Jim Koenigs, McIntire, IA, tried Bt corn when it was first introduced, in 1996. "We had heavy corn borer pressure and saw enough in our plot to know we wanted to plant half our acreage to Bt corn in 1997," says Koenigs. "The top two hybrids in our 1997 plot were Bt hybrids. We had light corn borer pressure, but we still saw an 8.3 bu./acre yield advantage for N4640Bt compared to the same hybrid without the Bt gene. Having corn borer resistance gives the plants more stress tolerance. Four out of five years, I expect to get a strong yield advantage from the Bt corn," he predicts. This spring, Koenigs will plant 75% of his corn acreage to Bt corn.

Reports of 20 to 25 bu./acre yield advantages for Bt corn in western Minnesota and northwestern Iowa, where European core borer (ECB) pressure was high, are bound to spur Bt corn plantings this spring. In fact, researchers estimate that, in 1998, 15 to 20 million acres of Bt corn will be planted - a three- to fourfold increase over last year. Entomologists worry that, as more farmers become enamored with this new technology and plant a majority of their acreage to Bt corn, ECBs will develop resistance to Bt corn. Many say that it's not a question of "if," it's a question of "when."

"Farmers must accept that corn borers can become resistant to Bt and that they may lose this powerful tool if they don't implement a resistance management plan when they plant Bt corn on their farm," says Kevin Steffey, extension entomologist, University of Illinois.

When the EPA granted conditional registration for the four different Bt corn events approved to date, it stipulated that companies must develop and implement an insect resistance management plan by the year 2001. Companies must also monitor ECB populations to determine if resistance is developing.

Legitimate concerns. Insects are known to be highly adaptable. More than 500 species of insects and mites have developed resistance to insecticides and miticides. While no insects have developed resistance to transgenic Bt plants, lab studies in Minnesota and Kansas have produced colonies of ECB that are moderately resistant to Bt insecticides.

"We don't know if field resistance will develop, but the lab studies show there is a genetic potential for ECB to develop resistance to Bt corn," says Bill Hutchison, extension entomologist, University of Minnesota. "To preserve Bt as long as possible farmers need to limit their Bt corn acres. If all farmers planted 95% of their corn acreage to Bt corn, there's a good chance we could have some form of resistance in five years."

High-dose/refuge strategy. Leading university and industry experts believe that planting a refuge of non-Btcorn on every farm where Bt corn is planted will help delay the onset of resistance. The goal is to have a high population of susceptible ECB moths live and reproduce with any rare resistant ones that survive the Bt corn.

Bt corn produces very high levels of Bt Cry proteins (the toxins that kill ECB). The intent is to kill all European corn borer larvae with no genes for resistance, plus those with one copy of a resistant gene. This high-dose/refuge strategy should result in a higher percentage of susceptible moths in the population and prolong the useful life of Bt corn.

20 to 40% refuge. Research and Extension entomologists of the North Central Regional Research Project (NC-205) released their resistance management recommendation last month in a publication titled Bt corn and European Corn Borer: Long-Term Success Through Resistance Management.

The NC-205 recommends a 20 to 30% refuge of non-Bt corn in areas of continuous corn and corn-soybean rotations. In areas where ECBs are typically sprayed with insecticides, the refuge should be increased to 40%.

The publication notes that in any given year, approximately 20 to 30% of the ECB larvae should not be exposed to Bt Cry proteins from transgenic plants or from insecticides. To be effective, ECB moths must emerge from the refuge at the same time that resistant moths emerge from the Bt field. And, they must be close enough to mate with resistant moths. Many moths fly less than a mile from their emergence site. Consequently, each farm should have one or more refuge within the same half-section (320 acres) as the Bt corn. The refuge should be planted at the same time as the Bt corn to a similar maturity hybrid, and it should be managed the same way as is the Bt corn.

While industry and university researchers agree on the high-dose/refuge strategy, they disagree on refuge size. Authors of the NC-205 report admit they don't know exactly how large the refuge should be, and that's one reason for the wide range (20 to 40%) they recommend.

"Research on ECB resistance management is ongoing. Our recommendations may be conservative but are based on our most current knowledge of ECB biology," says Hutchison. "We'll update our recommendations as we learn more."

Novartis Seeds and Mycogen Seeds recommend that growers follow the NC-205 insect resistance management recommendations when they plant Bt hybrids. "We support the NC-205 recommendations because they represent the consensus of the best scientific research on this subject," says Jack Bernens, director of product management, Novartis Seeds.

The other resistance management plan, promoted by Monsanto and most of the licensees of its YieldGard Bt gene, requires a minimum refuge of 5 to 20%, depending on whether an insecticide is used. "While we can debate the size of the refuge, it's important that farmers establish a non-Bt refuge on their farm when they plant Bt corn," stresses Steffey.

Take responsibility yourself. Some farmers may believe that, because non-Bt corn is being planted in their area, they don't need to plant a refuge on their farm. "Resistance management is each farmer's personal responsibility," says Steffey. "Refuges need to be planted adjacent to Bt corn, to hybrids with similar maturity, and they must be managed similarly. Relying on neighbors to provide this refuge is a recipe for disaster." If growers fail to implement resistance management plans, the EPA may not approve future transgenic crops like the much-anticipated Bt corn that will control corn rootworm, he suggests.

Farmers seem willing. An Iowa State University survey of 750 Iowa farmers who planted Ciba (now Novartis) and Mycogen Bt hybrids in 1996 showed farmers' willingness to follow a resistance management plan. Three out of four farmers said they would follow a recommended resistance management strategy, notes Marlin Rice, extension entomologist, Iowa State University. In response to another question on the survey, 53% of the farmers said they'd be willing to limit Bt acreage to half their total acres if scientists determined it necessary to prevent resistance.

Economic benefits of refuges. Preliminary research by economist Terry Hurley, Center of Agriculture and Rural Development at Iowa State University, shows that planting a refuge of non-Bt corn offers a strong economic benefit to farmers. "Computer model estimates indicate that planting Bt corn exclusively resulted in a $20 to 30/acre increase in annualized profits compared to planting conventional corn, but the return was even higher if farmers planted a non-Bt refuge," says Hurley. "Including a non-Bt refuge results in an estimated $50 per acre increase in annualized profits compared to planting all conventional corn." Without a refuge, farmers' returns are reduced over time because Bt corn becomes ineffective as resistant corn borers proliferate.

To obtain a copy of the NC-205 report (#BU-7055), contact your local extension service, or call 800/876-8638. Price is $3.50 plus shipping and handling. Updated versions will be available at the Northern Plains Crop Base Internet site at: www.mnipm.umn.edu