Predatory fauna in cornfields and response to imidacloprid seed treatment.

The composition and abundance of predatory fauna in corn, Zea mays L., were studied by field visual sampling and pitfall traps over a 5-yr period. In visual samplings, the most abundant groups were Araneae, Heteroptera, Carabidae, Coccinellidae, and Staphylinidae, whereas the prevalent predators caught in pitfall traps belonged to Carabidae, Araneae, Dermaptera, and Opilionidae. The most abundant species or genera in the prevalent groups, except in Arachnida, were identified. Application of the insecticide imidacloprid as a seed dressing is common in the study area to prevent wireworm and cutworm damage. By comparing predator composition and abundance in treated and untreated cornfields during the 5 yr, we assessed the impact of imidacloprid seed treatment on predatory fauna. Among the prevalent predator groups found in visual sampling, Araneae, Coccinellidae, and Staphylinidae were not affected by the imidacloprid treatment, whereas Carabidae was only moderately affected in one of the 5 yr studied. On the contrary, Heteroptera was more drastically reduced by the imidacloprid, but the effect varied with the year. Incidence of European corn borer, Ostrinia nubilalis (Hübner), may be increased as result of such heteropteran reduction. In pitfall traps, only Staphylinidae resulted in lower numbers as consequence of the treatment, whereas the rest of the most abundant predator groups, Carabidae, Araneae, Dermaptera, Opilionidae, Trombididae, and Heteroptera, were not caught in significantly different numbers in treated or untreated plots.