Effects of Insect Herbivory and Fungal Endophyte Infection on Competitive Interactions among Grasses

Interactions among plants may be influenced by pests of parasites that differentially affect one competitor. The purpose of this study was to determine the effects of fungal parasitism and insect herbivory, alone in concert, on plant competitive interactions. The effects of fungal endophyte (Acermonium spp.) infection and fall armyworm (Spodoptera frugiperda) herbivory on competitive interactions in one— and two—species mixtures of the grasses tall fescue (Festuca arundinacea), red fescue (F. rubra), and perennial ryegrass (Lolium perenne) were examined in greehouse experiments. In general, herbivory reduced plant biomass whereas endophyte infection increased plant biomass. Endophyte infected (E+) plants were less damaged by herbivory than uninfected (E—) plants of the same species. Studies on fall armyworm larval feeding and choice tests with the five grass species were generally consistent with the outcome of the competition experiments; E+ plants were less nutritious and less preferred than E— plants of the same species. There were significant interactions among factors so that the outcome of competition depended on the species identities and the presence or absence of endophytes and herbivores. In competition with Kentucky bluegrass (Poa pratensis), E+ and E— perennial ryegrass produced similar biomass in the absence of herbivory, but E+ and E— perennial ryegrass had nearly twice the biomass of E— plants when herbivores were present. E+ and E— tall fescue were poor competitors with orchard grass (Dactylis glomerata) when herbivores were absent, but E+ tall fescue was a better competitors than E— plants and orchard grass when herbivores were present. This study indicates that competitive hierarchies among grasses are altered by interactions with insect herbivores and fungal endophytes, which have typically been ignored in past studies.