cotton pest complex have caused concern for both short-term management and resistance management. From the perspective of short-term management, farmers have difficulty in distinguishing the tobacco budworm from the cotton bollworm during the egg and young caterpillar stages. In many areas, both species occur and farmers have trouble in determining when they can rely on Bt cotton for control and when they need to consider additional control tactics. That can lead to overuse of insecticides by farmers who are trying to decrease the probability of yield loss.

From the perspective of resistance management, the currently available cultivars of Bt cotton can fit well into the high-dose refuge approach to the tobacco budworm (section 2.9), but it falls short of producing adequate toxicity in the cotton bollworm. To slow the evolution of adaptation to Bt toxins by corn earworms, very large refuges of non-Bt cotton are needed (Gould and Tabashnik 1998).

The toxin titer in Bt cotton appears to be at least close to a high dose for the pink bollworm in early-season Bt cotton, but it is unlikely to achieve a high dose late in the season. Because Bt cotton has been so widely adopted by Arizona growers, refuges are on the verge of being too small, even if attainment of a high dose is assumed. Recent research indicates that pink bollworm has the genetic capacity to develop resistance to Bt cotton (Liu et al. 1999), so the overuse of Bt cotton in Arizona could lead to a rapid loss of the technology. Unlike the cotton bollworm, which is a pest on a wide variety of crops, the pink bollworm is specialized on cotton and its close taxonomic relatives. If pest resistance to Bt evolves in the pink bollworm, the problem will mostly be restricted to the cotton growers and is unlikely to have any impacts on organic or other farmers who rely on sprays of Bt bacterial formulations for the production of crops other than cotton.

3.1.3 Mammalian Toxicity Testing for Bt Crops

Only general summaries of the toxicologic assessments of Bt crops were available from EPA pesticide fact sheets. However, the Monsanto Company made available to the committee the toxicology data used in the registration process for its corn, cotton, and potatoes (see appendix B). The array of tests and the protocols used for the tests of Bt toxins from each of the three crops were generally similar. The committee therefore reviewed the three crops together. Three types of studies were conducted on each crop. The goals of the studies were

  • To establish equivalency of Bt proteins produced by the three crops and Bt proteins produced by appropriate B. thuringiensis strains or



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement