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Corn+Soybean Digest

Resistance Fighter

David Sieck never dreamed he'd be planting corn to keep European corn borers alive. Putting the Bt whammy on the tunneling, yield-robbing rascals was something to celebrate on his Glenwood, IA, farm.

Sure, the new technology cost $8-10 more per acre, but it worked. No more difficult first- and second-generation spray decisions. And it yielded. In 1997, a Bt hybrid took him into the national no-till yield contest winner circle, placing second with 227 bu/acre on non-irrigated ground.

But every soldier understands the importance of keeping the powder dry. The increasing popularity of Bt hybrids could lead to resistant European corn borer populations. To keep this powerful new weapon in the pest control arsenal, growers are required to fight against resistance by planting a refuge of susceptible varieties near Bt-protected crops.

While it's still questionable how non-Bt refuges will be enforced, seed corn companies are gathering signed grower agreements with each Bt corn sale.

"Growers need to demonstrate stewardship of this technology," says Kevin Steffey, University of Illinois extension entomologist.

"The EPA has made it clear it will begin rescinding labels by 2001 if growers are not implementing resistance management strategies."

So there's no dodging the duty. There are three basic factors to consider: the size of the refuge, the distance of the refuge from the Bt field and the importance of resistant and susceptible moths emerging at the same time.

Chances are you don't plant all your corn acres to Bt hybrids anyway. There's still plenty of debate about acceptable refuge size, but 20% is the figure most commonly recommended. If corn borer pressure is typically heavy enough to warrant insecticide treatment, then plant 40% of your corn acres to non-Bt numbers. Just make sure to use a non-Bt based insecticide spray or granules.

The corn industry recently attempted to come together on the refuge size issue, with most of the major seed companies agreeing on a 20% sprayed or unsprayed refuge. That's a much bigger percentage than most companies required last year. But EPA has not yet accepted the 20% figure, and may mandate even bigger refuges. Steffey believes 25% refuges are effective, whether sprayed or not.

Sieck avoids the controversy - spreading risk on his 1,400 acres of corn by planting 50% Bt and 50% traditional hybrids.

"The theory is either I'm half right or half wrong," he says.

He selects Bt numbers in longer-season corn because they will stay in the field longer.

"I don't have much bin space or plan to dry much corn in these economic conditions."

Some growers prefer to protect the fields most likely to face the heaviest corn borer attack or those with highest yield potential. That's fine, says Randy Higgins, Kansas State University entomologist, but manage them properly to avoid reducing the value of the refuge.

"Bt is usually the most expensive corn seed being planted, so it often goes on the best ground. Problems arise when growers plant refuges when they get around to it and/or on less productive soils," he explains. "To be effective, European corn borer moths must emerge from the refuge at the same time as resistant moths and be in close enough proximity to mate with resistant moths."

Simplify the timing issue by picking a refuge hybrid that closely resembles the Bt hybrid being grown and treat it as agronomically similar as possible.

"We recommend growers simply go with the hybrid's non-resistant equivalent," says Sue MacIntosh, AgrEvo product safety manager. Some Bt hybrids also carry herbicide-resistant traits and need careful matching when the refuge is located in the same field.

Corn borer moths tend to move down rows, not across. They're no social butterflies, either. While winds can blow them around, moths seldom fly more than a mile from their emergence site.

Placement of the refuge can take many forms, but the general rule is, the closer to the Bt field the better. You'll want to have a refuge placed within each 320-acre block of Bt corn. Don't count on the neighbor's field to produce moths at the right time, either.

"A refuge adjacent to the field, preferably bracketing both sides and within 1,500' of the middle of the Bt field is optimum," says Z.B. Mayo, University of Nebraska entomologist.

Illinois' Steffey suggests making it easy on yourself. Refuges can be blocks in the field or turn rows. Splitting planter boxes and planting strips at least six rows wide through the field is effective. "The problem is how to manage strips if control becomes necessary," observes Steffey.

Mixing seed is never a good idea. Baby borers are the most susceptible to the Bt toxin. After a couple of weeks munching away on non-resistant hybrids, they could crawl over to a resistant plant and feed without dying. Borers three instars or older can survive these sublethal doses and increase the chances of resistant individuals.

If he's racing to replant, Sieck isn't sure what will happen to the checkerboard resistance plan he has mapped out. But for now, he's doing his part in the fight against resistance. He just hopes the borers do theirs.

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