We used multiple regression modeling to investigate the numerical response by the predatory insects Hippodamia convergens Gué rin-Mé neville, H. parenthesis (Say), and C. septempunctata L. (Coleoptera: Coccinellidae), Chrysoperla plorabunda (Fitch) (Neuroptera: Chrysopidae), and Nabis americoferus Carayon (Hemiptera: Nabidae) to aphids during 5 yr in three geographically separated alfalfa Þelds in eastern South Dakota. Regression models for abundance of adults of all species were signiÞcant.

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... ression models for immature H. convergens, H. parenthesis, and C. septempunctata were signiÞcant, but regression models for immature C. plorabunda and N. americoferus were not signiÞcant. Regression parameters differed among the three Þelds for most predator species, indicating that the numerical response was dependent on geographical location. To obtain insight into why the numerical response by predators differed among Þelds we determined how the abundance of predators in alfalfa Þelds was inßuenced by the landscape surrounding a Þeld and the vegetation in it. Variables describing the complexity of the landscape surrounding alfalfa Þelds and the plant community in the Þelds entered into regression models for predator abundance and explained a greater proportion of the variance in predator abundance than aphid abundance did. We conclude that the structure of the landscape matrix plays an important role in determining the abundance of aphid predators in alfalfa Þelds, as does the plant community in a Þeld. These effects can sometimes overshadow the direct numerical response by predators to aphids.