Bt Corn Hybrids : Confirmed Resistance to Western Corn Rootworm – DTN

    Adult western corn rootworm.

    Few corn hybrids are fully protected from the western corn rootworm anymore — not even pyramided Bt corn hybrids.  

    In October, Corteva Agriscience informed EPA that the company has confirmed resistance to Cry34/35Ab1 (Herculex RW) in rootworm populations in the northeastern Iowa county of Delaware.

    Although the company called it the “first case” of such resistance, Iowa State University entomologist Aaron Gassmann documented low levels of rootworm resistance to this same trait in Iowa in 2016. At the time, he called it “an early warning” for industry and farmers to improve their stewardship of the technology — or risk losing it soon.

    Now, two years later, with no major changes to Bt management by farmers or industry, the problem has deepened.

    Cry34/35Ab1 is the underpinning of many popular pyramided Bt corn hybrids, which offer multiple belowground Bt traits targeting the rootworm. Because the other Bt traits in those pyramids, namely Cry3Bb1 and mCry3A, are already compromised by resistant rootworm populations, Cry34/35Ab1 has been the only effective Bt trait remaining in many cornfields in intensive corn-producing states like Iowa for several years now.

    “There is so much pressure being put on that trait in Iowa,” said Evan Sivesind, program manager for the Iowa Pest Resistance Management Program (IPRMP). “It is really what is being leaned on by anyone who grows Bt corn here.”

    This latest resistance report is unlikely to be the last, he added. “Evolution is going to proceed,” he said. “That’s why managing Bt resistance is not about eliminating resistance, it’s about minimizing it as much as possible to preserve current management options as long as possible.”

    For years, industry, government and academic scientists have promoted the same group of resistance prevention strategies to achieve this: crop rotation, trait rotation and rotation to non-Bt hybrids, with use of a soil insecticide.

    But there is little evidence that growers are adopting them, Sivesind acknowledged. It is the goal of IPRMP to figure out why and change that dynamic — through education, but also through addressing the economic and emotional hurdles that come with changing a crop production practice, he said.

    “It’s hard to make people do any long-term management that comes with increased costs upfront,” Sivesind said. “It’s like ignoring a roof leak — it will save you money this year, but it will cost you more three years down the road. We need better strategies for making adoption of these best management practices more feasible for growers, especially in tough economic times.”


    Cry34/35Ab1 showed the first official signs of weakening a few years ago, and Gassmann documented partial resistance to it in some Iowa fields in 2016

    See the DTN story here:…

    The trait is usually offered in pyramids like SmartStax, QROME, Intrasect Xtreme, AcreMax Xtreme and Agrisure 3122, along with another Bt rootworm trait, usually Cry3Bb1 or mCry3A.

    Since resistance to Cry3Bb1 and mCry3A has been documented in the Corn Belt for nearly a decade, many rootworm populations are only controlled by the Cry34/35Ab1 trait. And because the stack contains multiple belowground Bt traits, the refuge component is often only 5%, as compared to 20% required for single Bt hybrids used in the past, which puts additional selection pressure on Cry34/35Ab1. 

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    Syngenta’s pyramided Duracade hybrids, which contain eCry3.1Ab, paired with mCry3A, are running into similar problems.

    Two years ago, scientists from Iowa and Minnesota documented field resistance to eCry3.1Ab, likely because of its similarity to other Cry proteins on the market, rather than overuse. Gassmann and other researchers have found that rootworm populations with resistance to Cry3Bb1 and mCry3A are likely to be already resistant to eCry3.1Ab, even if they have never encountered the trait before.


    Most growers know the practices that can help slow resistant rootworm populations from taking over, Sivesind said.

    They can rotate to a non-host crop, usually soybeans, switch between different rootworm Bt traits, or better yet — switch to a non-Bt hybrid and use a soil insecticide.

    The problem is those options usually appear economically unappealing or downright difficult to adopt to growers, Sivesind said.

    Commodity prices and market demand often means corn pencils out best, year after year, in parts of Iowa, leading to continuous corn production.

    “We can say grow oats, but if there’s no good place for them to sell oats, that’s no help,” Sivesind noted.

    Switching to different traits is also tricky since licensing agreements allow different seed companies to use the same four belowground Bt traits labeled under different brand names and numbers.

    Michigan State University’s Handy Bt Trait table can help growers figure out which traits their hybrids actually have and what they do:…

    Finally, switching to a non-Bt corn hybrid and using soil insecticide can also be difficult. Many growers no longer have the equipment needed to add soil insecticides during planting. And although he has heard anecdotal reports of growers buying and using more soil insecticides, there is no way to be sure they aren’t just using them on top of Bt hybrids — which is not a recommended resistance management practice, Sivesind said.

    Nor are non-Bt corn hybrids of the same genetic caliber always readily available to growers each year. For seed companies, the process of turning out new, un-traited elite hybrids takes time and persistence.

    “These are large companies and this is a multi-year process to decide on new hybrids, contract with growers to grow them, produce them and sell them,” Sivesind said.

    Iowa Pest Resistance Management Program (IPRMP) is hoping to knock down some of these barriers in the state’s farming communities. The group, which is funded by state commodity groups and industry, quizzed Iowa corn growers on their Bt use and decision making this year via statewide surveys. (IPRMP is also tackling weed resistance and the growing phenomenon of soybean aphids resistant to pyrethroids.)  

    Sivesind said he hopes the latest news on resistance to Cry34/35Ab1 can serve as a “wakeup call” to growers that Bt rootworm technology may not last as long as they need it to — even if it seems to be working for the moment.

    “We need to slow this down as much as possible and give industry five to ten years buffer to develop new traits,” he said. “The worst thing will be if [Bt pyramids] burn out in the next couple of years, and there is a long gap with no new options available on the market.”

    “But it’s hard to get people to change something that’s working,” he added. “And that’s what we need to do — fix things before they break. And that has to become the usual decision-making process, to work in diverse practices, rather just pushing on the same one.”

    Emily Unglesbee can be reached at

    Follow her on Twitter @Emily_Unglesbee

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