A central question in the study of insect herbivore-plant interactions is: What are the factors responsible for variation in herbivore density and surviorship among conspecific host plants in a population? Although there are many possible mechanisms, variation in host plant resistance and in enemy impact are strongly suggested as important by previous work. Theory and empirical evidence suggests that interactions between plant resistance and variable enemy impact among plants may be common and important in plant-herbivore systems. The concept of three tophic level interactions and the enemy impact hypothesis pose three main pathways of interactions between plant resistance and enemy impact. The main goal of this research is to determine the relative importance of these pathways of interaction between host plant genotype and natural enemy impact in this system. Answers to these questions are important to the development of theory of plant-herbivore-natural enemy interactions and to understanding the processes that influence population and community variation of herbivorous insects among conspecific plants. The experimental program will determine: 1) the heritability and correlation of resistances to three herbivores on willow, 2) the presence of variation in enemy impact among plant genotypes and its correlation with plant resistance, 3) the relative importance of pathways by which plant genotype or other herbivore species directly or indirectly affect enemy impact, and 4) the relative importance of several mechanisms of direct and indirect competitive interactions between herbivores. Other experiments will examine details of specific pathways of interaction between plant genotype, other herbivore species, and natural enemy impact.
