda performance in a no-choice greenhouse experiment, nor did the endophyte alter plant growth in response to this herbivore. Our results suggest that the fitness consequences of endophyte symbiosis for host plants will be context-dependent on water conditions and the local composition of insect herbivores. Future work addressing herbivory and water availability in a single experiment is needed to access possible interactions between these factors for this symbiosis.