Fungal grass endophytes, grass cultivars, nitrogen deposition and the association with colonising insects

Abstract

Fungal endophytes associated with pasture grasses can have community-wide effects on insect consumers. Here we asked the question to what degree endophyte infection, simulated nitrogen deposition and grass cultivar influence the abundance of colonising herbivores and their natural enemies. In a fully randomised field experiment, consisting of four Lolium perenne monocultures of known endophyte infection status and a nitrogen addition treatment, we determined the abundance of colonising aphids, their parasitoids and predators, and other grass herbivores. The three colonising cereal aphid species did not respond to endophyte infection, possibly because peramine concentrations were relatively low (3.9 μg/g). There was a significant interaction between nitrogen addition and plant cultivar on the abundance of Sitobion avenae, suggesting a cultivar-specific response to nitrogen addition. Aphid predators were affected by an interaction between endophyte and plant cultivar, but abundance of aphid parasitoids and other grass herbivores was not affected by any treatment. The fungus Claviceps purpurea naturally infected our experimental plants and infection rates differed among cultivars and were more likely to occur on endophyte-infected plants, in particular on wild-type Samson. We conclude that strong effects of endophytes on insect abundance may not occur in systems built upon L. perenne because overall peramine levels rarely reach threshold levels for insect toxicity. Keywords: fungal endosymbionts, multitrophic interactions, field experiment, insect food webs, alkaloids