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UW Scientists Say Bt Corn Works, But They Urge Caution

University of Wisconsin-Madison agronomist Joe Lauer and entomologist John Wedberg are finding that new corn hybrids that make their own insecticide are protected from European corn borers — corn”s major insect pest. But the scientists stop short of giving the hybrids a blanket endorsement.

“The technology is truly amazing,” says Lauer. “It may well revolutionize how farmers grow corn. But I don”t think farmers should ”bet the farm” on these new genetically altered corn hybrids yet.”

A small amount of CIBA Seeds” new Bt corn was available to farmers this year. Northrup King will market its version of Bt corn next year. Wedberg estimates that the two companies will market enough Bt seed to plant 3 million to 6 million acres of corn in the Midwest next year.

Lauer and Wedberg are testing several of CIBA Seeds” and Northrup King”s new hybrids with the Bt gene. The gene, first identified in the bacterium Bacillus thuringiensis, codes for a crystal protein that is toxic. Scientists have identified about 70 toxins from different bacterial strains, according to Wedberg. In general, Bt toxins kill certain types of insects but are harmless to people and livestock.

Plant scientists in universities and industry have moved Bt genes into economically important plants, including corn, cotton, cranberries and poplar trees.

In 1995 Lauer and Wedberg evaluated Bt corn hybrids at the College of Agricultural and Life Sciences” Arlington, Hancock and Lancaster Agricultural Research Stations. This year they are again testing them at Arlington. Corn borer populations in both years have been very high.

The Bt hybrids from CIBA and Northrup King were virtually untouched by European corn borers in 1995, even though Wedberg inoculated some corn plants with borer egg masses four times during the summer. Lauer says the Bt hybrids stood up well and the grain dried down well.

Inoculating the corn with borers resulted in yields that averaged 14 bushels per acre more for the Bt hybrids than DeKalb DK512 and Golden Harvest H2387 at the three sites. The latter two non-Bt hybrid standards were grown with the Bt hybrids but not sprayed with insecticide for borers in last year”s trial.

Although the 1996 data won”t be available until December, both researchers expect the results to be similar to 1995. Despite these results, both have some reservations about Bt corn.

Lauer”s main concern is that inserting a gene into a crop hybrid often initially depresses its yield. He calls it yield lag. “It may take seed companies several years before their genetically engineered hybrids yield as much as existing hybrids,” he says.

Also, Lauer is frustrated that seed companies have not put their Bt lines up against other hybrids in University of Wisconsin field trials.

“We compare about 200 hybrids in trials across southern Wisconsin. We have no idea how the Bt hybrids would do in this mix, which includes hybrids that have been bred for improved stalk strength so they too can withstand borer attacks,” he says.

“To test the Bt corn, a farmer could plant 5 or 10 acres of the corn and see how it performs on the farm for at least two years before making a big investment in it,” Lauer says.

Even though the Bt corn seed is expected to cost only $5 to $8 per acre more than other hybrids, Wedberg wants to evaluate if farmers can be as profitable with current farm management as they can with the new Bt corn.

“There are parts of Wisconsin where corn fields historically average less than one borer per corn plant going into the winter,” Wedberg says. “In those areas, borers are not likely to cause more than a 5-percent reduction in yield. Farmers there may want to think twice about changing their management.”

Wedberg is very concerned that borer populations may become immune to the crystal toxin in the Bt hybrids. Unanticipated problems this year with Bt cotton in eastern Texas have raised fears that this tool for protecting plants from insects may not work as expected.

Wedberg and other scientists worry that widespread plantings of Bt crops may foster the development of insect populations resistant to Bt. This has already happened with many other insecticides, according to Wedberg.

Widespread use of a single insecticide initially kills most insect pests. However, it immediately begins to select for the few individual insects, which for unusual genetic reasons, can tolerate it. When these survivors breed, they quickly multiply and form a population resistant to that insecticide.

“We know now that we can”t rely on a single strategy to control insect pests,” Wedberg says.

Slowing down the selection of insects able to resist an insecticide, thus keeping pesticides effective, is part of an important new specialty called managing pest resistance.

“Farmers, educators and seed companies all have the same goal here,” Wedberg says. “We all want to keep this new tool available to farmers for as long as possible.”

Wedberg says the way to keep Bt corn effective against borers is to plant non-Bt hybrids on a substantial fraction of Wisconsin”s corn acreage. “When the seed becomes widely available, we will urge farmers to plant non-Bt hybrids on a quarter of their corn acreage,” he says. “We hope this will keep the corn borers from developing resistance to Bt corn.”