The susceptibility of parental and hybrid willows to plant enemies under contrasting soil nutrient conditions

We conducted an experimental study of the effects of nutrient addition on the susceptibility of two species of willows (Salix eriocephala and S. sericea) and their hybrid to a pathogen and several herbivores. We hypothesized that the relative susceptibility of parental and hybrid willows would depend upon soil nutrient availability and vary among plant enemies. Using potted plants in a common garden, we found that S. eriocephala was significantly more susceptible to attack by a fungal rust (Melampsora sp.), a leaf-chewing beetle (Popillia japonica), and a leaf-folding sawfly (Phyllocolpa nigrita) than was S. sericea. Conversely, S. sericea was significantly more susceptible to attack by a spider mite (Tetranychus sp.) and a leaf-mining caterpillar (Phyllocnistis sp.) than was S. eriocephala. Hybrid susceptibility to Melampsora sp. and to Phyllocnistis sp. resembled S. eriocephala's, while hybrid susceptibility to Phyllocolpa nigrita, Tetranychus sp., and Popillia japonica resembled S. sericea's. Susceptibility to a sixth enemy, another leaf-mining caterpillar, Phyllonorycter salicifoliella, did not differ among the parents and hybrid. Susceptibility to herbivores and pathogens increased along a gradient of increasing fertilizer application, and this effect was independent of plant taxon or enemy. The results of our study point to the contrasting influences of a taxonomically and functionally diverse enemy community, reinforce the hybrid dominance model of enemy susceptibility, and demonstrate that physiological stress and enemy susceptibility can be inversely related.