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 a nd alfalfa grown under high, intermediate, or low crop management intensity (CMI). CMI treatments differed in the amount of nitrogen fertilizer applie d and herbicide used and in the rigor of tillage operations. Cereal aphids (primarily Rhopalosiphum padi, Sitobion avenae, and Schizaphis graminum) co llectively 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 aid not var y by CMI. Seven taxa of aphid predators predominated: Nabis spp., Chrysoper la spp., Coleomegilla maculata, Hippodamia convergens, H. tredecimpunctata tibialis, H. parenthesis, and Coccinella septempunctata. Coccinella transve rsoguttata richardsoni, a species in decline in eastern South Dakota, was n ot collected. Nabids were generally the most abundant predatory taxon. In 1 992, coccinellid adults were more abundant in high than low CMI plots. In 1 993, H. tredecimpunctata tibialis adults were significantly more abundant i n 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. Regressi ons between the number of aphid-infested tillers and abundance of some pred ator taxa were significant in 1990, 1991, and 1992. In 1990, most regressio ns showed that counts of predators (except Chrysoperla spp. adults) were in versely proportional to aphid infestation levels, whereas significant regre ssions in 1991 and 1992 showed that the abundances of predators were weakly proportional to aphid infestation levels. Adjusted r(2) Values for all sig nificant regressions ranged from 0.07 to 0.27. Relationships between crop m anagement, cereal aphid infestation, and aphidophagous predators are discus sed.