Rootworms show resistance to Bt corn in Midwestern fields

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Isolated findings of resistant rootworms in Iowa emphasize that planting a refuge is more critical than ever for maintaining the durability of Bt corn, a Purdue Extension entomologist says.

Bt corn does not kill all larva that feed upon it, and very slight feeding damage from corn rootworm is typical, said Christian Krupke. But after researchers at Iowa State University were alerted to high levels of feeding damage in some fields, they began to test Bt corn hybrids that expressed the Cry3B1 toxin. They found that rootworms from those fields were able to survive exposure in the lab.

"This is not a cause for alarm for Indiana producers, and it was something that we suspected would occur eventually," Krupke said. "Producers should keep doing what they are doing for now as the vast majority of Bt continues to perform well for producers. This is more of a warning to be vigilant"

Currently, other Bt toxins appear to be effective against the pest.

He said growers in areas with histories of high rootworm pressure should scout fields in early to mid-July to look for lodged corn and adults, which are gold and black beetles. Farmers seeing anything unusual should dig up the plants and look for root damage. By August or later, roots can regrow and disguise damage.

If farmers see widespread damage, they should contact their seed representatives to determine if their corn was supposed to be protected by the Bt technology.

"Producers should only be more concerned if there is considerably more damage than usual," Krupke said. "Also, it's worth a second look to ensure they aren't looking at refuge plants instead of the hybrid."

Growers who think they have a potential problem should report it to their Purdue Extension county educators. All farmers should follow best management practices when using any hybrid varieties targeting pests.

The most important thing corn growers can do this season and in the future is follow refuge guidelines, Krupke said. Refuges develop a population of susceptible adults and allow mating between those and any potentially resistant beetles that emerge from Bt plants. Compliance with refuge recommendations has declined in recent years.

"The refuge is crucial as the Bt hybrids do not kill all larva; some will inevitably become adults. We don’t want those survivors to mate with one another and pass on the traits that helped them survive," Krupke said. "By limiting the number of mating adults we have, we can hopefully dilute the genetics of these resistant individuals and prevent the population from becoming resistant."

Rotating with soybeans, and diversity in cropping systems in general can also delay resistance. Fields with continuous Bt corn and high rootworm pressure should be closely monitored, as they provide the greatest pressure for resistance development.

For more information and a link to the Iowa State article, read the Aug. 5 issue of Purdue Pest and Crop Newsletter at http://extension.entm.purdue.edu/pestcrop/



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