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Corn+Soybean Digest

Refuge Reform

Get your arms around the concept of sheltering the enemy. Corn farmers planting Bt-enhanced seed to control European corn borer are finally getting some consistent guidelines on how much of a haven to plant to prevent insect resistance.

And that refuge may be more acreage than you've dedicated in the past.

Major seed companies and the National Corn Growers Association (NCGA) have signed a unified corn insect management plan for the year 2000. The agreement will require growers to plant a 20% non-Bt refuge and spray only when corn borers reach economic thresholds. Growers will be required to sign statements stipulating they will follow these guidelines before they buy seed.

With one-third of the nation's corn crop planted to Bt hybrids in 1998 and additional acreage expected in 1999, the issue of insect resistance has become critical, says Kevin Steffey, University of Illinois entomologist.

"Do it right or suffer the consequences," he warns. "If the EPA is convinced by 2001 that growers are not implementing resistance management strategies for Bt corn, it will begin rescinding labels. That was the stipulation when these products were introduced in 1996."

It's important to demonstrate a willingness for voluntary compliance, says Scott McFarland, NCGA's director of industry relations. "We've had a lot of mixed messages out there. It's time for a simple, consistent strategy that protects the technology and allows growers maximum use of it."

How the plan will be monitored is still to be ironed out, and EPA has sanctioned the effort.

The industry understanding represents significant concessions by companies that had once suggested 5% refuges. But it fails to meet scientific community recommendations of a 20-30% unsprayed refuge; 40% with insecticide use.

The stewardship agreement struck by Dekalb, Monsanto, Mycogen/Dow AgroSciences, Novartis and Pioneer was not stringent enough for AgrEvo.

"We support the attempt to come together on this topic," says Sue MacIntosh, AgrEvo product safety manager. "But we continue to support the higher refuge levels established by universities.

"All Bt-corn products are not created equal," she adds. "Some are low dose, some are high, and soon, pyramided Bt toxins with different sites of action will be found in a single corn plant. Products in higher risk categories should have different refuge strategies."

In any population of corn borers, there will be a few rare individuals unaffected by the crystal-like protein that kills when it mixes with enzymes in the insect's gut. Under normal circumstances, the resistant borers would mate with non-resistant moths and produce normal offspring.

Problems arise by using too much of a good thing. If nearly all corn borers are killed, only the resistant borers are available for mating.

"Borers with low levels of resistance have been found in the field," says Z.B. Mayo, University of Nebraska entomologist. "It's not a matter of if resistance happens, it's when.

"It's still a giant laboratory out there," says Mayo. "Movement studies show these unmated moths don't fly very far. Depending on the genetics of the resistance mechanisms, it is possible computer models could indicate refuges need to be even larger than we are currently considering."

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