A Multiyear, Large-Scale Comparison of Arthropod Populations on Commercially Managed <I>Bt</I> and Non-<I>Bt</I> Cotton Fields

Abstract

Field studies were conducted in 2000 Ð2002 to compare foliage-dwelling arthropod populations on Bacillus thuringiensis Berliner (Bt) (Bollgard) cotton and non-Bt (conventional) cotton season-long in South Carolina, Georgia, northern Alabama, and southern Alabama. For each of these four regions, three or four paired Þelds were sampled weekly in each of the 3 yr. Each pair of Þelds consisted of a Bt and a non-Bt cotton Þeld, both at least 5 ha in size. The dominant arthropod taxa collected included target pests (heliothine moths and Spodoptera spp.), nontarget pests (stink bugs and plant bugs), and generalist natural enemies (Geocoris spp., Orius spp., Solenopsis invicta (Buren), ladybeetles, and spiders). Where target pests were present, particularly Helicoverpa zea (Boddie), their numbers were consistently signiÞcantly lower in the Bt cotton Þelds. Natural enemy populations generally were not signiÞcantly different between the Bt and the non-Bt cotton Þelds (50% of all comparisons) and, where signiÞcant differences were present, natural enemy abundance usually was higher in theBt than the non-Bt cotton Þelds. These differences were correlated with lower insecticide use on the Bt than the non-Bt cotton Þelds, particularly in South Carolina, where target pest pressure was heaviest. When presented with insect eggs or larvae as prey items, the larger natural enemy populations inBt cotton Þelds exhibited signiÞcantly higher predation rates. These results show that Bt cotton has no signiÞcant adverse impacts on the nontarget arthropod populations studied and, compared with insecticide-treated non-Bt cotton, Bt cotton supports higher natural enemy popula- tions with signiÞcant positive impacts on biological control.