The diversity of plant-animal interactions provides evidence demonstrating the iterative adaptive changes in interacting species attempting to enhanced fitness and survival. A central theme in ecology is understanding the factors that influence interacting animal and plant species. This study focuses on the role of a plant host as a driving force for phenotypic and genetic changes in herbivores. I have examined a suite of lepidopteran larvae on several different tree species in forests of eastern Maryland for phenotypic and genetic host-associated differentiation. Larvae were collected from at least three different tree species at three different sites in eastern Maryland. Phenotypic traits such as development time, pupal weight, and fecundity were measured for the larvae. Larval and pupal traits were recorded and an initial set of reciprocal crosses on host plants were run for a sample of lepidopteran larvae. In addition, DNA was extracted from the adult moth for analysis of genetic differentiation. Using cytochrome oxidase 1 mitochondrial DNA, each species was examined for host-associated genetic differentiation. As in other ecological studies of assemblages or guilds, changes in some species are distinct from those of other species in the assemblage on each tree host species. In addition, changes in clusters of species, of those that comprise the assemblages on trees, parallel each other but are distinct from other clusters, both in regard to phenotypic and genotypic changes.