The prevalence of invasive species poses a major threat to natural and agricultural systems worldwide. Biological control can be an effective means of managing pests while minimizing the use of pesticides, but the method suffers from unpredictability. The success rate of biological control is likely to be improved through a more complete understanding of the ecological and evolutionary relationships between an invasive pest and its native natural enemies. The mealy plum aphid, Hyalopterus pruni, is an invasive pest of California's dried plum industry, and the parasitoid wasp Aphidius transcaspicus is being considered for introduction as a biological control agent. In the following dissertation, I use molecular genetic methods and laboratory experiments to examine aspects of these species' evolution, genetic structure, and ecology, with the aim of facilitating biological control of H. pruni. Among the major findings are: (1) host plant specificity is an important evolutionary force maintaining diversity in Hyalopterus, with analyses revealing three cryptic species that are each associated with a different Prunus host plant; (2) genetic analyses of A. transcaspicus from Spain and Greece reveal no patterns of host associated diversification like that observed in their hosts; (3) patterns of population structure of H. pruni in its native and introduced ranges provide evidence for multiple invasions of North America, with H. pruni in California introduced from Spain and aphids in the eastern USA and western Canada originating from central/northern Europe; (4) A. transcaspicus is genetically structured into several distinct regional populations (eastern Mediterranean, Italy, western Mediterranean, and Middle East) that should be further examined for their utility in biological control, though (5) laboratory parasitism experiments thus far provide little evidence that these A. transcaspicus populations inherently differ in their virulence against invasive H. pruni; and (6) mating experiments reveal partial reproductive isolation between populations from Spain and Israel, suggesting they may represent unique species or sub-species. The overall implications of these finding for management of H. pruni and for biological control in general are discussed.