Functional Non-target Differences between Bt and Conventional Cotton

A five-year field study was conducted in Arizona to assess the long term impact of transgenic cotton expressing the Cry1Ac dð-endotoxin of Bacillus thuringiensis (Bt) on population densities of 22 taxa of foliar-dwelling arthropod natural enemies and on the effect of the natural enemy community on key pests in the system. Multi-year analyses of arthropod abundance revealed small, but statistically significant, reductions in five common arthropod predator taxa in unsprayed Bt compared with unsprayed non-Bt cotton. In contrast, the use of conventional insecticides led to large reductions in 13 predator taxa. Furthermore, functional studies conducted over a three-year period indicated that the small reductions in abundance observed in Bt cotton may have little ecological meaning. Sentinel eggs and pupae of P. gossypiella experienced the same rates of mortality, primarily from predation, in both Bt and non-Bt cotton and cohort-based life tables for B. tabaci demonstrated that rates of sucking predation, parasitism and dislodgement (chewing predation in part) were unchanged between Bt and non-Bt cotton. Results demonstrate that long-term and multi-factor studies are required to examine meaningful non-target effects in the field.