By Mari N. Jensen
University of Arizona

Biotech corn carrying a gene that confers protection from insects can pollinate corn plants as far as 100 feet (31 meters) away, reports a pair of researchers.

The gene, known as Bt, codes for a toxin that kills corn-munching caterpillars, including European corn borer and corn earworm.

The findings suggest measures are needed to reduce pollen spread from Bt corn to corn fields that should be Bt-free, according to the researchers.

The discovery is important because planting non-Bt corn, which is susceptible to insect attack, near Bt corn delays pest resistance to the Bt toxin. Such fields of non-Bt corn are called refuges.

However, this research indicates a need to revise the current Environmental Protection Agency guidelines for interspersing non-Bt corn with Bt, or transgenic, corn. The gene is from the bacterium called Bt--short for Bacillus thuringiensis.

"It's the first documentation of gene flow from a transgenic crop into a refuge," said Bruce E. Tabashnik (photo), head of the entomology department at the University of Arizona in Tucson and co-author on the research paper. "This will almost certainly cause a revision of some of the regulations," adding, "I think it's a problem that once observed, recognized and accepted can be readily overcome."

Tabashnik, who works on the evolution of resistance in insects, was involved in devising the refuge guidelines. Using such biotech crops can reduce the need for chemical insecticides, he said.

"If Bt crops were grown wall-to-wall, everyone would expect resistance in insects to evolve overnight," he said. "The EPA rules say that if you grow Bt corn, you must plant a refuge of non-Bt corn for at least 20 percent of your crop."

Caterpillars that can survive on Bt corn are rare at first, and only a few resistant adult moths emerge from Bt corn fields. But refuges of non-Bt corn produce oodles of susceptible moths. The idea is that the uncommon resistant moths will mate with the more abundant susceptible moths. Their hybrid progeny would be killed by feeding on Bt corn. Thus, Bt resistance would not increase quickly.

Non-Bt corn refuges must be close to Bt corn so Bt-resistant moths will almost certainly mate with only with Bt-susceptible moths from refuges.

Until now, researchers didn't consider that the Bt and non-Bt corn plants were also close enough to mate, potentially reducing the amount of non-Bt corn in the refuge.

The research article, "Contamination of refuges by Bacillus thuringiensis toxin genes from transgenic maize," will be published the week of May 10 in the online early edition of the Proceedings of the National Academy of Sciences. First author on the paper is Charles F. Chilcutt of Texas A&M University's Texas Agricultural Research & Extension Center in Corpus Christi. Research support was provided by the University of Arizona and Texas A&M University's Texas Agricultural Research & Extension Center.

Chilcutt questioned whether pollen from Bt corn moved into refuges when he noticed that ears of white non-Bt corn had some yellow kernels. Yellow kernels meant the plants had been pollinated by yellow, not white, corn. The plot of white corn had been planted near yellow Bt corn.

So he tested those yellow kernels for the Bt toxin and found it in high levels.

To see how far Bt corn pollen could spread, he planted eight rows of Bt corn next to 36 rows of non-Bt corn. The rows were planted 38 inches apart. At the end of the growing season, he took ears from the non-Bt corn and tested them for Bt toxin.

In the first few rows of corn that was supposed to be Bt-free, the ears had almost half as much Bt as the Bt corn. Although corn in more distant rows had less Bt, there was detectable Bt in the ears of corn planted 32 rows away from the plot of Bt corn.

Chilcutt said, "There's very good chance that if any grower is growing four rows of Bt corn and four rows of non-Bt corn -- 4-4-4-4 -- essentially all the refuge plants could be contaminated."

Current regulations allow such spacing between Bt and non-Bt corn.

He added, "It could increase the speed with which insect populations become resistant to the toxin."

Tabashnik said, "The possibility of toxin production in the refuge plants is something that needs to be incorporated into the science and the regulations."

Because corn is wind-pollinated, refuges could be planted upwind of Bt corn, suggests Tabashnik. Another possibility would be blocking cross-pollination by planting a variety of Bt corn that produces pollen when the non-Bt corn is not receptive.

Tabashnik said, "The problem will take more research to be fully understood, but it's not catastrophic and can be overcome with relatively minor refinements."

Proceedings of the National Academy of Sciences,  USA, 10.1073/pnas.0400546101

Contamination of refuges by Bacillus thuringiensis toxin genes from transgenic maize

Charles F. Chilcutt, Department of Entomology, Texas A&M University, 10345 Agnes Street, Corpus Christi, TX 78406, and Bruce E. Tabashnik, Department of Entomology, University of Arizona, Tucson, AZ 85721
Communicated by William S. Bowers, University of Arizona, Tucson, AZ, January 23, 2004 (received for review December 11, 2003)

Transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) are widely used to control pests, but their benefits will be lost if pests evolve resistance. The mandated high-dose/refuge strategy for delaying pest resistance requires planting refuges of toxin-free crops near Bt crops to promote survival of susceptible pests. We report that pollen-mediated gene flow up to 31 m from Bt maize caused low to moderate Bt toxin levels in kernels of non-Bt maize refuge plants. Immunoassays of non-Bt maize sampled from the field showed that the mean concentration of Bt toxin Cry1Ab in kernels and the percentage of kernels with Cry1Ab decreased with distance from Bt maize. The highest Bt toxin concentration in pooled kernels of non-Bt maize plants was 45% of the mean concentration in kernels from adjacent Bt maize plants. Most previous work on gene flow from transgenic crops has emphasized potential effects of transgene movement on wild relatives of crops, landraces, and organic plantings, whereas implications for pest resistance have been largely ignored. Variable Bt toxin production in seeds of refuge plants undermines the high-dose/refuge strategy and could accelerate pest resistance to Bt crops. Thus, guidelines should be revised to reduce gene flow between Bt crops and refuge plants.

http://www.pnas.org/cgi/content/abstract/0400546101v1

Related article from The Scientist

Resistance found in GM refuges: wind pollination carries Bt genes from GM maize into neighboring plants
by 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.”