Abundance of Cereal Aphids (Homoptera: Aphididae) and Their Predators in Spring Wheat-Alfalfa Intercrops Under Different Crop Management Intensities

Natural infestations of cereal aphids and abundance of their predators were compared from 1990 through 1993 among plots of intercropped spring wheat and alfalfa grown under high, intermediate, or low crop management intensity (CMI). CMI treatments differed in the amount of nitrogen fertilizer applied and herbicide used and in the rigor of tillage operations. Cereal aphids (primarily Rhopalosiphum padi, Sitobion avenae, and Schizaphis graminum) collectively infested a mean of 0 to 5.9 of 15 wheat tillers sam­pled per plot on various dates from 1990 through 1993, but aphid infestation did not vary by CM!. Seven taxa of aphid predators predominated: Nabis spp., Chrysoperla spp., Coleomegilla maculata, Hippodamia convergens, H. tredecimpunctata tibialis, H. parenthesis, and Coccinella septempunctata. Coccinella transversoguttata richardsoni, a species in decline in eastern South Dakota, was not collected. Nabids were generally the most abundant predatory taxon. In 1992, coccinellid adults were more abundant in high than low CMI plots. In 1993, H. tredecimpunctata tibialis adults were significantly more abundant in high CMI plots on the first three sampling dates but became more abundant in the low and intermediate CMI plots by the fifth sampling date. Regressions between the number of aphid-infested tillers and abundance of some predator taxa were significant in 1990, 1991, and 1992. In 1990, most regressions showed that counts of predators (except Chrysoperla spp. adults) were inversely proportional to aphid infestation levels, whereas significant regressions in 1991 and 1992 showed that the abundances of predators were weakly proportional to aphid infestation levels. Adjusted r2 values for all significant regressions ranged from 0.07 to 0.27. Relationships between crop management, cereal aphid infestation, and aphidophagous predators are discussed