Volunteer corn in soybean fields is making a comeback due to Roundup Ready soybeans being planted following Roundup Ready corn.

The genetically modified corn and soybean varieties that are resistant to Monsanto Co.'s trademarked and patented herbicide Roundup are popular among farmers. The broad-spectrum herbicide kills all weeds, but not the crop with the inserted genes that make it resistant to Roundup, or the volunteer plants from the previous Roundup Ready crop. Not only does that make for untidy fields, but entomologists warn that leaving volunteer corn that also contains the Bt protein could speed up the emergence of Bt-resistant insects.

Bt corn has been genetically engineered to express a protein found in the soil bacteria Bacillus thuringiensis. John Obemeyer, integrated pest management specialist in the Department of Entomology at Purdue University in Indiana, explains when larvae or soft-sided insects ingest the protein, it crystallizes in the animal's stomach, perforating the stomach lining and killing it. Inserting the protein expression effective against corn borers into corn genetics has been successful. "In 1996, the corn borer Bt became available," Obemeyer says. "It is expressed at such a high dose that we've not seen one corn borer live through it." After a decade, corn borer populations are dropping.

Bt corn effective against corn rootworm was introduced three years ago. "With corn rootworm, it's a different expression and a different dosage," Obemeyer says. "We're getting roughly a 90 percent kill." While that's high enough to satisfy farmers wanting to reduce insect damage, it's also low enough to worry entomologists. "My winter meetings have been about making sure farmers understand this is a different animal, a different Bt and a whole new game," he says. "Normally, this is a very boring subject for farmers." He says he welcomes the farmers' interest. "They're asking far more questions than I've ever had on the subject," Obemeyer says.

The main strategy to slow down the development of insect resistance is to leave refuges of non-Bt corn in every Bt cornfield. The goal is to reduce the chance of an insect that survived eating Bt corn mating with another survivor, which would increase the chances of resistance. Entomologists encourage farmers to plant non-Bt corn in 20 percent of the field, preferably in strips. "Farmers got a little lax with refuges," Obemeyer says, adding that farmers joked they weren't putting in the recommended refuges because their neighbor's field would be the refuge.

While no insect resistance has been detected, Purdue researchers are concerned about the heightened potential for resistance developing from rootworms feeding on volunteer corn. A survey of volunteer corn plants found 90 percent carried the Roundup Ready genes into the next generation. Two-thirds of that group expressed the rootworm Bt protein, and one-third of that was severely fed upon by rootworms. "As it loses hybrid vigor, there's something being lost as far as protein expression [goes]," Obemeyer says. The first-year data needs to be supported by further research. In the meantime, Obemeyer is stressing the importance of controlling volunteer corn early enough in the season to starve out the rootworm larvae before they mature into beetles—by mid-June in the central Corn Belt and later in June in the northern Corn Belt.

Companies with herbicides effective at killing volunteer corn are helping to spread the message, such as Gowan Co. Steve Hotovy, regional sales representative, says this new information increases the need for volunteer corn control. "Volunteer corn is a yield robber," he says. "Especially with current commodity prices, producers are looking for effective control." Gowan's Targa herbicide is a post-emergence grass herbicide that controls both conventional and glyphosate-tolerant volunteer corn in soybeans, as well as perennial grasses, and can be tank-mixed with glyphosate. Monsanto is also taking steps to address resistance potential by engineering corn varieties with multiple proteins to slow down the insect's ability to develop resistance.

Obemeyer raises another cautionary note about the increased potential for insect resistance building in continuous cornfields. "Imagine a field of corn with Bt for controlling rootworm and volunteer corn plants scattered throughout," he says. "The rootworm will feed on this low-expression volunteer corn and can easily move over to the corn expressing the Bt protein. Once [the rootworms] are beyond the just-hatching stage, they can eat right through the protein, and it no longer affects them."