Polyploidy is a dynamic process underlying a significant component of angiosperm diversity. This thesis research explores ploidy level as a source of genetic variability within a plant species, Solidago altissima L. (Asteraceae) and how this variation influences its herbivore community. More specifically, this thesis quantifies the spatial scale at which ploidy variation exists within and among population geographic distributions of diploid (2N), tetraploid (4N), and hexaploid (6N) races of S. altissima and assesses whether host use by seven taxonomically diverse herbivorous insect species is non-random with respect to ploidy level across this distribution. Sampling of S. altissima populations along a 400 mi longitudinal transect across Iowa and neighboring states revealed that substantial ploidy variation exists across the region as well as within local S. altissima populations. An initial phylogeographic study of ploidy races in four populations across the transect using AFLP markers indicates that ploidy races have a complicated geographical and genetic structure, suggesting that they have multiple origins. Genetic variation among plants can influence host choice and rates of development in insect herbivores. Ploidy-specific host use was found to be significantly non-random for six of the seven insects in at least one of the sites surveyed. Strong trends were evident in all species at z nearly all sites, but in many cases sample sizes were insufficient to establish statistical significance. Geographic variation in ploidy levels suggests that reciprocal co-adaptation of plants and their herbivores are likely to be highly dynamic.