Flea beetles in the genus Altica are herbivorous, urban agricultural pests that are morphologically difficult to distinguish. Host plant associations, therefore, have been used as an important species character in field studies. Indigenous weeds in the Onagraceae, genus Oenothera, are known to serve as developmental hosts for the flea beetle Altica litigata Fall. Although host plant specificity in herbivorous beetles is labile and adult A. litigata have been reported to aggregately feed on plants in the nonindigenous Lythraceae, genus Lagerstroemia, there is no evidence that these ornamental trees serve as developmental hosts. Because adult A. litigata feed on host plants from species in two plant genera, this study was designed to test two hypotheses. The first hypothesis that was tested was whether adult flea beetles collected from primrose and crape myrtle plants across four ecoregions are phenotypically (morphology) and genotypically (genotype) A. litigata. The second hypothesis that was tested was whether two unlinked loci, cytochrome oxidase subunit I and internal transcribed spacer, are phylogenetically concordant for flea beetle species. If so, they could be used to determine the intraspecific geographic history of A. litigata collected from Oenothera and Lagerstroemia species. We discuss how these markers, in conjunction with morphology and host plant feeding behavior, can not only help to validate morphologically difficult taxa but also can illuminate herbivore-plant genetic structure through phylogeny analyses.