COMBINED EFFECT OF FOLIAR AND MYCORRHIZAL ENDOPHYTES ON AN INSECT HERBIVORE

Abstract

Many foliar endophytic fungi deter herbivores from feeding on their host plants, but a previous study has suggested that their deterrent effect may be reduced in the presence of arbuscular mycorrhizae. In our study, factorial experiments determined the effects of infection of perennial ryegrass, Lolium perenne, by the foliar endophyte Neotyphodium lolii [present (+E) or absent (−E)] and the mycorrhizal fungus Glomus mosseae [present (+M) or absent (−M)] on larvae of the noctuid moth Phlogophora meticulosa. When host plants received adequate phosphorus (P), mycorrhizae and the foliar endophyte decreased the survivorship of larvae between their second and fifth instar in an additive manner. Sixth-instar larvae feeding on −M−E plants attained greater mass than those feeding on plants with one or both symbionts. There was an M × E interaction with the total amount of grass consumed per larva. The foliar endophyte increased relative consumption rate and reduced efficiency of conversion of ingested food of larvae, while the mycorrhizal fungus had no effect on insect nutritional indices. Larvae preferred −E over +E leaf blades regardless of mycorrhizal infection status. They showed no preference with respect to plant mycorrhizal status. When foraging among mixed stands of intact plants, larvae inflicted more damage on −E than on +E plants. Mycorrhizae reduced feeding damage, but only in −E plants and to a lesser extent than the foliar endophyte. The relative difference between +E and −E plants, in terms of grazing damage, was decreased but not eliminated by mycorrhizae. When P was limiting to plant growth, mycorrhizae still decreased the survivorship of larvae between their second and fifth instar, independently of the foliar endophyte. They accelerated larval development slightly. The foliar endophyte increased the survivorship of fifth-instar larvae, independently of mycorrhizae. We concluded that both foliar endophytes and mycorrhizae affected larval growth and survivorship. Some effects were additive, while others were nonadditive and depended to some extent on host P nutrition. Our findings suggest that the beneficial effect of the foliar endophyte on the host plant, in terms of insect resistance, was reduced but not eliminated by mycorrhizae.