Evolutionarily significant units in natural enemies: Identifying regional populations of Aphidius transcaspicus (Hymenoptera: Braconidae) for use in biological control of mealy plum aphid

Understanding the genetic structure of a natural enemy in its native range may provide important information for biological control practitioners by aiding the identification of populations that might exhibit local adaptations to environmental conditions or organisms with which they interact. The parasitoid wasp Aphidius transcaspicus is being investigated as a candidate biological control agent for the invasive mealy plum aphid, Hyalopterus pruni, in California, USA. For A. transcaspicus collected from throughout the Mediterranean and Middle East, mitochondrial sequence data (COI) revealed two major clades, one widespread throughout the sampled range, and another restricted to the eastern Mediterranean. Microsatellite data provided greater resolution, revealing five regional population clusters associated with Spain and northwest Africa, Italy, the eastern Mediterranean, Iran, and Pakistan, respectively. Within these clusters, parasitoids exhibited some geographic structure (FSC = 0.16), but between-cluster genetic structure was much more substantial (FCT = 0.45). Geographic or environmental barriers to gene flow, rather than isolation by distance alone, are likely to be important determinants of population structure in A. transcaspicus, and population clusters are isolated to such an extent that they may differ in their utility for biological control. The presence of a population localized to Spain and northern Africa corresponds to a similar subdivision observed in Hyalopterus pruni that was the probable source of invasion into California. A focus on parasitoids from this region may prove useful for biological control of this pest of dried plum and pluot in California.