Bt choices expanding, resistance developing

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

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

The most important thing corn growers can do 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.”

Ron Hammond and Andy Michel, Ohio State University Extension entomologists, have been looking into the resistance developing in the western corn rootworm to one of the Bt proteins and what this means, if anything, for Ohio growers. They also highlighted a few new Bt choices that have come on the market.

“There are two new hybrids from Syngenta, Agrisure 3220 and Agrisure 3122. Agrisure 3220 was released earlier this year. Agrisure 3220 is a stacked hybrid with Cry1Ab, Cry1F, and Vip3A Bt proteins providing resistance to many of the various lepidopteran (above-ground) pests, including the western bean cutworm. This product does not offer control of rootworms,” the entomologists wrote in a recent OSU CORN Newsletter.

“Only a 5% structured refuge is required (this product is not refuge-in-a-bag). The second, newer product is Agrisure 3122, which contains the Cry1Ab, mCry3A, Cry1F, and Cry34/35Ab1 Bt proteins offering control of both the above-ground lepidopteran pests along with corn rootworms. This product will be marketed as Agrisure 3122 Refuge Renew trait stack requiring only a 5% structured refuge. We would mention that Syngenta has submitted a 5% blended refuge-in-a-bag version of Agrisure 3122 for EPA approval.”

Pioneer is offering a second group of options in addition to their Optimum AcreMax 1 and Optimum AcreMax RW, the latter being a refuge-in-a-bag product for rootworm. “They have recently released two new products, Optimum AcreMax, containing Cry1F and Cry1Ab for lepidopteran or above-ground insect control, and Optimum AcreMax Xtra which includes those same two proteins as well as Cry34/35Ab1 for control of corn rootworms. Both Optimum AcreMax and AcreMax Xtra are integrated (refuge-in-a-bag) refuge products. No separate refuge is required,” Hammond and Michel wrote. “A major difference that growers should be aware of is that while Optimum AcreMax has a 5% integrated refuge, Optimum AcreMax Xtra has a 10% integrated refuge because of the addition of the genes for rootworm control. However, because the refuges are in the bag, no differences in planting will be evident. You open the bag and plant.”

The Bt transgenic corn hybrid table on OSU’s agronomic crops insects website ( has been updated. The table, developed and updated by Eileen Cullen, University of Wisconsin, and Chris Difonzo, Michigan State University, can be found under the corn subheading.  Although the table is as updated and accurate, the entomologists still urge growers to discuss the various Bt hybrids with their seed dealers for proper use.

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