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Tests show biotech corn rules need revision


A corn earworm (Helicoverpa zea) caterpillar damages corn by devouring kernels and spreading green mold (Aspergillus flavus).
Photo credit: Texas A&M University

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, 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."

Bruce Tabashnik | University of Arizona
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