The European corn borer invasion has escalated the past 10-12 years.

But how did borers gain reinforcements? And what does that - in light of a Bt corn defense - mean for growers?

Borers attacked with a vengeance after growers started planting earlier and earlier, says Kevin Steffey, University of Illinois extension entomologist.

"When I came to Illinois 20 years ago, early planting was mid-April. Now that's normal. People in some areas of our state plant in late March," he says.

With the advent of cold-tolerant, long-season hybrids and subsequent earlier plantings, first-generation borer moths have found plenty of well-established corn in late May or early June to lay eggs on.

And that has led to increased borer numbers, according to Illinois data comparing corn borer densities dating from the 1940s to 1996.

Steffey hopes, this winter, to use that data to provide a history of corn borer populations in Illinois.

"People now are trying to figure out what has been the history of corn borer populations in different areas because it will help them make decisions about whether or not they need to plant Bt corn."

In northwestern Illinois, for example, economic infestations occur seven out of 10 years, historically speaking. In the southern part of the state, over a 10-year period, only two or three years show severe outbreaks, he says.

"So in areas with frequent economic infestations, Bt corn as a long-term management approach makes sense," Steffey says.

"In areas with infrequent economic infestations, Bt corn doesn't make a lot of sense, because Bt corn is going into the ground before anybody knows what corn borers are going to be like."

Ken Ostlie, University of Minnesota extension entomologist, thinks growers will have a hard time deciding whether to pay the Bt price premium next year.

"Last year they would have grabbed everything they could get," says Ostlie of western Minnesota growers. "With the reduced commodities prices and the lowered corn borer pressure, you will have farmers debating how much they want to have in terms of borer insurance."

Ostlie has seen a dramatic fluctuation in borer numbers the past few years.

In 1995, parts of Minnesota were hit by an unexpectedly strong second generation of the pest. By the next year, borer populations mushroomed around the state, with a hint of subsiding in its eastern part.

Then Bt corn entered the picture.

"In 1997, sites in the eastern side of the state showed virtually no yield benefits to Bt corn, or very small amounts. But sites on the western side were showing yield benefits that ranged all the way from 10 to 45 bu/acre in our trial."

Populations fluctuate in time, and fields differ in borer pressure, Ostlie says. "So we're trying to instill the idea that growers can't expect to see a significant yield increase every year.

"That's why I think a year like this year is very important. They've seen very strong yield response for two years now. But this year, it may not be there. It'll introduce a little dose of realism into how farmers are wrestling with the Bt decision," he adds.