Crop Scientists Want Farmers To Limit Bt Corn To 75 Percent Of Their Acreage

Article originally published in July 1998

University entomologists from across the corn belt have a message that may leave many farmers shaking their heads.

The message has two parts. First, the easy part: University trials show that new corn varieties with the gene for Bt toxin work great to control the European corn borer. Now, the confusing part: Farmers who plant these Bt varieties should plant non-Bt varieties next to them on 25 percent of their acreage.

If Bt corn hybrids are so great, why should farmers voluntarily limit their use?

The experts say that farmers who fail to follow these guidelines risk producing new corn insect populations that are immune to the Bt toxin. If that happens, farmers would lose the most promising new insect management tool in decades.

"Farmers, educators and seed companies all have the same goal here," says University of Wisconsin-Madison entomologist John Wedberg. "We all want to keep this new tool available as long as possible."

Wedberg was one of 28 scientists who recently contributed to a report on Bt corn and the European corn borer. The report explains what has to be done to keep the new Bt hybrids successful in the long term.

The Bt hybrids are so named because they contain a gene from the bacterium Bacillus thuringiensis. Corn plants with the gene produce a protein that is toxic to certain insects that eat it, but harmless to people and livestock.

Wedberg and UW-Madison agronomist Joe Lauer have tested new Bt hybrids at the College of Agricultural and Life Sciences Arlington, Hancock and Lancaster Agricultural Research Stations over several years. The trials with Bt corn have produced very clean plants, according to the two CALS and UW-Madison Extension crop specialists. The Bt hybrids were virtually untouched by European corn borers, even when Wedberg inoculated corn plants with borer egg masses during the summer.

"The technology is truly amazing," Lauer says. "It may well revolutionize how farmers grow corn."

Scientists, however, are certain that unless farmers manage Bt corn wisely, its widespread use will lead to insect populations that are resistant to the Bt toxin.

Widespread and repeated use of an insecticide kills most insect pests initially. However, such use immediately begins to select for the few individual insects, which for unusual genetic reasons, can survive and reproduce despite that compound. When these survivors mate with each other, they pass on the genes for insecticide resistance, producing a population of resistant insects. Insect pests have become resistant to many insecticides and to Bt where it was applied as a spray, Wedberg says.

Slowing down the selection of insects able to resist an insecticide - and keeping pesticides effective - is called managing pest resistance. Wedberg says the best thing farmers can do to keep Bt corn effective against borers is to plant non-Bt hybrids on a substantial fraction of their corn acreage.

"Entomologists from across the region recommend that growers plant non-Bt hybrids on approximately 25 percent of their corn acreage," he says. "We want the non-Bt acres mixed in with the Bt corn, not acres planted far away. Ideally these would be blocks or strips in the Bt field."

The scientists call these non-Bt corn areas refuges because they protect the corn borers that can be killed by the Bt toxin. Maintaining field populations of borers that are susceptible to Bt slows or stops the development of resistance. When the occasional Bt-resistant borer is ready to mate, it is then much more likely to mate with one of the more numerous Bt-susceptible moths than another relatively rare Bt-resistant one.

"We hope this will keep corn borers from becoming resistant to Bt corn," Wedberg says.

Although the Bt corn seed costs only about $5 per acre more than other hybrids, Wedberg thinks some farmers can be as profitable with current farm management as they can with the Bt corn.

"Over the past 20 years, the borer has been a serious pest on only about 10 percent of the corn acreage in Wisconsin,'' Wedberg says. "Most of us only remember the abnormally high populations during 1995 and 1996. In parts of the state corn fields historically average less than one borer per corn plant going into the winter. In those areas, borers are not likely to cause more than a five percent reduction in yield. Farmers there may want to think twice about changing their management."