Field studies to assess effects of transgenic crops on nontarget organisms primarily have focused on nontarget abundance and diversity, whereas few have examined effects on nontarget community function. A 5-yr field study in Arizona assessed whether transgenic cotton producing the Cry1Ac toxin of Bacillus thuringiensis (Bt) had an effect on the natural enemy community’s impact on three key pests. Predator:prey ratios for Bemisia tabaci (Gennadius) and Lygus hesperus Knight were similar in unsprayed Bt and non-Bt cotton. Insecticide applications in positive control plots inconsistently altered ratios for B. tabaci. Predation indices based on the known feeding activity of selected predators showed that potential predator impact was unaltered by Bt cotton but was consistently depressed with insecticide applications. Sentinel eggs and pupae of Pectinophora gossypiella (Saunders) experienced the same rates of predation in both unsprayed Bt and non-Bt cotton. Cohort-based life tables for B. tabaci showed that rates of sucking predation, parasitism, and dislodgement (chewing predation in part) were unchanged by Bt cotton. Underlying experimental designs were sufficient to consistently detect changes of <20% in predator:prey ratios and predation indices and changes of <10% in rates of P. gossypiella and B. tabaci mortality with >80% power (α = 0.05). These long-term studies showed essentially no effects of Bt cotton on natural enemy function in Arizona cotton and further showed that minor reductions in density of several predator taxa in Bt cotton observed in a companion study may have little ecological meaning relative to natural enemy impact on key pests in the system.
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