Host specificity of Aphelinus species collected from soybean aphid in Asia

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is native to Asia where it is an occasional pest of soybean, Glycine max (L.). Aphis glycines was found during 2000 in North America and since then has spread throughout much of the area where soybean is grown. In Asia, A. glycines seldom reaches damaging levels; however in North America, it has become the most important insect pest of soybean, decreasing yields and incurring large control costs. Field surveys and exclosure experiments in China showed that natural enemies can limit soybean aphid abundance. A project to find, evaluate, and introduce Asian natural enemies into North America was initiated in 2001, with an emphasis on parasitoids. To ensure that introductions of exotic parasitoids would have minimum impact on non-target species, we tested host specificity of all candidates for introduction. Here we report results of experiments on host specificity of 13 populations in seven species from three species complexes in the genus Aphelinus (Hymenoptera: Aphelinidae). In no-choice laboratory experiments, four species had broad host ranges and one species had an intermediate host range. However, two species, Aphelinus glycinis and Aphelinus rhamni, had narrow host ranges, being restricted to some species in the genus Aphis. We also report the results of experiments on the mechanisms of host specificity in the parasitoid species with intermediate to narrow host ranges. Most of this host specificity can be explained by differences in the behavior of females when they encountered different aphid species. Females of these species rarely approached, stung, oviposited or host fed on aphids outside the genus Aphis. Even within the genus Aphis, Aphelinus glycinis and Aphelinus rhamni parasitized primarily Aphis glycines and closely related species. From these results, we conclude that Aphelinus glycinis and Aphelinus rhamni are excellent candidates for introduction into the North America to control Aphis glycines.