EFFECTS OF AN INVADING COCCINELLID ON NATIVE COCCINELLIDS IN AN AGRICULTURAL LANDSCAPE

Seven native coccinellid species inhabited alfalfa, corn, and small gr ain fields in eastern South Dakota prior to invasion and establishment of Coccinella septempunctata L. Six species occurred in all crops, ho wever, Adalia bipunctata (L.) occurred only in corn. The structure of native coccinellid communities differed significantly for years prior to compared with years after establishment of C. septempunctata in fie lds of the three agricultural crops. Differences in community structur e were accounted for mainly by reduced abundance of two species, C. tr ansversoguttata richardsoni Brown and Adalia bipunctata (L.). Annual a bundance of C. transversoguttata richardsoni averaged 20-32 times lowe r during post-invasion years than in years prior to invasion, dependin g on crop; while annual abundance of A. bipunctata averaged 20 times l ower in corn after invasion. Addition of C. septempunctata to the comm unity did not result in a significant increase in total abundance of c occinellids in the crops. Coccinellid abundance in agricultural crops may be limited by the total abundance of prey or by the availability o f other requisites in the landscape as a whole. Therefore, introductio n of a new species, while resulting in reductions in native species po pulations, may not increase total coccinellid abundance, and may there fore have no net effect on biological control of aphid pests.