Characteristics of ground beetles (Coleoptera: Carabidae), including their distribution, diet breadth, and importance as generalist predators, make them candidates for evaluating potential unintended effects of transgenic crops. The abundance and composition of carabids collected from pitfall traps placed in hybrid dent corn were used to determine which species are consistently present and abundant in Iowa carabid communities and to test for population differences in these species caused by transgenic (Bacillus thuringiensis [Bt]) or insecticide-based pest management. Power analyses were also used to evaluate the adequacy of the experimental design. Carabid collections indicate Harpalus pensylvanicus DeGeer is the best choice to sample based on its apparent ubiquity and abundance in Iowa and other corn-producing states. However, population levels were time-dependent, and composition of carabid communities differed between locations. Considering the numerical dominance of a few species per field, the two to four most abundant species might be used to effectively represent local carabid communities. H. pensylvanicus populations were impacted by insecticide use, but no effects of Bt were found. Power analyses indicated that with the experimental design and replication employed, only large effects were detectable; based on the variation between plots, increased replication is needed to make detection of either moderate (30–50%) or small (<30%) effects likely. The recent release of transgenic corn with coleopteran toxicity highlights the importance of these results when evaluating potential unintended effects of these crops on ground beetles. One species not previously recorded in Iowa, Lebia pulchella Dejean, also was collected.
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