Pearce: Rootworm injury in first-year corn a concern in U.S. Midwest

Reports in the past month citing corn rootworm injury to certain hybrids in first-year corn in the U.S. Midwest are providing a red flag to growers — including reports of some emerging issues here in Ontario.

Last month, University of Illinois entomology professor Dr. Mike Gray, sounded something of an alarm regarding the discovery of significant injury from western corn rootworm larval feeding in first-year corn fields.

In several fields, pruning of corn roots, as well as some lodging, was discovered. Many of those fields had been planted to certain Bt rootworm hybrids which express the Cry3Bb1 protein.

The reason for the concern is that up until recently, the pruning and lodging seen throughout counties in east-central and central Illinois was more of an issue in second- and even third-year corn fields. Now those same fields are displaying what could be rotation-resistant corn rootworms (and further research and identification have to be performed for that confirmation).

Asked if there similar concerns of the same trend developing in Ontario, provincial entomologist Tracey Baute expressed her doubt that it will become as prevalent as those incidents in the U.S. Midwest. At the same time, there is room for caution.

“We have a very low incidence of first-year rootworm or what we would call the rotation variant here in Ontario,” says Baute, who works out of the Ontario Ministry of Agriculture and Food office at Ridgetown.

“With a three-cropping system, it decreases our chances of seeing it here in Ontario, though there has been an increase in volunteer corn in soybean fields, especially this year, which doesn’t help.”

Volunteers

That’s something many crop advisors and provincial specialists have been warning against for several years, but it’s been more as a question of how to control volunteer corn, not as an indicator of corn rootworm.

“We are not technically rotating out of a corn crop in that situation and you can easily find rootworm on roots of those volunteer plants in fields that were infested with adults last year,” says Baute.

“Growers really need to do their best to control volunteer corn to help reduce the risk of seeing the rotation variant” and resistant rootworm, for that matter, since the volunteer corn, if it was a rootworm trait, has less Bt toxin in it and can help encourage resistance to happen, she says.

In the situations in the U.S., Gray notes that bioassays are needed to determine whether there is resistance to the single Cry3Bb1 protein. If resistance is confirmed, then farmers are better off planting “pyramided” hybrids (those expressing more than one rootworm Cry protein).

In the year ahead for Midwestern growers, it’s suggested they find hybrids with multiple Cry proteins and that if they’re relying on a single-protein hybrid, it’s best to use a seed treatment at planting to help lengthen the odds in the farmer’s favour.

— Ralph Pearce is a field editor for Country Guide at St. Marys, Ont.

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