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Corn+Soybean Digest

Wrangling Over Refuges

Will non-Bt refuges delay corn borer resistance? Some scientists are charging that the present refuge strategy for protecting the effectiveness of Bt corn could be full of holes. They say the policy needs more testing.

Others are confident that it will delay the development of resistant European corn borers.

The strategy requires farmers who grow Bt corn to plant refuge areas of at least 20% non-Bt corn. That's to ensure a safe haven for Bt-susceptible corn borers. Those borers will mate with any resistant moths and produce offspring that are susceptible to Bt corn.

Or will they?

"To a surprising degree, the current refuge strategy is based on assumptions," says Ann Clark, crop physiologist at the University of Guelph, Ontario. "Most of the models are based heavily on three key assumptions and none of the assumptions has been verified."

- The first assumption is that major resistance genes to Bt are rare. But Clark cites research showing that at least eight insect species have already developed resistance.

- The second assumption is that resistant genes are nearly recessive. But, according to Clark, a 1999 study showed that resistance in European corn borers is partially dominant.

- The third assumption: Susceptible moths will mate with resistant moths and the offspring will be susceptible. Clark says even if that were true, the timing would be off.

"Corn borers that consume Bt - and live - typically suffer delayed development," she notes. "They reach reproductive maturity out of phase with neighboring borers from the refuge and aren't able to mate anyway. This has been shown in Bt cotton and Bt potatoes."

University of Minnesota entomologists Bill Hutchison and David Andow believe there is some validity in Clark's points. But they say the assumptions behind the refuge strategy aren't totally unproved.

"Our research to date indicates resistance is probably rare," says Hutchison.

"Although we don't have absolute evidence that resistance is rare, it is on the verge of being proved," Andow adds.

In regard to the assumption that resistance genes are nearly recessive, Hutchison cites a Kansas State University study. Kansas State researchers have the only corn borer colony with a high level of resistance.

"They have found that resistance in that population could be partially dominant," notes Hutchison. "This needs further study."

Andow says evidence indicates that resistance is recessive, but that evidence is not strong. "Other resistances to Bt, such as with diamondback moths and cabbage, have been recessive, although not fully recessive," he explains.

"Theoretically, it is correct to say resistance could be dominant. However, my current thinking is to say it is recessive, while closely monitoring for dominance."

As for the charge that susceptible and resistant corn borers would be out of sync for mating, Hutchison says it's a concern. But there has been no definitive research on this subject.

Andow acknowledges that resistant borers likely would be slower to mature. However, he points out that the flight period for borer moths can be extended. That would improve the chances for mating.

In summary, says Hutchison, "Some aspects of the current refuge strategy are still unproved. That's the reason we need further studies on genetic inheritance and moth dispersal and mating."

Researchers at Cornell University have found that a refuge planted to non-Bt plants can be effective in delaying insect resistance to a Bt crop. They used diamondback moths and Bt broccoli as a model system to test the theories behind the resistance management strategy that uses refuges.

The scientists found that a refuge helps maintain susceptible insects within the field, and that a separate block (of non-Bt plants) is more effective than a mixed refuge (a scattering of non-Bt plants throughout the field) in maintaining a needed population of susceptible insects.

Additional greenhouse and lab data have shown that resistant diamondback moths display similar levels of weight gain, growth and survival on Bt plants as on non-Bt plants.

While the Cornell study was the first field test to show the usefulness of the refuge strategy, the researchers point out the need to develop the second generation of Bt and other insecticidal plants that have the potential to delay resistance much more effectively.

Getting smarter about all facets of corn and soybean production just got easier.

It's possible with a new crop-management "bible" called Modern Corn & Soybean Production, which is hot off the presses.

If the title sounds familiar to some farmers, that's because the forerunners of this book were Modern Corn Production, by S.R. Aldrich and E.R. Leng from the University of Illinois (U of I), published in the mid-1960s, and Modern Soybean Production, written by Aldrich and W.O. Scott some time later.

Both books are still found in many farmers' libraries. Now they've been combined into a brand-new, 350-page hardcover book.

Bob Hoeft, Emerson Nafziger and the late Richard Johnson undertook the task of producing the book, and they, along with Aldrich, are the principal authors. The scientists enlisted the help of other leading authorities to write those parts for which additional expertise was needed. All of the authors are current or former U of I scientists.

"The trouble-shooting chapter alone," says Hoeft, "contains more than 130 color photographs of common diseases, insects, nutrient deficiency and toxicity symptoms, herbicide injury symptoms and other problems that occur in corn and soybean fields."

Adds Nafziger, "The book emphasizes the economics of production, and it provides many examples that will assist producers in reducing cost without sacrificing yield."

The book is available for $79.95 plus applicable tax and $7 shipping and handling. To order, call 866-803-6277. Web address:

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