Invasions of exotic insects represent a substantial threat to agriculture in the United States. Classical biological control, the introduction of natural enemies from a pest’s region of origin, is one of the most desirable means to mediate the effects of invasive pests without the use of chemical pesticides. Unfortunately, biocontrol's impact has often been limited. Introductions might be improved, however, by addressing fundamental concerns, such as the proper identification of pests and natural enemies, determining the geographic source and invasion history of the exotic pest and its potential for range expansion or host shifts in the invaded region, and the isolation of the most virulent natural enemy strain(s) for introduction. I am currently examining these issues using a combination of molecular biology and ecology in a model biological control system: the mealy plum aphid (Hyalopterus pruni), an invader from the Mediterranean that threatens prune production in California, and its parasitoid (Aphidius transcaspicus). The first step in this process has been to address taxonomic issues in the genus Hyalopterus, such as the occurrence of cryptic species or biotypes. Initial results from mitochondrial COI sequences show the presence of three largely host-plant associated groups within Hyalopterus collected in Spain and Greece from plum, almond, peach, and apricot, with indication that the one ‘species’ present in California has the potential to feed on multiple trees in the genus Prunus. These results could have important implications for agriculture in California, as a mealy aphid host expansion from prune would have significant impacts on the state’s stone-fruit industry.