VARIABLE ENEMY IMPACT AT 3 SCALES - HERBIVORE SPECIES, PLANT-SPECIES,AND PLANT GENOTYPE

We tested predictions of the Enemy Impact and Tritrophic Level Hypothe ses by examining survival and causes of mortality of two closely relat ed, coexisting species of gall-forming sawflies, Phyllocolpa, among th ree species of willow that grow in the same habitat and among potted c lones of one willow species. We hypothesized that: a) susceptibility o f the Phyllocolpa species to shared natural enemies would vary among t he three willow species, b) Phyllocolpa species would differ in suscep tibility to their natural enemies on the same willow species, and c) w illow genotype would influence the susceptibility of the Phyllocolpa s pecies to their natural enemies. The sawfly species differ in their ga ll 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. Galls formed by sawfly species B have gaps more frequently at the base of the fold th an galls formed by sawfly species M on an willow species. We predicted that sawfly species B would be more susceptible to egg parasitoids si nce its oviposition site is at the base of the fold where it appears t o be more exposed to parasitoids. The susceptibility of sawfly species M and B to natural enemies varied among willow species, supporting th e first hypothesis. Both sawfly species survived best on Salix eriocep hala in 1990 but survival was highest on S. discolor in 1991, apparent ly due to increased ''other'' mortality on eggs on S. eriocephala in 1 991. Sawfly species M and B differed in survival and causes of mortali ty on S. eriocephala (41% vs 34% - 1990; and 12% vs 6% - 1991), suppor ting the second hypothesis. There was significantly higher egg parasit ism on sawfly species B (13% and 16%) than on sawfly species M (2% and 3%) On S eriocephala in both 1990 and 1991. Survival of sawfly specie s B was significantly lower (7% vs 28%) and egg parasitism was signifi cantly higher (56% vs 18% than for sawfly species M on potted clones o f S. sericea. There were significant differences in survival and egg p arasitism for both sawfly species among different genotypes of S. seri cea.