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Neotyphodium(Clavicipitaceae: Balansieae) fungal endophyte infection does not always confer temperate grass resistance to insect herbivores, although reports indicate that over 40 species are adversely affected by its infection. Laboratory and glasshouse experiments were conducted to improve our knowledge of the anti-insect properties ofNeotyphodium-infected (E+) non-commercial grasses, and E+ wild barley (Hordeum) specifically.Neotyphodiuminfection of four plant inventory (PI) lines of wild barley conferred resistance toMayetiola destructor(Say) (Diptera: Cecidomyiidae), whereas none of the E+ wild barley accessions reduced the survival ofRhopalosiphum padi(L.) (Homoptera: Aphididae).Metopolophium dirhodum(Walker) (Homoptera: Aphididae) densities were significantly lower on the E+ clones ofHordeum brevisubulatumssp.violaceum(Boissier and Hohenacker) (PI 440420), compared with densities on endophyte-free (E–) plants of this species in population growth experiments.Neotyphodiuminfection of threeHordeum bogdanii(Wilensky) PI lines did not confer resistance toM. dirhodum; however, one of these E+ lines (PI 314696) was resistant to this aphid in a second population growth experiment. Our results provide additional evidence that the outcome of a grass–endophyte–insect interaction is influenced by the host grass species or genotype,Neotyphodiumspecies or genotype, and the insect species involved. They also reinforce this phenomenon for non-commercial grass–endophyte–insect interactions and underscore the potential role of endophytes in mediating wild barley–insect interactions and their potential to act as defensive mutualists. [ABSTRACT FROM AUTHOR]