American Scientist

Farmers, industry, scientists and environmentalists—all are fighting about how to control insect pests. That's nothing new.

What is new—and ironic—is that all the groups claim to fight for the same thing: for farmers to continue, or increase, their use of one of the first major products of agricultural biotechnology, Bacillus thuringiensis, or "Bt", for environmentally safe control of insect pests. The bacterium, found naturally in the soil, produces proteins that are toxic to many insects. Farmers and gardeners have used Bt in organic insecticide sprays for 50 years.

Scientists have spliced genes of B. thuringiensis into crop plants, creating transgenic varieties that produce insect stomach poisons fatal to crop-damaging pests such as cotton bollworms but harmless to "good" bugs such as ladybugs, which prey on crop pests.

In 1997, U.S. farmers planted almost 9 million acres of the three transgenic Bt crops now approved for commercial use. About 7 million acres were in Bt corn, 1.7 million in Bt cotton, and 250,000 acres in Bt potatoes. Bt varieties of at least 15 other crops are now being field-tested. By the 1999 growing season, "many tens of millions" of acres of Bt crops may be in the ground, one environmental group predicts.

It's easy to see why. Ted Posey of Rotan, Texas, sprayed his irrigated cotton with pesticides 14 or 15 times to control boll weevils and bollworms in 1996 and 1997. "I also sprayed three times for aphids, because the boll weevil pesticide killed the beneficial insects that normally control them," the farmer says. Posey then planted Bt-protected cotton in 1998—and didn't spray at all. He considers the $43 an acre he spent for Bt seed and technology a bargain. Nearby farmers spent $125 an acre on spraying.

However, from an evolutionary standpoint, Bt protection of crops works too well. Extensive planting of Bt corn varieties, for example, kills almost all corn borers—but a few might survive because of mutations that confer resistance to Bt toxins. Matings of Bt-resistant survivors may produce hardy new strains that devastate crops previously protected by Bt—rendering useless what many call the world's most important natural pesticide. Because insect populations are continuously exposed to the toxins, not periodically as they were with Bt sprayings, the evolutionary dynamics are expected to be more dramatic now that resistance is engineered into the crops.

The Environmental Protection Agency has therefore mandated resistance-management strategies to prevent or delay the evolution of Bt-resistant insects. A key strategy is the planting of "refuges"—plots of varieties that do not produce the Bt toxin in the crop seed—near Bt-protected crops. Pests such as the European corn borer, the United States' most damaging corn pest, will escape Bt annihilation in the refuges—in far higher numbers than the few tough bugs that survive in the protected fields—easing the selection pressure. Sheer numbers mean that most matings will be among Bt-susceptible insects. Most offspring of the few matings with resistant bugs will be Bt-susceptible, and will die if they attack Bt varieties.

All parties agree that refuges are a good thing—but have drawn battle lines over whether the government should make them mandatory, and regulate their size and location. Monsanto, which owns the patent for all Bt cottonseed sold in the United States, has written refuge requirements into its seed-purchase contracts. Farmers who plant Bt cotton must put 4 percent of the land in non-Bt refuges with no chemical protection for the Bt-targeted pests, or plant 25 percent in refuge, with permission to spray there.

Mandatory refuges for all Bt-protected crops were recommended in a 1998 report, Now or Never: Serious New Plans to Save a Natural Pest Control, authored by six entomologists with the Union of Concerned Scientists. Now or Never also called for refuges as large as 50 percent of the total acreage of some Bt-protected crops, if farmers are allowed to spray them.

Stringent regulations, however, might restrict Bt-protected crops "to the point that they will no longer be practical for farmers to use," stated a September 24 press release from the National Corn Growers Association (NCGA) in St. Louis, Missouri. "We believe in the need for a refuge and will strongly encourage our members to plant one," said NCGA President Ryland Utlaut of Grand Pass, Missouri. "But we fear if the refuge requirements are too onerous, growers will not be able to justify using the technology from an economic perspective."

If farmers must spray large areas of corn, "they might as well spray it all to realize the economic efficiency," said Tim Hume, NCGA Board member from Walsh, Colorado. "Bt corn has shown us just how much yield we are losing to corn borers, and we are not willing to accept that yield loss anymore. Raising the refuge requirement moves growers away from Bt and back to sprays, and I don't think that's what anyone envisioned when we first heard about…this new, environmentally compatible technology….We want to protect this technology for years to come, but we also want to be able to use it on our farms."

More than 40 chemical and seed industries and farmers' associations signed a September 30 letter to the EPA protesting the agency's " . . . intentions of imposing additional limitations on the planting of Bt crops" and called EPA's new directions "particularly disturbing" because " . . . there is still no empirical evidence to confirm the development of resistance to Bt crops currently in the field."

That statement is "technically true—but misleading," responds Dr. Bruce Tabashnik, head of entomology at the University of Arizona. About 500 insect species have evolved to resist synthetic pesticides, and several key pests have developed Bt resistance in the laboratory, the Now or Never author says.

"One insect, the diamondback moth—I call it 'the moth that roared'—has evolved resistance to Bt sprays in the field in cabbage, broccoli, and watercress in Hawaii, Pennsylvania and Florida," Tabashnik says. Resistance has also been reported in Central America and Asia. "It's only a matter of time until we have pest resistance to Bt transgenic crops. The best time to slow the evolution of resistance is before it begins."

Fred Gould, professor of insect ecology at North Carolina State University, and a Now or Never author, points out that the tobacco budworm has increased resistance to the toxin in Bt cotton by more than a thousandfold in selection experiments in his laboratories. Without refuges, Bt-resistant insects could evolve in 4 years, the entomologist says.

The location of refuges is another issue. To date, regulations have not required a refuge to be inside—or even beside—Bt-protected fields. But the EPA required that refuges be planted within half a mile of the two newest Bt corns.

"We ask that the refuges be close enough to ensure that any Bt survivors can mate with susceptible insects from the refuges," says Gould.

"The history of the war between farmer and pest," he says, "teaches us not to underestimate the capacity of any pest species to resist attempts to destroy it, whether it be a weed, a pathogen or an insect."—Thomas R. Hargrove