Are endophyte-mediated effects on herbivores conditional on soil nutrients?

Neotyphodium endophytes are assumed to have mutualistic relationship with their grass hosts, mainly resulting from mycotoxin production increasing plant resistance to herbivores by the fungus that subsists on the plant. To study importance of often ignored environmental effects on these associations, we performed a greenhouse experiment to examine the significance of endophyte infection and nutrient availability for bird-cherry aphid ( Rhopalosiphum padi) performance on meadow fescue ( Lolium pratense). Naturally endophyte-infected (E+), uninfected (E-), or manipulatively endophyte-free (ME-) half-sib families of meadow fescue were grown on two soil nutrient levels. Endophyte infection reduced aphid performance in general. However, to our knowledge, this is the first study to demonstrate experimentally that herbivore performance decreases on E+ host plants with increasing availability of nutrients in soils. Potential improvement in herbivore performance in high nutrient soils and decreased plant performance in low nutrient soils in ME- plants, compared to E- and E+ plants, suggests that loss of endophyte infection after long coevolutionary relationship may be critical to plant fitness.