Consumer Policy Institute/Consumers Union, Environmental Defense Fund,
Union of Concerned Scientists, Benbrook Consulting Services
June 23, 1999
Ms. Susan Wayland
Dear Ms. Wayland:
We, the undersigned, are writing to urge the Environmental Protection Agency to: 1) withdraw all registrations for transgenic Bt corn plants that produce only moderate doses of Bt toxin to Ostrinia nubilalis, the European corn borer, and 2) initiate an intensive examination of the resistance threat posed by Bt corn and cotton that produce only moderate doses to Helicoverpa zea-also known as the corn earworm and cotton bollworm. Moderate-dose plants exacerbate the development of insect resistance and will hasten the loss of Bacillus thuringiensis (Bt), a natural and effective pest control used by organic farmers, gardeners, and vegetable growers.
European corn borer resistance management.
A study published recently in Science [F. Huang, L.L. Buschman, R.A. Higgins, W.H. McGaughey, Inheritance of resistance to Bacillus thuringiensis toxin (Dipel ES) in the European corn borer, Science 284:965-7, 5/7/99] underscores the threat to resistance management represented by moderate-dose Bt crops. The study shows that resistance to moderate doses of Bt toxin is a partially dominant trait in a laboratory species of the European corn borer, one of the target pests of Bt corn.
The finding of a resistance gene in a laboratory colony recently collected from the field makes it likely that such genes exist in natural populations of corn borers. The fact that the laboratory colony was started with only a few hundred individuals suggests that the allele is not rare. In addition, the fact that the trait is partially dominant increases the likelihood that pests resistant to moderate doses of Bt will multiply rapidly in the population.
EPA has long been aware of the disadvantages of moderate-dose transgenic crops and has repeatedly publicly espoused the use of high doses as a cornerstone of resistance management. High doses for purposes of the strategy are those high enough to kill almost all the insects in a population, leaving behind only a few resistant individuals whose impacts can be diluted by mating with susceptible insects harbored in nearby refuges. Moderate-dose crops have no place in such a strategy because they tend to cull out the susceptible organisms and leave the population enriched for resistance. (Moderate-dose plants, in fact, would be the foundation of a program intended to encourage resistance development.) They are even more likely to elicit resistance if the resistance traits are dominant or partially dominant as in the case of the research noted above.
Yet, in addition to apparently high-dose transgenic corn varieties, EPA has approved moderate-dose corn varieties. These include all those based on Event 176 and DBt418, which produce moderate doses during the second generation of corn borers. These products are registered to Novartis, Mycogen, and DeKalb (now owned by Monsanto).
If the Agency is serious about resistance management, it should not approve Bt crops that unequivocally fail to meet the requirements of the high-dose strategy. The Agency should rescind the registrations for all Bt-corn varieties based on moderate doses for target pests and should establish a policy barring the approval of any such varieties in the future.
If EPA chooses not to rescind the registrations, it should at a minimum require refuges of non-Bt corn no smaller than those recommended in the November 1998 report from the International Life Sciences Institute (An Evaluation of Insect Resistance Management in Bt Field Corn: A Science-Based Framework for Risk Assessment and Risk Management)-40 percent of the Bt acreage, if untreated with pesticides, or 80 percent, if treated. Sound science demands no less.
Cotton bollworm/corn earworm (H. zea) resistance management.
Helicoverpa zea is a pest of a wide range of crops, including corn, cotton, tomato, pepper, bean, eggplant, alfalfa, sorghum, and soybean. Bt-resistant H. zea would pose an extremely serious problem for many traditional users of Bt spray formulations as well as farmers planting transgenic corn and cotton.
Known as the cotton bollworm on cotton and the corn earworm on corn, H. zea is also subject to moderate Bt doses in these crops. Exposure to less-than high doses of Bt toxin in both corn and cotton exacerbates the selection pressure for resistance. EPA approved Bt cotton knowing that it produces only a moderate dose of Bt toxin against H. zea, one of three target cotton pests. Moreover, some Bt-corn cultivars (i.e., those producing the CryIA toxin in corn kernels) also produce only a moderate dose against H. zea.
Resistance management for Bt corn has to date ignored H. zea, at least in part because entomologists think that the insects migrate north from Mexico in the spring and then die in the northern United States once autumn weather turns cold. As a result, H. zea has relatively little opportunity to evolve resistance to Bt corn, since moths feeding on plantings of Bt corn throughout much of the corn belt do not survive to produce subsequent generations. However, this analysis may not hold true. New, unpublished research by Fred Gould of North Carolina State University suggests that at least some H. zea migrate in August and September from northern corn fields to southern cotton fields, where the moths can overwinter. Thus, Bt corn may exert a much stronger selection pressure for resistance in H. zea than once thought.
Given the preliminary information on H. zea migration and the double exposure to moderate doses, EPA needs to move promptly to consider resistance management for H. zea. We urge EPA to convene an SAP meeting on resistance management for H. zea in fall 1999.
Thank you for your consideration of our recommendations.
Margaret Mellon, Ph.D., J.D.
Rebecca Goldburg, Ph.D.
Jane Rissler, Ph.D.
Michael Hansen, Ph.D.
Charles Benbrook, Ph.D.
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Last Updated on 6/23/99
By Karen Lutz