The importance of ecological specialization as an evolutionary force involved in population differentiation and speciation has been well studied over the last several decades. The phenomenon of resource mediated differentiation is one example of such an ecological force, perhaps best known in phytophagous insects. However, less is known about how host plant associated genetic structure in these insects affects differentiation among populations of their natural enemies, including arthropod predators and parasitoids. The aphid genus Hyalopterus was recently discovered to comprise several species that show remarkable genetic isolation on different host plants in the genus Prunus. The parasitoid wasp Aphidius transcaspicus is a natural enemy of Hyalopterus aphids throughout the Mediterranean and can be found attacking aphids on all of their host plants. Here, using mitochondrial DNA and microsatellites, we examine A. transcaspicus individuals collected from sympatric and allopatric populations of four different Prunus species in Spain and Greece to determine whether host plant associated genetic structure in the aphids is transferred among trophic levels. We find that A. transcaspicus is not subdivided into cryptic strains or species specialized on different closely related Hyalopterus, but appears to represent a panmictic species capable of utilizing aphids from all host plants. Results suggest interesting possibilities for how evolutionary forces shape parasitoid host utilization and what effect this could have on their diversification. Implications of this information for the use of A. transcaspicus in pest management of Hyalopterus are also discussed.