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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 function of the natural enemy community. Environmental Entomology 34(5) 1211-1223. 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. Most studies have examined non-target abundance and biodiversity, but few have examined potential effects on the function of the non-target community. A five year field study was conducted in Arizona to assess the long term non-target impacts of Bt transgenic cotton on the function of the natural enemy community by examining their effects on populations of three key pests in the system. Predator:prey ratios for whitefly and Lygus bugs were unchanged in Bt compared with non-Bt cottons. The application of insecticides in positive control plots inconsistently altered these ratios for B. tabaci. Predation indices based on the feeding activity of selected arthropod predators from gut-content analyses showed that potential predator impact was unaltered with the use of Bt cotton, but was consistently depressed with the application of insecticides. Sentinel eggs and pupae of pink bollworm experienced the same rates of mortality, primarily from predation, in both Bt and non-Bt cotton. Cohort-based life tables for whitefly showed that rates of predation and parasitism were unchanged between Bt and non-Bt cotton. The modest reductions in populations of several predator taxa in Bt cotton observed in a companion study would appear to have little ecological meaning relative to natural enemy impact on key pests in the system. Technical Abstract: A five year field study was conducted in Arizona to assess potential effects of transgenic cotton expressing the Cry1Ac toxin of Bacillus thuringiensis on the function of natural enemy community on three key pests Weighted and unweighted predator:prey ratios for Bemisia tabaci and Lygus hesperus were unchanged in Bt compared with non-Bt cottons. The application of insecticides in positive control plots inconsistently altered these ratios for B. tabaci. Predation indices based on the feeding activity of selected arthropod predators from gut-content analyses showed that potential predator impact was unaltered with the use of Bt cotton, but was consistently depressed with the application of insecticides. Sentinel eggs and pupae of Pectinophora gossypiella experienced the same rates of mortality, primarily from predation, in both Bt and non-Bt cotton. Cohort-based life tables for B. tabaci demonstrated that rates of sucking predation, parasitism and dislodgement (chewing predation) were unchanged between Bt and non-Bt cotton. Power analyses showed that 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 mortality of P. gossypiella and B. tabaci with >80% power. The modest reductions in populations of several predator taxa in Bt cotton observed in a companion study would appear to have little ecological meaning relative to natural enemy impact on key pests in the system. |
