Those volunteer corn plants standing in your soybean fields are more than a weed problem. They could be a recipe for rootworm resistance, Minnesota and Indiana entomologists warn.

Volunteers that contain the Bt gene, and are also glyphosate-tolerant, could hasten the development of Bt-resistant corn rootworms, says University of Minnesota Entomologist Ken Ostlie. Now, he and other corn rootworm experts are taking a closer look at the effects of volunteer corn on these rootworms.

Volunteer corn plants allow larvae hatching from eggs laid in last year's corn to feed and survive within soybean fields, creating “pockets of continuous corn rootworm,” says Bruce Potter, a University of Minnesota integrated pest management specialist.

If left standing until July, volunteer corn plants also serve as sites for northern corn rootworm beetles to lay their eggs, Potter says. These beetles commonly forage in soybean fields, but are thought to lay eggs exclusively on corn. That boosts rootworm populations the following year, intensifying damage potential. And it weakens the value of crop rotation for managing rootworms.

“We've seen this happen historically,” Ostlie says. “Thirty years ago, it was known that if you had volunteer corn in soybeans, you had more problems with western corn rootworm.” In the eastern Corn Belt, western corn rootworm beetles now lay their eggs in soybeans and other crops. “The question is: Is the same development occurring with northern corn rootworms?” asks Ostlie.

But that's not the worst of it.

IN 2007, ABOUT one-fourth of the nation's 93.6 million acres of corn were planted in herbicide-tolerant varieties, according to the USDA. Just over 20% of acres were planted in Bt varieties and another 28% were stacked varieties with Roundup and Bt traits.

But the Bacillus thuringiensis gene in last season's Bt-rootworm varieties may not be fully expressed in the following year's volunteer corn plants, Ostlie says. So larvae that feed on those plant roots could be exposed to less toxic doses of insect-protected corn. That raises natural selection pressure, making it more likely that some larvae will survive to mate, lay eggs and produce offspring that are less susceptible to the Bt gene, Potter says. “It has the potential to force or accelerate development of insect resistance.”

You have the same issue in continuous cornfields, Ostlie adds, in years following harvesting problems that cause dropped ears and kernels. “You could end up with a mosaic of Bt-gene expression in the field.”

In Indiana, Purdue University Entomologist Christian Krupke found that “in areas where triple-stack corn was planted in 2006 and soybeans in 2007, we had a great deal of volunteer corn in some of those fields. Most of that volunteer corn showed up as being Roundup Ready and as having the Bt gene for rootworm,” he says.

“The problem is that the Bt, for whatever reason, isn't expressed at the same level as Bt that you'd get in off-the-shelf corn. In fact, our analysis shows that it is about five-fold less toxic. As a result, you get a lot of rootworm larvae feeding upon and possibly surviving exposure to it.”

Scientists don't yet know how significant the issue of volunteer corn is, Krupke says. “There's no evidence that resistance is happening, but we always worry if we have sub-lethal levels of any insecticide, whether sprayed or in the plant itself. This is the way that resistance has historically been known to develop.”

In 2008, Purdue researchers will be looking at how many soybean fields have volunteer corn, how abundant it is within fields and how many beetles are emerging from those fields. “We're trying to get a handle on the scope of the problem throughout the state,” Krupke says.

At the University of Minnesota, researchers are investigating the effects of volunteer corn in soybeans on both northern and western corn rootworms. Potter and Ostlie are tracking the number of eggs laid in soybean research plots that have been seeded with different amounts of corn to simulate volunteer growth. They are also tracking adult beetle emergence from those plots. Initial observations suggest that “the more corn you have in soybeans, the higher the beetle populations,” Potter says. This summer, they'll compare larvae feeding in the subsequent corn crop.

MEANWHILE, “PAY ATTENTION to volunteer corn in soybeans,” Potter says.

Just over 90% of U.S. soybeans last year were Roundup Ready varieties, and that makes management of glyphosate-tolerant volunteer corn a bit tricky, he says. Growers need to use another postemergence grass herbicide, such as Select or Assure, alone or tankmixed with glyphosate, Potter says. “The extended emergence of volunteer corn makes proper herbicide timing important,” he adds.

“Take it out of the field as soon as possible,” Ostlie urges. Krupke recommends treating by mid-June to be conservative, so there's nothing to feed on. If you wait until July, “some of the larvae may already have pupated or emerged as adults.”