Variable Enemy Impact at Three Scales: Herbivore Species, Plant Species, and Plant Genotype

Abstract

We tested predictions of the Enemy Impact and Tritrophic Level Hypotheses by examining survival and causes of mortality of two closely related, coexisting species of gall-forming sawflies, Phyllocolpa, among three species of willow that grow in the same habitat and among potted clones of one willow species. We hypothesized that: a) susceptibility of the Phyllocolpa species to shared natural enemies would vary among the three willow species, b) Phyllocolpa species would differ in susceptibility to their natural enemies on the same willow species, and c) willow genotype would influence the susceptibility of the Phyllocolpa species to their natural enemies. The sawfly species differ in their gall characteristics and oviposition sites. Phyllocolpa nigrita (species B) oviposits at the base of the leaf fold, and Phyllocolpa sp. nov. (species M) oviposits near the middle of the leaf fold