Natural enemy impact varies with host plant genotype

Abstract

The concept of Three Trophic Level interactions (Price et al., 1980) states that interactions between members of two trophic levels will have effects on the third trophic level. Variation among host plants is predicted to affect traits of herbivores in ways that, in turn, influence the susceptibility of the herbivores to their natural enemies. Genetic variation among crop plant cultivars has been shown to affect natural enemy impact on herbivores, but the effect of plant genetic variation on natural enemy impact has seldom been demonstrated for natural populations of host plants, herbivores, and natural enemies. Previous work showed that survival and enemy impact on a gall-forming sawfly (Phyllocolpa sp.) varied significantly among potted clones of the willow (Salix lasiolepis Benth.) in Arizona (Fritz & Nobel, 1990). I tested the hypothesis that in a different willow-herbivore system, plant genetic variation would also have a significant effect on the susceptibility of two herbivore species to natural enemy impact. I also tested the hypothesis that densities of heterospecific herbivores are correlated with natural enemy impact.