Few corn hybrids are fully protected from the Western corn rootworm anymore--not even pyramided Bt corn hybrids.
In October, Corteva Agriscience informed the EPA the company had 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.
Now, two years later, with no major changes to Bt management by farmers or industry, the problem
MANAGEMENT MAZE. Pyramided Bt corn hybrids offer multiple belowground Bt traits targeting the rootworm. Cry34/35Ab1 is a key component of those pyramids. But, rootworm populations have already compromised the other Bt traits in those pyramids, namely Cry3Bb1 and mCry3A. So, Cry34/35Ab1 has been the only effective Bt trait remaining in many corn fields in intensive corn-producing states such as Iowa.
“There is so much pressure being put on that trait in Iowa,” says 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, Sivesind adds. “Evolution is going to proceed. 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
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 acknowledges. 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 says.
“It’s hard to make people do any long-term management that comes with increased costs up front,” Sivesind explains. “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.”
Traits Topple. Cry34/35Ab1 (Herculex) showed the first official signs of weakening a few years ago, and Gassmann documented partial resistance to it in some Iowa fields in 2016.
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.
Because the stacks contain multiple belowground Bt traits, the refuge component is often only 5% compared to the 20% required for single Bt hybrids used in the past. That puts additional selection pressure on Cry34/35Ab1.
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.
Change is hard. Most growers know the practices that can help slow resistant rootworm populations from taking over, Sivesind says.
They can rotate to a nonhost crop, usually soybeans, switch between different rootworm Bt traits or 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 says.
In some regions, commodity prices and market demand often mean corn pencils out best.
“We can, say, grow oats, but if there’s no good place for them to sell oats, that’s no help,” Sivesind notes.
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.
Table Talk. Michigan State University’s Handy Bt Trait Table is designed to help growers figure out which traits their hybrids have. Visit www.texasinsects.org/bt-corn-trait-table.html.
Finding suitable non-Bt corn hybrids can be difficult, and many planters no longer have soil insecticide boxes. 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 says.
IPRMP is hoping to knock down some of these barriers in the state’s farming communities. The group, which is financed by state commodity groups and industry, quizzed Iowa corn growers on their Bt use and decision-making this year via statewide surveys. It is also tackling weed resistance and the growing phenomenon of soybean aphids resistant to pyrethroids.
Sivesind says he hopes the latest news on resistance to Cry34/35Ab1 can serve as a “wake-up 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 10 years buffer to develop new traits,” he says. “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 adds. “And, that’s what we need to dfix 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.”
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