Enemy-reduced space is hypothesized to promote host shifts and the formation of host races of phytophagous insects. However, few empirical studies have tested this hypothesis, and their results show varying patterns of natural-enemy attack in space and time. Furthermore, recent findings suggest that differentiation of natural enemies following host-race formation by their hosts (i.e., sequential radiation) may be a widespread phenomenon, thus potentially diminishing the advantage of shifting to a novel host plant. We found behavioral and genetic evidence that confirm the existence of host races in the gall midge Dasineura folliculi (Diptera: Cecidomyiidae) that induces bud galls on Solidago rugosa and S. gigantea. The gall midges showed significant preference to mate within the same host-associated population and to oviposit on their natal host, with more oviposition mistakes and lower rates of gall induction in the S. gigantea-associated population. The populations also formed genetically distinct clades based on mtDNA analysis. The galls are attacked by parasitoids, inquilines and predators, all of which were significantly more abundant in S. rugosa than in S. gigantea galls. Together with our behavioral results, this finding suggests that S. gigantea is the more recent host, offering enemy-reduced space to the population of gall midges associated with it. The lower levels of natural-enemy attack on S. gigantea were consistent at several Pennsylvania field sites, including sites where the S. rugosa host-race was absent. The inquiline Macrolabis americana (Diptera: Cecidomyiidae) that attacks D. folliculi galls showed no genetic divergence according the differentiation of its host. We are currently examining whether sequential radiation has occurred among other natural enemies in this system and whether enemy-reduced space on S. gigantea is consistent in time and space.