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Bt Crops May Be Prone to Insect Attack

Studies find theoretical risk of toxin-resistant bugs

THURSDAY, Aug. 2, 2001 (HealthDayNews) -- Could a common family of genetically modified crops be showing weakness against pests?

It sure looks that way, says new research.

Two studies show how insects that attack plants engineered to express the so-called Bt toxin may become invulnerable to the poison.

The research, reported in tomorrow's issue of Science, involves only laboratory studies so Bt-resistant pests still are only a theoretical concern. The latest findings expose the possibilities for resistance and offer new ways to track the potential problem in the field.

Bt is short for Bacillus thuringiensis, a bacterium that produces a poison that's safe for humans but lethal to insects. It works on bugs by eroding their intestinal tracts. Crops engineered to carry the bacterium have better protection against predators, experts say.

Farming with Bt has sharply reduced pesticide spraying. The Environmental Protection Agency (EPA) says in 1996 alone, the cotton industry's insecticide use fell by about 1 million gallons, and switching to Bt crops globally could shave nearly $3 billion off the $8.1 billion annual pesticide cost.

Major crops that incorporate the borrowed genes have been planted worldwide. They include corn, cotton and potatoes. This year, 37 percent of America's planted cotton and 19 percent of its corn were Bt plants.

To reduce the risk of bug resistance, farmers grow Bt crops in tightly regulated patterns, sowing a patch of non-Bt plants beside the larger acreage of modified plants, which produce massive amounts of toxin.

In the first study, Linda Gahan and her colleagues at Clemson University isolated a gene that shields lab-raised tobacco budworms from Bt toxin.

So far, budworms, which enjoy both tobacco and corn, haven't seemed to develop resistance to Bt corn in the fields. But it's only a matter of time, says Gahan.

Michael Gray, an entomologist at the University of Illinois, says rapid DNA testing easily could find which pests have developed genes resistant to the toxin. That could tell farmers whether they will have a problem, he says.

The research is likely to catch the eye of the EPA, which reviews its conditional registration of Bt corn next month, Gray says.

"EPA is really trying to gather input and scientific counsel to make sure that the resistance management plans in place are solid ones," he says.

The information could influence the agency's willingness to register additional Bt crops later.

In the second study, University of California scientists uncovered a mechanism by which roundworms can become immune to two forms of Bt toxin.

Raffi Aroian and his colleagues found gene mutations lead to the loss of an enzyme that cobbles carbohydrates to protein and fat in the gut. That suggests carbohydrates act as a sort of beacon for Bt toxin, signaling where in gastric cells to attack. If the poison doesn't recognize a signal, it passes harmlessly through the critter.

"By losing a carbohydrate enzyme, you're potentially knocking out the ability of a toxin to bind to multiple receptors," Aroian says.

Although neither of the two Bt toxins are now used in crops, Aroian says the discovery is "threatening."

Bryan Hurley, a spokesman for Monsanto, a leading maker of Bt crops, says no cases of resistance to the modified plants have been reported in this country or abroad.

Still, Hurley says Monsanto is extremely interested in the latest research. Gahan's study, in particular, could help the company keep closer tabs on the effectiveness of its products, he says.

"This won't take us there directly, but it's a step, and it's an additional tool that we'll have," he says.

Monsanto, a subsidiary of the drug giant Pharmacia, has new seeds in the pipeline that could further reduce the odds of pest resistance to Bt, Hurley says. These include crops that contain two deadly toxins instead of one, making it tougher for an insect to outwit the plant.

What To Do

For more on Bt crops, try the National Agricultural Library or the U.S. Department of Agriculture.

For more on genetic engineering in agriculture, visit Purdue University.

SOURCES: Interviews with Linda Gahan, Ph.D., assistant professor, biological sciences, Clemson University, Clemson, S.C.; Raffi Aroian, Ph.D., assistant professor of biology, University of California at San Diego; Michael Gray, Ph.D., professor, agricultural entomology, University of Illinois, Urbana-Champaign, and Bryan Hurley, spokesman, Monsanto, St. Louis, Mo.; Aug. 3, 2001 Science
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