August 3, 2009

Volunteer corn helps rootworms beat insecticides

Christian Krupke, an assistant professor of entomology, said western corn rootworm larvae feed on volunteer corn, unwanted plants that grow from seed dropped during the previous year's harvest. Volunteer corn doesn't have a full dose of the insecticide Bt, which can help the rootworms build up resistance.

"Now they're exposed to a sublethal source of Bt that didn't exist before," Krupke said. "That becomes problematic."

In field tests, Krupke and Bill Johnson, a professor of weed science, found that more than half of the volunteer plants expressed some amount of Bt and, of those, some had severe rootworm damage. The concern is that the rootworms could build a tolerance that, if passed to offspring, could allow the pests to eventually survive a full dose of the insecticide in commercial corn hybrids. Their results were published in a recent edition of Agronomy Journal.

Volunteer corn in soybeans is easy to spot as it towers above the other plants. But killing it can be costly since the corn is resistant to the popular herbicide Roundup.

"A grower has to add a new herbicide to control a volunteer crop. They use Roundup to kill a dozen weeds, but this adds a big expense for a grower to control just one," Johnson said. "It's essentially developing a new weed problem."

In continuous corn rotations, the problem is worse because volunteer corn is visibly indistinguishable from the wanted plants. And since the volunteers carry the same genes, there isn't a herbicide that would only kill those unwanted plants.

Johnson suggested scouting before planting to eliminate volunteers as early as possible and making sure combines are set properly to ensure little corn escapes and becomes volunteers in the next growing season.

Krupke and Johnson said this line of research would focus in the future on how to determine which plants are volunteers in continuous corn rotations and how many rootworms survive and build tolerance. The Indiana Soybean Alliance funded the research.

Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu

Sources: Christian Krupke, 765-494-4912, ckrupke@purdue.edu

Bill Johnson, 765-494-4656, wgj@purdue.edu

Ag Communications: (765) 494-8415;
Steve Leer, sleer@purdue.edu
Agriculture News Page

Genetically modified (GM) corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] dominate the North American agricultural landscape and are becoming increasingly important as biofuels. However, as herbicide-tolerance and insecticidal traits are often simultaneously expressed by individual plants, glyphosate [N-(phosphonomethyl) glycine]-resistant (GR) volunteer corn is becoming a widespread problem as a weed in corn-soybean rotational systems. We show that these volunteer corn plants not only have herbicide-tolerance genes but also express insecticidal "Bt" protein. We also report high levels of damage to these plants from larvae of the target pest, the western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte). This suggests that volunteer herbicide-tolerant Bt corn has the potential to present problems both for weed management and insect resistance management, as it may facilitate more rapid evolution of Bt resistance in corn rootworm populations.

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