Stu Ellis, FarmGate blog  |   June 18, 2012 resize text

Many Corn Belt farmers planted corn a month earlier than usual this year, and the corn rootworms were on the same schedule. The adults are emerging a good month earlier than usual; and there is a lot of anxiety among the farming and entomological communities about what—if any—Bt hybrids are able to resist the onslaught. Paired with the field analysis comes recommendations from rootworm experts that some hybrids should have a—are you sitting down—50 percent refuge.

Current Field Research
Corn rootworm experts are picking up where they left off last year, with the discovery that some of them have developed resistance to the Bt toxins that once were fatal to them. Specialist Mike Gray at the University of Illinois said his colleagues were working with counterparts at Iowa State who identified the resistance last year in Bt hybrids that carried the Cry3Bb1 protein. In his weekly newsletter, Gray said, “Thus far, we have not confirmed resistance to this protein in Illinois. We are cooperating with Dr. Gassmann’s laboratory to determine if the Illinois fields in 2011 were infested with a resistant population. Results should be forthcoming in August of this year.”

Gray said his Illinois research this year found rootworm adults scavenging for food, since silks, tassels, and pollen are not yet available. They have been removing the top cell layer of corn leaves, causing the corn leaf to take on the appearance of rough paper. In fields where adults are being found, Gray says he is also finding substantial root pruning, which is evidence of heavy populations of rootworm larvae. 

While the corn tested positive for the Cry3Bb1 protein, Gray says he is not issuing an indictment against the hybrid, “All plants tested positive for the Cry3Bb1 protein. This does not mean that a resistant western corn rootworm population has been confirmed in Illinois. The registrant of this technology has been notified and will conduct some follow-up investigations in these fields. So, at this point, precise reasons for the continuing performance challenges of some Bt hybrids expressing this protein remain elusive. However, producers should remain vigilant and report any performance issues that surface with their Bt hybrids regarding corn rootworm injury this growing season.” 

Although Gray wanted more research, he said the suspect fields have been in continuous corn for at least 10 years, and using the same hybrid with the Cry3Bb1 genetics for the past six seasons.

More Refuge Acres
Gray points to the importance of “integrating management practices such as rotating corn with other crops, rotating Bt traits from season to season, considering the use of a non-Bt hybrid along with a soil insecticide at planting, and not neglecting the use of a refuge if a Bt hybrid is planted.” On the issue of a refuge, Gray pushes some recommendations of other corn rootworm experts to the forefront. 

In another publication, he quotes the authorities as saying the current refuge requirements are insufficient because the corn does not meet a high-dose standard and rootworms have rapidly developed resistance in the laboratory, greenhouse and field. Accordingly, we recommend increasing the minimum refuge for Bt corn targeting corn rootworms to 50% for plants producing one toxin active against these pests and to 20 percent for plants producing two toxins active against these pests. Increasing the minimum refuge percentage can help to delay pest resistance, encourage integrated pest management, and promote more sustainable crop production.”

Will the EPA step into the picture and either remove the hybrids with the Cry3Bbi protein from the market or push your acreage requirements up to 50 percent? Gray says the EPA response is “murky.” He said a scientific advisory panel had initially suggested a 50 percent refuge, but the EPA instead initiated a 20 percent requirement. Gray says a new technology to fight corn rootworms is at least as far away as the year 2020.

Summary:
Although corn fields are being found with the Cry3Bbi protein that seemed to have lost its toxicity to rootworms in Iowa last year, entomologists are not ready to proclaim its worthlessness in Illinois. However, many fields that bear that gene as the only measure of protection have been found with pruned corn roots and many adults on growing corn stalks. Along with the research on potential loss of resistance come suggestions that refuges be enlarged to 50 percent, of a producer is planting corn that has only a single protein designed to fight corn rootworm.

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