Grass–herbivore interactions altered by strains of a native endophyte

Summary•  Many plants support symbiotic microbes, such as endophytic fungi, that can alter interactions with herbivores. Most endophyte research has focused on agronomically important species, with less known about the ecological roles of native endophytes in native plants. In particular, whether genetic variation among endophyte symbionts affects herbivores of plant hosts remains unresolved for most native endophytes. Here, we investigate the importance of native isolates of the endophyte Epichloë elymi in affecting herbivory of the native grass host, Elymus hystrix . •  Experimental fungal isolate–plant genotype combinations and endophyte‐free control plants were grown in a common garden and exposed to natural arthropod herbivory. •  Fungal isolates differed in their effects on two types of herbivory, chewing and scraping. Isolates exhibiting greater sexual reproduction were associated with greater herbivore damage than primarily asexual isolates. Endophyte infection also altered patterns of herbivory within plants, with stroma‐bearing tillers experiencing up to 30% greater damage than nonstroma‐bearing tillers. •  Results suggest that intraspecific genetic variation in endophytes, like plant genetic variation, can have important ‘bottom‐up’ effects on herbivores in native systems.