ronmental Protection Agency (EPA) requires most plants expressing Bt to produce a sufficiently high dose to enable effective resistance management. Is it scientifically reasonable to assume that the gene for drought tolerance, which alters plant physiology, will not also alter expression of the Bt gene? Under the statutory authority of Federal Insecticide, Fungicide and Rodenticide Act, the EPA can and may even be required to evaluate Bt toxin expression in such a drought-tolerant variety. Under the Federal Plant Pest Act and the Federal Plant Quarantine Act, however, APHIS may not be able to make such an evaluation.
At the field or farming systems level, interaction of traits in transgenic plants could involve complex indirect causal pathways. One example is how, in some cases, stacked transgenic traits might interfere with the practice of integrated pest management (IPM) and insecticide resistance management. One tenet of IPM is that pest suppression tools are only used when the level of the pest has increased above an economic threshold. The fact that Bt genes are typically expressed constitutively in itself raises concerns about their impacts on IPM.
A more serious concern was raised during the 2000 field season, when there was a shortage of transgenic cottonseed with only the single trait for herbicide tolerance to glyphosate (Roundup). The demand for glyphosate-tolerant cotton was much higher than the supply, so farmers in a number of southeastern states had to plant either a conventional nonherbicide-resistant cultivar or a transgenic cultivar with stacked Bt and herbicide tolerance genes. In 1999 approximately 22% of the cotton acreage in North Carolina was planted to Bt-expressing cultivars; in 2000, it was 54%. Cotton entomologists (Bradley, 2001, personal communication) have determined that about two-thirds of the increase was caused by farmers who wanted to use glyphosate-tolerant cotton and therefore needed to purchase a cultivar that contained the Bt gene. Similar increases in acreage planted to Bt cotton occurred in some areas of Texas and elsewhere. Such artificial increases in the area of Bt cotton can jeopardize resistance management in all target pests by creating geographic pockets where resistance could evolve rapidly. In addition, one important cotton pest, Helicoverpa zea, the cotton bollworm, is at high risk of evolving resistance, and unnecessarily high use of Bt cotton increases this risk. Stacking Bt and herbicide tolerance genes has actually caused an increase in the risk of resistance evolution.
Within the next three years, industry intends to commercialize corn cultivars with Bt genes that control corn rootworms. It is likely that these rootworm Bt genes will be stacked with the currently commercialized Bt genes that control the European corn borer in some of the new varieties. In 2000, approximately 20% of corn grown in the United States contained Bt genes that control the European corn borer (USDA-NASS 2000). The primary reason for the large proportion of nontransgenic corn is that