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Higher yields and reduced pesticide impacts are needed to mitigate the effects of agricultural intensification. A 2-year farm-scale evaluation of 81 commercial fields in Arizona show that use of transgenic Bacillus thuringiensis (Bt) cotton reduced insecticide use, whereas transgenic cotton with Bt protein and herbicide resistance (BtHr) did not affect herbicide use. Transgenic cotton had higher yield than nontransgenic cotton for any given number of insecticide applications. However, nontransgenic, Bt and BtHr cotton had similar yields overall, largely because higher insecticide use with nontransgenic cotton improved control of key pests. Unlike Bt and BtHr cotton, insecticides reduced the diversity of nontarget insects. Several other agronomic and ecological factors also affected biodiversity. Nevertheless, pairwise comparisons of diversity of nontarget insects in cotton fields with diversity in adjacent noncultivated sites revealed similar effects of cultivation of transgenic and nontransgenic cotton on biodiversity. The results indicate that impacts of agricultural intensification can be reduced when replacement of broad-spectrum insecticides by narrow-spectrum Bt crops does not reduce control of pests not affected by Bt crops.agricultural sustainability ͉ environmental impact ͉ transgenic crops T he increasing world population and changes in consumption patterns may necessitate significant agricultural intensification in the next 50 years (1, 2). Unless crop yield is improved and release of fertilizers and pesticides from croplands is reduced, such intensification could augment contamination and perturbation of managed and natural ecosystems, ultimately harming biodiversity and public health (1-4). It was proposed that transgenic Bacillus thuringiensis (Bt) crops could be valuable tools for increasing agricultural productivity while minimizing the environmental impacts of agriculture (1, 2). However, the potential effects of transgenic crops on nontarget arthropods have caused concern, especially in regions where agricultural land is important to sustain biodiversity (5-7).Although Bt crops are grown extensively worldwide (8), no large-scale studies had been performed to simultaneously test whether they have favorable agricultural effects and minimal impacts on nontarget arthropods. Here, we report results of a 2-year farm-scale evaluation of the effects of transgenic cotton on biodiversity, pesticide use, and yield. We studied 81 commercial fields in a region of 6,600 km 2 in Arizona, where Bt cotton represented 48% and 62% of the cotton planted in the first and second year of the study, respectively. Forty fields were planted to nontransgenic (nonTr) cotton, 21 fields to transgenic cotton producing the Bt toxin Cry1Ac (Bt), and 20 fields to cotton with Bt protein and herbicide resistance (BtHr). Bt cotton with Cry1Ac controls the pink bollworm (Pectinophora gossypiella), a major insect pest of cotton (9, 10). Results and DiscussionEffects of Transgenic Cotton on Pesticide Use. Transgenic cotton was treate...