sealPurdue News

April 1998

Overuse of Bt corn could eliminate effectiveness

WEST LAFAYETTE, Ind. -- Planting Bt corn fence row to fence row could potentially eliminate it from farmers' list of options for pest control, two Purdue University entomologists warn. The corn is genetically enhanced to produce a naturally occurring pesticide in the plant itself.

"We were on a treadmill with chemical insecticides, and there is the potential for a similar situation with genetically enhanced pest-resistant crops," says Eldon Ortman, professor of entomology and acting director of the Office of Agricultural Research Programs at Purdue.

"With broad scale use of genetically enhanced crops, the insects will develop resistance. It is important for researchers and practitioners to give attention to the development of insects that are resistant to Bt-engineered plants, or we will lose a very valuable technology and control method."

Although almost no genetically enhanced crops were used as recently as 1996, this year one-third of the nation's corn fields are expected to be planted in Bt-enhanced varieties. Bt-enhanced corn was developed to fight the European corn borer, which as the No. 1 pest of corn causes losses each year exceeding $1 billion in the United States.

Bt corn uses a portion of the genes of the bacterium Bacillus thuringiensis, which produces a crystal-like protein that kills the insect when it mixes with enzymes in the insect's gut. The crystal protein has no effect on people, animals, or even other insects other than the Lepidoptera group of stalk-boring pests. Organic insecticides containing Bt have been used on many crops for more than 30 years.

Scientists have moved the gene for this crystal protein from the Bt bacterium into corn plants, which allows the leaves and stalks of the plants to produce the crystal protein.

Tom Turpin, professor of entomology and an expert in population dynamics of corn insects, says the fact that Bt has been used as an insecticide for decades is a double-edged sword. On the upside, there are virtually no concerns about its safety. "You can take this stuff and throw it on the cabbage plants in your garden and then eat them tomorrow," Turpin says. On the downside: "There is already some resistance to the Bt toxin in insect populations because of the use of the insecticide."

The problem is that in any population of corn borers, there are a few insects that aren't affected by the crystal protein. These resistant corn borers are so rare that they typically aren't any concern to farmers. Under normal circumstances, the resistant corn borers will most likely mate with others that aren't resistant, and will produce offspring that also aren't resistant.

The problem comes from using too much of a good thing. If nearly all of the corn borers are killed by the Bt, only resistant insects will be available for mating. Soon, all of the borers produced in that area are resistant to the Bt-corn, and the enhanced corn loses its effectiveness.

According to Turpin, the new method of delivering insecticides via the crops themselves has increased benefits and potential problems. "We've never been very good at delivering the insecticide to the insect," he says. "We'd spray fields, but only a certain percentage of the insects actually came into contact with the insecticide and died.

"Now we're putting the insecticide in their food. Blind luck is no longer an option -- escape isn't a possibility for the insect. So we're not allowing random chance to keep resistance out of the gene pool. The result will be that resistance will show up much more."

There have been more than 500 examples of insects that have developed resistance to various chemical insecticides, and widespread overuse of genetically enhanced crops could cause the same thing to happen with those control methods. There are no known incidents of corn borers developing widespread resistance to Bt crops, but scientists know that it is possible. Several research studies have been able to create Bt-resistant caterpillars in the laboratory.

"Companies are developing other versions of Bt as well as other plant resistance factors that can be incorporated into a new variety," Ortman says. "However, this comes at a significant research and development cost and puts our insect-control system on a resistance-factor treadmill."

Turpin says that dealing with insect resistance by constantly bringing out new products isn't new. "If you look through the history of technology, the producers of a new technology have never moved to optimize the long-term use of the product but instead to maximize the immediate sales of the product," he says. "The farmers and companies could have managed the use of insecticides so that resistance was never a problem, but no one ever did it. There was always another insecticide in the pipeline ready to go to the market. It's the 'good to the last drop theory' -- you use it until it's gone and then you bring out something else."

Ortman participated in production of a report by the National Association of State Land-Grant Universities and Colleges' North Central Regional Technical Committee on "Bt Corn and European Corn Borer." The report was prepared by a team of 35 entomologists from 20 land-grant universities, eight seed companies, the Environmental Protection Agency and the U.S. Department of Agriculture.

The report urges producers to create "refuges" of 20 percent to 30 percent of non-Bt-enhanced corn in each farm. In areas where the corn borer is such a large problem that it has to be controlled with insecticides, the report suggests refuges of 40 percent of total acreage. The report notes that because the corn borer seldom travels more than one mile, it is important for these refuges to be set up on each farm, and not on widely distributed sites.

On these refuges, the corn borer would have to be left alone, or treated only with chemical insecticides. That would ensure that a population of non-Bt-resistant corn borers would be available to breed with insects in neighboring fields that had developed resistance.

The environmental group Union of Concerned Scientists has released a report called "Now or Never: Serious New Plans to Save a Natural Pest Control," that insists that the government impose structured refuges that comprise 20 percent to 50 percent of total crop acreage, depending on the crop.

A full, updated copy of the report "Bt Corn and European Corn Borer" is available at Printed copies of the report also are available for $3.50 from the University of Minnesota Extension Service's Distribution Center at (612) 624-4900. Ask for NCR Report 602, item number BU-7055-NR1.

Institutions participating in the report were the Agricultural Extension Services and Experiment Stations in Colorado, Delaware, Illinois, Indiana, Iowa, Kansas, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New York, North Carolina, North Dakota, Ohio, Pennsylvania, South Carolina, South Dakota, Wisconsin, and the U.S. Department of Agriculture Research Service in Ames, Iowa.

Also providing input for the report were AgrEvo, Monsanto Co., DowElanco, Dekalb Genetics Corp., Garst Seeds, Mycogen Seeds, Novartis Seeds, Pioneer Hi-Bred International, Inc., the U.S. Environmental Protection Agency and the USDA Cooperative States Research, Education and Extension Service.

Sources: Eldon Ortman, (765) 494-8363; e-mail,;
Tom Turpin, (765) 494-4568; e-mail,
Writer: Steve Tally, (765) 494-9809;;
Purdue News Service: (765) 494-2096; e-mail,

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