How do farmers decide whether to plant corn hybrids containing a Bt gene for resistance to European corn borers?

"Yield is the bottom line," answers Purdue University entomologist Larry Bledsoe. "Farmers should pick the hybrids that yield the best on their farms.

"The Bt gene simply preserves the yield potential of a hybrid," Bledsoe adds. "A farmer has to look at the total genetic package in choosing which hybrids to grow."

"You have to know the situation on your farm," echoes Kevin Steffey, University of Illinois extension entomologist. "If you have yield-depressing levels of corn borers five or more years in 10, you'll be interested in Bt corn. Growers in northwestern Illinois have a lot more corn borer pressure than growers in southern Illinois."

Growers will need to find out if it will cost more or less to control corn borers with scouting and spraying in problem years - or to grow borer-resistant varieties every year. The cost of spraying is perhaps $15 per acre in problem years; the extra cost of Bt hybrids is roughly $10-15 per acre every year.

Since Bt corn is relatively new, growers have to start with the facts. They are:

* The Bt gene works, but the Bt expression is not the same in all hybrids. Some have season-long borer protection; in others, protection decreases after pollination.

"You may see corn borers in these hybrids at the end of the season," says Steffey.

Yield, however, may not be affected.

* There appears to be no yield drag.

Hybrids with the Bt gene yield as well as conventional hybrids with the same genetics but without the Bt gene. Entomologists say seed companies have done well at introducing the Bt gene into hybrids while keeping yield and other key characteristics intact.

* Corn borer outbreaks are not predictable.

Steffey says growers have to look at odds, and if the odds favor Bt corn, plant it every year.

"This is not a year-by-year decision," he comments. "Growers have to look at their records and university information to determine how many years they can expect problem levels of corn borer."

* Some cornfields are more vulnerable than others.

If you're the first grower in your area to get corn planted, that field will be the tallest and lushest when the first-brood corn borer moths fly. Late-planted fields, on the other end, may be pollinating at the second-brood flight - and also be attractive for egg laying.

There are other, unknown factors. Some growers have fields with more damage than others, and they don't know why. Nobody does.

In the long run, Bt corn hybrids will reveal their power in field trials on farms and in university test plots. Agronomists point out that choosing hybrids is a trial-and-error process that can take several years.