The exceptional diversity of phytophagous insects may be due in part to their propensity for speciation via host-race formation. In our research, we have focused on two closely-related goldenrods (Solidago altissimaand S. gigantea) and their insect herbivores as a model system to study host shifts (and eventual speciation) by multiple evolutionarily independent insect lineages on the same host plant pair. Our ecological and genetic analyses establish host-race formation as an important mechanism of diversification in phytophagous insects (Nason et al. 2002b; Stireman et al. 2005) and their insect parasitoids (Stireman et al. 2006; Kolaczan et al. 2009). Although models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host, we show that such “enemy-free space”, like many ecological and evolutionary forces, operates as a complex geographic and temporal mosaic (Heard et al. 2006). Genetic differences among individual host plants may also influence attack rates and our cytological and genetic studies of S. altissima indicate co-occurring 2N, 4N, and 6N plants to be an example of neopolyploidy, with the different cytotypes originating locally (Halverson et al. 2005). We have found cytotype variation to differentially influences attack rates among phylogenetically diverse insect herbivores and to do so in a consistent manner across sites despite its recurrent and independent origins (Halverson et a. 2006).