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Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 8/11/2005 Publication Date: 10/1/2005 Citation: Naranjo, S.E. 2005. Long-term assessment of the effects of transgenic bt cotton on the abundance of non-target arthropod natural enemies. Environmental Entomology 34(5): 1193-1210. Interpretive Summary: Although transgenic crops expressing the insecticidal proteins of Bacillus thuringiensis have many potential benefits for improved pest management they also pose potential risks. A six year field study was conducted in Arizona to assess the long term non-target impacts of Bt transgenic cotton on populations of 22 taxa of arthropod natural enemies. Analyses of individual years showed that 0-2 taxa declined significantly (26-50%) in Bt compared with non-Bt cotton in each year. In contrast, the application of broad-spectrum insecticides for control of caterpillars and other pests in both non-Bt and Bt cotton had much greater negative effects (45-84%) on many more taxa. Community level patterns were examined with a novel multivariate method called Principal Response Curves which showed no effect from using Bt cotton but large and long-lasting effects from the use of insecticides. Multi-year analyses revealed significant reductions in five arthropod predator taxa in Bt cotton averaging 19%. Similar analyses showed an average reduction of nearly 48% in 13 taxa for plots receiving insecticides. A companion study examining the impact of natural enemies on three key pests in the system suggested that the modest reductions in abundance of predator taxa in Bt cotton have little ecological meaning. Plot size, sampling method, and replication issues were examined to provide guidelines for improving non-target field studies in Bt crops. Technical Abstract: A six year field study was conducted in Arizona to assess the long term impact of Bt transgenic cotton on populations of 22 taxa of arthropod natural enemies. Univariate analyses of individual years showed that 0-2 taxa declined significantly (26-50%) in unsprayed Bt compared with non-Bt cotton in each year. In contrast, the application of insecticides for control of caterpillars and other pests in both non-Bt and Bt cotton had much greater negative effects (45-84%) on many more taxa. The experimental design was sufficient to detect a change in density of 50% with >80% power for some taxa in all years. Principal response curves for individual years reinforced the findings of univariate analyses for the entire natural enemy community combined showing no effect from using Bt cotton but large and long-lasting effects from the use of insecticides. The increased power of multi-year analyses allowed the delineation of significant reductions in five common arthropod predator taxa in Bt cotton averaging 19%. Similar analyses showed an average reduction of nearly 48% in 13 taxa for plots receiving insecticides. On average, a three year study with at least four replicates per year would likely be sufficient to discern changes of <20% with 80% power in a multi-year analysis. A companion study examining natural enemy function suggests that the modest reductions in abundance of predator taxa in Bt cotton have little ecological meaning. Guidelines for improving non-target field studies in Bt cotton are discussed. |
