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. Members of the aphid genus Hyalopterus feed on different species of the genus Prunus (mainly plum, almond, peach, and apricot). The parasitoid wasp Aphidius transcaspicus is a natural enemy of Hyalopterus aphids throughout the Mediterranean and can be found attacking Hyalopterus on all of their host plants. Here, using mitochondrial DNA and microsatellites, I examine the population structure of Hyalopterus and A. transcaspicus individuals collected from sympatric and allopatric populations of four different Prunus species in the Mediterranean. I demonstrate that while Hyalopterus is strongly divided into host plant specific species, there is no evidence that A. transcaspicus has evolved cryptic strains or species associated with Hyalopterus or their Prunus hosts. Instead A. transcaspicus appears to represent a panmictic species, though there is some evidence for geographically isolated populations. Results suggest interesting possibilities for how evolutionary forces shape aphid and parasitoid host utilization and what effect this could have on their diversification.