The natural enemy complex on Russian wheat aphid (Hemiptera: Aphididae) in winter wheat in dryland agroecosystems

In the years immediately following its introduction in 1986, the Russian wheat aphid Diuraphis noxia (Kurdjumov) became a significant pest of wheat in the western United States. In recent years, however, there have been few reports of economically damaging populations and lower numbers of Russian wheat aphid collected in suction traps. Here, we present evidence that the existing natural enemy complex may significantly reduce Russian wheat aphid populations even if no single species can be shown to be highly effective. In this study, we used 3 exclusion cage treatments (fully caged, partially caged, and uncaged plots) to explore the effects of the existing natural enemy complex on Russian wheat aphid populations in winter wheat in 3 fields across a north-south gradient in eastern Colorado: Fort Collins (northeastern Colorado), Akron (eastern Colorado), and Lamar (southeastern Colorado). Natural enemies were collected and identified. Fewer aphids and symptomatic wheat tillers were found in the uncaged treatment and partially caged treatments than in the caged treatment, which excluded the possibility of natural enemy attack on aphids, suggesting that natural enemies were responsible for suppressing Russian aphid populations in the open cage treatments. Predators were collected on both early and later growth stages of wheat, whereas parasitoids were found only on the later growth stage. Together, these findings suggest that the natural enemy complex is responsible for reducing Russian wheat aphid abundance among different dryland agroecosystems and climatic conditions.