Prior to Bt technologies farmers lost $1 billion annually to corn rootworm (CRW)—in the form of chemical costs or actual yield loss. That’s $11 per acre based on USDA’s recent 89.1-million-acre count. With resistance developing to traits that once killed the pest, it might just nibble its way back to a billion-dollar price tag.
“Back in the day, CRW control was unbelievable,” says Tim Reinhart, corn and soybean farmer in Champaign, Ill. “We’re seeing more pockets where we should have complete control and don’t—it’s concerning.”
CRW poses a double threat: The adult snips corn silks and, if unchecked, could prevent successful pollination and kernel development. The larvae munch on roots, which leads to risk for disease and plant stress. With resistance on the rise, it’s time to find a solution.
“The first documented case [of CRW resistance] was reported in Iowa in 2011 to Cry3Bb1 [corn rootworm I],” says Dalton Ludwick, post-doctoral researcher at Virginia Tech who works with USDA. “Now we have documented issues in Iowa, Illinois, Minnesota, Nebraska and North Dakota.”
Researchers suspect resistance has spread to other states and just hasn’t been formally documented yet. The first case of resistance was actually discovered in 2009 but took two years of research to confirm.
“Typically, if we see resistance it’s because the same trait has been used year after year, such as in northeast Iowa where it was first detected,” says Marlin Rice, Syngenta product biology technical manager. “Rotating to another trait or crop helps break that cycle, which greatly diminishes the risk of resistance.”
In lab studies, researchers proved it only takes up to four years of pressure to select for CRW resistance on three of the four proteins on the market. When the pest develops resistance to a protein, the Bt technology within a specific hybrid can be rendered ineffective or crippled.
“All available hybrids with pyramided traits for CRW use either Cry3Bb1 [Monsanto] or mCry3A [Syngenta] in combination with a second toxin, either Cry34/35Ab1 [Pioneer and Dow AgroSciences] or eCry3.1Ab [Syngenta],” says Joseph Spencer, insect behaviorist at the Illinois Natural History Survey at the University of Illinois. “This means where resistance is present in the population, there might be at best only one effective toxin at work.”
There are essentially only two modes of action used in the four proteins. The Cry3Bb1, mCry3A and eCry3.1Ab represent one mode of action and display the greatest amount of resistance, according to Ludwick. The Cry34/35Ab1 protein took longer to show resistance in lab testing.
“The Cry34/35Ab1 [Herculex] is a unique protein complex; it’s a combo of two proteins that work together inside the insect,” says Clint Pilcher, Corteva Agriscience integrated solutions manager. “This dual protein complex may lend itself to being more durable than other CRW proteins.”
Monsanto is poised to release SmartStax Pro with a third mode of action. The company partnered with Corteva Agriscience so the trait can be developed in its corn lines as well.
“We’re still waiting for a couple regulatory approvals,” says Tom Clark, Monsanto global insect trait platform lead, corn and soy technology. “We can’t give an exact date but our plan is to hopefully launch within a couple of years. We don’t expect this to be our final CRW product; we’ll continue to research solutions.”
The new trait, Corn Rootworm III, is formally referred to as MON87411. Monsanto is using RNAi technology to create this trait, which includes Cry3Bb1 and glyphosate tolerance, and is doing resistance testing before launch, per the norm for new traits.
University of Illinois researchers are also investigating the promise of two naturally-occurring resistance genes in corn. One interacts with nematodes in the soil and tells them to attack the rootworm larvae. The other is related to the plant’s ascorbate synthesis pathway that produces free radicals that injure feeding insects.
“We were screening [corn lines] for insect resistance. There were not many, but we found some,” says
Martin Bohn, corn breeder in the Department of Crop Sciences at the University of Illinois. “We had to look into lines from Argentina, Brazil and the Caribbean Islands to find it.”
Until alternate forms of resistance come to the market, farmers will have to use tried-and-true methods to manage the pest.
“Crop rotation is the best option. If you can’t rotate, plant something with two modes of action. If you can’t do that, use a single mode of action and a soil-applied insecticide—that’s the bare minimum,” Ludwick says. “If you can’t do that, and have CRW present, you’re out of luck.”
Reinhart keeps resistance manage-ment top of mind. “We use SmartStax on our corn-on-corn acres and are ready with insecticides if we see a lot of beetles,” he says.
While only five states have officially documented resistance, it’s likely present in other states. Since it only takes up to four years of pressure for the pest to become resistant to most of the proteins, the rest of the U.S. is a ticking time bomb.
Not All Bt Traits Offer the Same Control
Corn rootworm (CRW) isn’t the only pest that demands control. Prior to CRW resistance, corn borer control came in the form of a Bt trait in 1996.
“I think with corn rootworm traits, the initial expectation was that the traits would blow corn rootworm out of the soil because of what we saw with European corn borer—the first Bt trait,” says Marlin Rice, Syngenta product biology technical manager. “The European corn borer trait was the silver bullet, a high dose trait, and after more than 20 years it has no documented resistance.”
Alternatively, CRW traits are low-dose traits. This means you’ll occasionally have survival or unexpected injury from large populations of the pest.
Both traits—CRW and corn borer—were created using biotech. Bt stands for Bacillus Thuringiensis, a bacterium found in the soil with natural insecticidal properties. There are hundreds of strains of Bt.
“We identify Bt proteins that are very specific to certain insects, isolate that gene and insert it into the corn genome,” says Clint Pilcher, integrated solutions manager, Corteva Agriscience.
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