West Lafayette, Indiana, USA
August 3, 2009
Volunteer corn can act as a safe
harbor for some pests by expressing lower doses of the
insecticide found in newly planted corn, according to
Purdue University
researchers.
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.
ABSTRACT
Volunteer Corn Presents
New Challenges for Insect Resistance Management
Christian Krupke, Paul Marquardt, William Johnson,
Stephen Weller, and Shawn P. Conley
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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