Natural enemy impact on the Russian wheat aphid in southeastern Colorado

The effectiveness of predators and parasitoids of the Russian wheat aphid was experimentally evaluated using mechanical exclusion in production winter wheat, Triticum aestivum L., fields at four locations in southeastern Colorado. Three types of enclosure were used: complete exclusion enclosures, partial exclusion enclosures that permitted entry by parasitic Hymenoptera, and environmental exclusion enclosures that reduced the effects of wind and rain on Russian wheat aphids and trapped emigrating alate Russian wheat aphids so that they could not return to plants within the enclosure. Russian wheat aphids in nonenclosed plots were also studied. Russian wheat aphid density varied among treatments in the following order: complete exclusion greater than or equal to partial exclusion > environmental exclusion > nonenclosed plots. The trapping of alatae within enclosures and reduced adverse stresses such as rain and wind within enclosures were partially responsible for the greater Russian wheat aphid density in complete and partial exclusion enclosures compared with environmental exclusion enclosures and nonenclosed plots. The aphidophagous coccinellid, Hippodamia convergens Guerin-Meneville, and the generalist Nabis spp. were the most abundant predators during the increasing phase of Russian wheat aphid population development, but they did not substantially reduce Russian wheat aphid numbers. H. convergens, Coccinella septempuntata L., and H. sinuata Mulsant were the most abundant predators during the declining phase of Russian wheat aphid population growth. The dominant parasitoid was Lysiphlebus testaceipes (Cresson), but parasitism rates were very low. Canonical correspondence analysis showed close associations between the abundance of predators and Russian wheat aphid density, Russian wheat aphid density and wind during the increasing phase of Russian wheat aphid population development, and Russian wheat aphid density and rainfall late in the growing season.