New Worries About Moderate-Dose Bt Corn

The Gene Exchange May99

A new study from the University of Kansas confirms the potential of Bt crops expressing only moderate doses of Bt toxin to elicit resistance to Bt. The study shows that a pest targeted by Bt crops may carry partially dominant genes for resistance to moderate doses of Bt toxins. The results strengthen the arguments against the approval of Bt crops that do not meet the criterion of high dose, the cornerstone of Environmental Protection Agency (EPA) resistance-management plans. They also raise concerns about the efficacy of the high-dose strategy.

The Kansas study, done with laboratory-reared European corn borers resistant to moderate doses of Bt toxin, shows a pattern of inheritance consistent with a partially dominant gene. Dominant genes are more likely to increase in the population in subsequent generations than recessive genes and are of much greater concern for resistance management. If extended to the field, these results suggest that corn borer populations resistant to moderate doses of Bt could soon predominate. The findings exacerbate concerns about transgenic crops expressing moderate doses of Bt. Scientists do not favor such crops for resistance management because they tend to cull out susceptible insects but leave substantial populations of insects enriched for resistance traits.

The EPA has adopted a strategy that relies on high-dose crops plus nearby refuges of non-Bt corn. The high doses are levels that kill nearly all of the insects in a population. The non-Bt refuges produce susceptible insects that swamp the very few resistant insects remaining after high-dose exposure. In theory, high-dose plants plus refuges could substantially slow the spread of resistance traits.

Despite its espousal of the high-dose-plus-refuge strategy, the EPA has approved applications submitted by Mycogen, Novartis, and DeKalb (now owned by Monsanto) for transgenic corn varieties expressing moderate doses of toxin.

The discovery of European corn borers possessing dominant traits for resistance to moderate doses also raises the question of whether dominant genes exist for resistance to very high doses of Bt toxins. Such genes would completely undercut the high-dose-plus-refuge strategy. Scientists believe that such traits exist but that they are exceedingly rare and for practical purposes can be ignored. This study intensifies to some extent worry about such traits. Since the mechanisms for resistance to high and low doses are likely to be different, however, the Kansas study results do not necessarily increase the likelihood that such genes will be found.

Source: F. Huang, et al., "Inheritance of resistance to Bacillus thuringiensis toxin (Dipel ES) in the European corn borer," Science 284: 965-7, May 7, 1999.

UNION OF CONCERNED SCIENTISTS
2 Brattle Square
Cambridge, MA 02238
617-547-5552
ucs@ucsusa.org

source: http://www.ucsusa.org/Gene/may99.moderate.html