Assessing the contribution of geography and host use to genetic structure in New York populations of the parasitoid wasp, Aphidius ervi

Host-associated differentiation (HAD) been proposed as a general mechanism for differentiation of many groups of parasitic organisms including plant-feeding insects and their natural enemies. In particular, parasitoid wasps that attack herbivorous insects have many life habits similar to other parasitic taxa, suggesting that HAD also may be important in differentiation. We tested for the population genetic signature of HAD in a parasitoid wasp, Aphidius ervi (Haliday), a biocontrol agent that uses many species of aphids throughout Europe, but is mainly limited to the clover and alfalfa host-races of the pea aphid Acyrthosiphon pisum (Harris) in North America. We assessed allelic variation from 6 microsatellite loci across 16 localities along a 200 km transect in New York State to examine genetic structure in relation to pea aphid host race use and geography. Results from AMOVA and pairwise FST values indicate that there is no genetic structure in A. ervi due to HAD, and there was a general lack of genetic structure across the geographic range. These findings suggest that A. ervi localities are connected by high levels of gene flow that likely swamp out selection for specialization on the pea aphid host races that differ in defenses and resource quality as hosts for A. ervi. The spatiotemporal distribution of hosts as well as dispersal characteristics of parasitoids in general need to be integrated into consideration of the potential role of HAD in parasitoid taxa.