Host specificity of Aphelinus species considered for introduction to control Diuraphis noxia

Diuraphis noxia, the Russian wheat aphid, has become a major pest of wheat and barley since first being detected in the western USA in 1986. However, it is rarely a pest in Eurasia, its area of origin, and research has shown that natural enemies can limit its abundance there. Among the most important of natural enemies of D. noxia in Eurasia are parasitoids in the genus Aphelinus. Here we report results on host specificity of ten populations of seven species from two species complexes in the genus Aphelinus. Host specificity was not related to host plant species or the phylogenetic relatedness of the aphids or the parasitoids. While some species had very broad host ranges and others had intermediate host ranges, Aphelinus hordei had a very narrow host range, being restricted primarily to species in the genus Diuraphis, and especially D. noxia. We also report the results of experiments on the mechanisms of this host specificity. Most of the host specificity of A. hordei can be explained by differences in the behavior of females when they encountered different aphid species. Females of A. hordei rarely approach, sting, oviposit or host feed on aphids outside the genus Diuraphis, and they oviposit most frequently in D. noxia. From these results, we conclude that A. hordei is an excellent candidate for introduction into the USA to control D. noxia.