One billion dollars. Prior to Bt technologies farmers lost $1 billion annually to corn rootworm—in the form of chemical costs or actual yield loss. With resistance to traits that once killed the pest on the rise, it might just nibble its way back to a billion-dollar price tag.
Corn rootworm (CRW) poses a double threat—the adult snips corn silks, and if unchecked could prevent successful pollination and kernel development, and the larvae munch on roots which leads to risk for disease and plant stress. CRW was once controlled by traits but with resistance on the rise is now at risk of running rampant: it’s time to find a solution to slow the spread of resistance.
“The first documented case [of CRW resistance] was in Iowa in 2011 to Cry3Bb1,” says Dalton Ludwick, post-doctoral researcher at Virginia Tech and working with USDA. “Now we have documented issues in Iowa, Illinois, Minnesota, Nebraska and North Dakota—there are other areas it’s been talked about, but no formal documentation has been published.”
In the wild, CRW feeds on corn, every time. Only in lab conditions will the pest feed on alternate hosts—but CRW have to be forced.
Farmers who practice corn-on-corn are at the greatest risk of developing 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 result is the Bt technology within a specific hybrid can be rendered ineffective or crippled.
“All available hybrids with pyramided traits for CRW use either Cry3Bb1 or mCry3A in combination with a second toxin, either Cry34/35Ab1 or eCry3.1Ab,” 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.”
Taking a close look at each of the proteins (see the “Handy Bt Trait Table” for common names), there are essentially only two modes of action, even though there are four proteins. The Cry3Bb1, mCry3A and eCry3.1Ab represent one mode of action and are the most compromised, according to Ludwick. The Cry34/35Ab1 protein took longer to confer resistance in lab testing.
Public and private researchers are seeking new solutions. Monsanto is poised to release SmartStax Pro, which features a third mode of action against the pest. The company recently partnered with Corteva so the trait can be developed in its corn lines as well. Monsanto hasn’t announced an official launch date for the product—instead explained it is in “phase four” of development.
The new trait, Corn Rootworm III, is formally referred to as MON 87411. Monsanto is using RNAi technology to create this trait and is doing resistance testing before launch.
University of Illinois researchers are 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 University of Illinois. “We had to look into lines from Argentina, Brazil and the Caribbean Islands to find it.
“Our previous research showed that there is no inherent resistance in the elite hybrids grown by most farmers in the Midwest,” he adds.
Until alternate forms of resistance come to the market, farmers will have to use tried and true methods to manage the pest.
“EPA, industry and others have created best management practices for CRW,” Ludwick says. “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. If you can’t do that, and have CRW present, you’re out of luck.”
While only five states have officially documented resistance, it’s likely present in other states or will be soon. Because it takes four or less years of pressure for the pest to become resistant to most of the proteins, the rest of the country is just a ticking time bomb.
Joining the fight against resistance, the National Corn Growers Association recently launched the “Take Action” Insect-Resistance initiative. The goal is to fight resistance and preserve control with the current Bt proteins on the market, as well as any coming down the pipeline.
Here are the steps Take Action encourages to preserve usefulness:
- Plant the required refuge. Take into account the product and geography you’re in—corn-growing states’ refuge is 5% (in-bag) or 20% (structured refuge), and cotton-growing states are 20% (in-bag) and 50% (structured refuge).
- Use insect resistance management strategies: rotate crops, use pyramided traits, rotate traits and rotate and use multiple modes of action for insecticide seed treatments, soil-applied insecticides and foliar-applied insecticides.
- Actively scout to see if control methods are working, if there are escapes or possible resistance. Take additional action to control pests when necessary.
Farmers have learned a hard lesson about resistance with herbicide-resistant weeds. Learn from those mistakes and principles and other management methods to fight the good fight against CRW resistance.