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GENETICALLY MODIFIED PEST-PROTECTED PLANTS: SCIENCE AND REGULATION
interpreted with caution however, because, as stated in the USDA study, “attributing differences in yields, pesticide use, and profits between adopters and nonadopters observed in the data solely to adoption of genetically engineered crops is nearly impossible because many other factors also affect yield and pesticide use.” However, in summary the committee concludes that
The introduction of Bt cotton appears to be bringing environmentalbenefits through the reduced use of insecticides.
Monsanto has provided EPA with data on potential effects of the Cry1Ac toxin on nontarget species (for example, Sims 1994) and has made the information available to this committee. The range of nonpest species tested is similar to that of species tested for registrant 's corn, and the test protocols were also similar. None of the studies showed biologically significant impacts on nontargets other than lepidopterans. The only ecological study in which a comparison of corn and cotton testing produced contrasting results was the study of toxin degradation in soil (Ream 1994a; Sims and Sanders 1995). In the corn study, the half-life of Cry1Ab Bt toxin from pulverized corn placed in vials of soil was 1.5 days, and 90% of the activity was lost in 15 days. In the similar study of cotton plant material that produced Cry1Ac toxin, the half-life was 41 days, and 90% of the activity was expected to be lost in 136 days (on the basis of extrapolation). These differences may reflect differences in tissue degradation perhaps resulting from the differences in woody cotton tissue compared to com tissue. In comparison with insect-control practices now used in nontransgenic cotton, the possible minor effects of that Bt cotton may have on beneficial insects are expected to be insignificant.
The lepidopteran pests that attack cotton differ by region. In North Carolina, South Carolina, and Virginia, the cotton bollworm, Helicoverpa zea, is a predominant pest. In the Midsouth, the tobacco budworm, Heliothis virescens, is typically the most important caterpillar pest. In Arizona, the pink bollworm, Pectinophora gossypiella, is the only lepidopteran pest of economic importance (Gould and Tabashnik 1998). Each of those pests has a different response to the one Bt toxin, Cry1Ac, that is present in commercial cotton cultivars. The concentration of this toxin in cotton decreases toward the end of the season but still seems to be enough throughout the season to fit the EPA SAP criteria (SAP 1998) of a “high dose” for the tobacco budworm. However, this same toxin concentration range causes only 60-95% mortality in the cotton bollworm and cannot be considered a high dose for this pest. Although high mortality in the pink bollworm has been reported in the field, at least some pink bollworms survive late in the season. Those differences in Bt cotton efficacy against the