Resistance found in GM refuges

Wind pollination carries Bt genes from GM maize into neighboring plants

May 11, 2004
Cathy Holding(

US Environmental Protection Agency (EPA) guidelines on the planting of non-transgenic “refuges”—areas in which a non-transgenic crop is grown to allow survival of susceptible insects—adjacent to genetically modified (GM) crops could actually increase the risk of pests acquiring resistance to the GM crops, according to a report published in the May 10 online edition of PNAS.

The results would also “throw away” the idea of using GM and non-GM mixed seeds in developing countries as an alternate solution for land-hungry refuges not available to small-scale Third World farmers, according to Charles F. Chilcutt and Bruce E. Tabashnik, authors of the report. Such a mixed method was thought to create “mini-refuges” among the GM crop.

Along with six non-transgenic commercial hybrids, the authors studied six transgenic hybrids producing Bacillus thuringiensis (Bt) toxin Cry1-Ab—a benign pesticide that does not affect mammals, birds, or fish, said Chilcutt. “It's a very useful pesticide especially for organic farmers,” Chilcutt, assistant professor of entomology at Texas A&M University Agricultural Research and Extension Center, told The Scientist.

“If growers planted wall-to-wall corn that had Bt [bred in], we would expect that [insect] resistance would evolve rapidly,” said Tabashnik, professor in the Department of Entomology at the University of Arizona. The use of refuges “can slow resistance down considerably,” he said.

However, regulations governing the planting of refuges had never been tested, according to Chilcutt. “No one really went into the idea of what will happen if you plant the non-Bt refuge too close to the Bt crop. And I'm not sure why that is,” he said.

The authors found that low to moderate levels of Bt toxin were detectable in ears of non-transgenic maize growing up to 31 meters away from the GM crop. At least 43% of the levels found in the transgenic crop were found in the closest planted rows, with levels decreasing with distance, which implies that pollen-mediated transgene flow from Bt maize caused contamination of non-Bt maize refuge plants, the authors say.

Pests eating the kernels of the contaminated refuge plants would not be exposed to the very high level of Bt toxin found in the transgenic crop, but to an intermediate level, said Ian Denholm, head of the Division of Plant and Invertebrate Ecology at Rothamsted Research, UK.

Insects heterozygous for a resistance gene would not be expected to be as resistant as a homozygote, said Denholm, who was not involved in the study. “Yet because they may encounter conditions under which they can survive [in the contaminated refuge], the potential risk of resistance developing to an appreciable frequency in the pest population is definitely increased,” he said.

It was notable as well that the authors were detecting Bt toxin in the refuge plants up to 31 meters from the transgenic crop, Denholm said. “But they note that the current [EPA] recommendations for the minimum separation distance between the transgenics and the refuge is only about 4 meters.”

Denholm said that the implementation of refuge strategy in developing countries is “nothing like as rigorous as it is in countries such as in the US and in Australia, for example.”

The marketing of seed mixes, proposed to be an alternative to managing refuges as distinct areas for crop in developing countries, would mean there will always be a proportion of non-transgenic plants within the crop, said Denholm. “This phenomenon which the authors describe in the paper [however] almost excludes seed mixes as a tactic,” he said.

Denholm said that the transgene toolbox is very small at the moment. “It's basically a few Bt toxins, and those are structurally quite similar, so there's a risk that resistance selected by one would extend to the others as well,” he said. “Without a new supply of equally effective toxins, resistance developing to Bt plants would effectively preclude the technology.”

David Deegan, a spokesperson at the EPA's Office of Public Affairs, said that the EPA had not yet had an opportunity to fully evaluate the study. He said, however, that “at all times EPA has the ability to take action on a pesticidal crop or chemical pesticide if new information comes to light that indicates that there are significant health or ecological concerns that were not previously identified.”