CONCLUSIONS

Environmental effects at the farm level have occurred as a result of the adoption of GE crops and the agricultural practices that accompany their cultivation. The introduction of GE crops has reduced pesticide use or the toxicity of pesticides used on fields where soybean, corn, and cotton are grown. Available evidence indicates that no-till practices and HR crops are complementary, and each has encouraged the other’s adoption. Conservation tillage, especially no-till, reduces soil erosion and can improve soil quality. The pesticide shifts and increase in conservation till-age with GE crops have generally benefited farmers who adopted them so far. Conservation tillage practices can also improve water quality by reducing the volume of runoff from farms into surface water, thereby reducing sedimentation and contamination from farm chemicals. Given that agriculture is the largest cause of impaired quality of surface waters, that may constitute the largest benefit of GE crops, but the infrastructure for tracking and understanding this does not exist.

The effects of Bt crops on nontarget invertebrates, including predators, are favorable or neutral, depending on the degree to which Bt crops replace insecticide treatment and on whether additional insecticide treatments are applied to the Bt crop. Evidence indicates no effect of Bt toxins on the honey bee, a widespread pollinator in agricultural systems. For HR crops, the effects on the abundance of arthropods in the fields correlate with whether weeds are controlled more effectively. Shifts in the weed communities have occurred in response to weed-management tactics used for HR crops, in particular when weeds in glyphosate-resistant crops are treated only with glyphosate. Similarly, glyphosate-resistant weeds have evolved where the glyphosate application is repeated and constitutes the only weed-management tactic used. The evolution of resistance to glyphosate in particular kinds of weeds and shifts in the weed community may increase production costs for farmers, require more tillage for weed control, and lead to at least a partial return to the use of different and often more toxic herbicides. The development and establishment of more diversified control strategies for managing weeds in HR crops is needed.

The first generation of IR crops commercialized in the United States produced a single Bt toxin for the control of insect pests. Since the commercialization of those crops, EPA has mandated the refuge strategy to delay the evolution of resistance in major insect pests that are controlled by Bt corn and cotton. After 14 years of use of Bt crops, two insect pests have evolved resistance to Bt crops in the United States: Cotton bollworm (Helicoverpa zea) evolved resistance to Cry1Ac and Cry2Ab in Bt cotton, and fall armyworm evolved resistance to Cry1F in Bt corn. The evolution in bollworm of resistance to Bt cotton did not have serious agronomic, economic, or environmental consequences. Information for assessing the



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