Western Corn Rootworm Resistance to Bt Confirmed in Four States

03:47PM Mar 28, 2014
corn rootworm beetle
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Entomologists in additional states in the Midwest and Northeast suspect resistance is present in farmers’ fields and are conducting laboratory analysis to make a final determination.

University Extension entomologists report that field-based western corn rootworm (CRW) resistance to the Bt toxin Cry3Bb1 is now found in four states, with confirmation announced recently in Nebraska. Resistance has been confirmed by researchers previously in Illinois, Iowa and Minnesota.

"It’s now a regional problem," notes Lance Meinke, an entomologist at the University of Nebraska-Lincoln. Meinke says the determination in Nebraska was made during the 2012-13 seasons.

Scientists in Colorado, Kansas, Missouri, New York, South Dakota and Wisconsin also are reporting significant damage in fields planted to corn hybrids containing the single Bt toxin. However, researchers in those states have not made a final determination.

Eileen Cullen, University of Wisconsin Extension entomologist, says she has seen levels of "unexpected damage" in the southwestern part of the state during the past several years. She says "progeny from suspect fields are in on-plant corn assays" that the USDA-ARS has underway.

"Results are still a few months away," she adds.

Entomologists define resistance as a "heritable trait passed on to offspring." Determining that requires laboratory analysis of eggs collected from adults in the field. The process is costly and time intensive, often a 12- to 18-month endeavor, and has slowed researchers’ efforts to confirm CRW resistance in their respective states.

While the CRW resistance problem in the Midwest has garnered the most attention, it is not isolated to that region. Entomologists in Pennsylvania and New York are seeing an uptick in problem fields as well. Elson Shields, Extension entomologist at Cornell University reported a suspect field in Cayuga County, N.Y., last September. Shields notes that the "hybrid in question had extensive goose necking, lodging and severely pruned roots characteristic of corn rootworm injury."

Furthermore, while Cry3Bb1 has been suspect in most of the CRW resistance cases, the practice of continuous corn and the repeated use of a single toxin are putting increased stress on all of the Bt toxins currently available.

Aaron Gassmann, Iowa State University researcher, confirmed that he had found CRW resistance to the mCry3A toxin in Iowa last year. He had suspected resistance was occurring. "Bioassay data from 2011 showed that whether the rootworm was from a field with Cry3Bb1 or mCry3A, they were able to survive exposure to either of the two toxins," he says.

Ken Ostlie, University of Minnesota Extension entomologist, is finding similar problems in that state as well. Ostlie notes: "We continue to receive reports of Cry3Bb1 performance problem fields, although with VT Triple and VT Triple Pro being phased out, the issue will be masked by Cry34/35Ab1 in SmartStax. We've also verified cross resistance with mCry3A at several locations. We also have had some troubling reports the last two years of performance problems with SmartStax and even Herculex Xtra, but don't have confirmed resistance to Cry34/35Ab1 yet."

How extensive the CRW problem will be in 2014 is anyone’s guess for now. Most scientists are concerned the resistance issue will continue to worsen if farmers don’t adopt a more comprehensive integrated pest management (IPM) program for their operations. Entomologists offer these recommendations to reduce the CRW resistance risk:

1. Rotate your crops. If you plant continuous corn, make it a point to rotate fields on a schedule every three to four years. Rotation not only eliminates that rootworm population the following season, but it has many agronomic benefits.

2. If you must grow continuous corn, rotate modes of action, just as you would with herbicides. Avoid using the same Bt year after year by planting a hybrid with a different Bt trait or multiple Bt traits for rootworm. Alternatively, plant a conventional hybrid with a soil insecticide. The use of a soil insecticide on top of a Bt hybrid is not recommended by corn entomologists. Research trials show little or no yield benefit from applying soil insecticides to Bt corn. Insecticides may also mask a problem with the Bt hybrid.

The use of insecticides over traits can be a relatively complex issue, Meinke says. "We have shown in our field trials that insecticide on top of a single trait only improves relative root protection if the single trait is starting to fail in the field (i.e. due to resistance evolution)," he explains. "In this case, root protection improves but selection for resistance to the trait continues, so this is not a good long-term solution to the problem."

Meinke adds: "Root protection from single traits that work well does not increase when insecticide is added on top of the trait. Also, we have not seen an increase in root protection when an insecticide is added on top of a pyramid containing Cry34/35Ab1. So in most cases, we do not recommend using a soil insecticide on top of a pyramid that contains Cry34/35Ab1."

3. Last but not least--detect problems early. Scout continuous cornfields for beetles and damage this summer and report problems immediately to your university Extension entomologist.