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Beyond Bt: Battling insect resistance for healthy corn

Corn biotech traits play a crucial role in modern agriculture, providing effective solutions for pest management and crop protection. For more than 25 years, Canadian corn growers have had access to Bt corn technology as a highly effective means of insect control. However, growers must also be aware of changing pest susceptibility and new innovations to keep their corn crops protected, healthy and profitable.

In a recent webinar, hosted by Top Crop Manager and presented by Bayer Crop Science and DEKALB seed, Dr. Jocelyn Smith, research scientist in field crop entomology and adjunct professor at the University of Guelph, joined Brock Smith and Mark Groen, Bayer Technical Solutions Agronomists, to share insights into Bt technology, common corn pests, insect resistance and available solutions for growers.

Understanding Bt and insect resistance
Bt corn is a type of genetically modified corn that produces a protein toxic to specific insect pests, such as corn borers and rootworms. This protein is derived from a naturally occurring soil bacterium called Bacillus thuringiensis (Bt).

In 1996, Cry1Ab was the first protein launched, followed by Cry1F in 2022, and Cry1A.105 x Cry2Ab2 in 2008, and most recently the Vip3A protein, Dr. Smith explains.

“The primary target pest was the European corn borer with these proteins, and the first three of those that came along in that timeline are highly toxic to European corn borer,” Dr. Smith explains, adding that corn borers have never been susceptible to the Vip3A protein, meaning it’s typically stacked with an additional protein.

“For about 25 years, there was no indication that European corn borer was developing resistance to any of the proteins,” Dr. Smith continues. “Unfortunately, in 2018, we got a call to look at corn fields near Truro, N.S. [in plants producing the Cry1F protein], where we found anywhere from 30 to 70 per cent of plants in those fields had corn borers in them with lots of feeding injury.”

After lab tests, Dr. Smith and her colleagues confirmed Cry1F resistance, discovering the first case of Bt-resistant European corn borer in the world. Since then, Bt-resistant European corn borers have also been discovered in Quebec and Manitoba.

While there are many potential reasons behind Bt resistance, Dr. Smith notes that short-season markets don’t have the same Bt corn hybrid options as larger corn-growing markets do, where hybrids with two or three proteins are more often used. But, this isn’t failsafe, she continues. “We now have three of the major Cry proteins in jeopardy, so there are some shifts happening with European corn borer in Canada.”

European corn borers aren’t the only problematic pests. Despite the efficacy of Bt technology, pests like Western bean cutworm, corn earworm and corn rootworm have also shown resistance to certain proteins.

“Western bean cutworm are not susceptible to the Cry1Ab protein. We thought there was some susceptibility to Cry1F, but we do have resistance to that. The Vip3A protein is the only one left effective against this insect, and there are more hybrids coming to market all the time that produce that protein,” Dr. Smith says.

“Corn earworm isn’t considered a major field corn pest, but it does seem to show up a lot lately,” she continues. “It has developed resistance to Cry1Ab over time, and the same thing has happened with Cry1A.105 and Cry2Ab2. Like Western bean cutworm, the Vip3A protein is the only one that will control corn earworm.”

Corn rootworm is one of the only below-ground pests that is targeted with Bt corn, Dr. Smith notes, and the story is similar. Over the years, companies have stacked proteins into plants to prevent the development of resistance, but this pest has been more persistent than its above-ground acquaintances.

“One factor is that some of the Cry3 proteins have a lot in common, so when resistance to Cry3Bb1 develops, it confers cross resistance to other proteins,” Dr. Smith explains.

Important traits and new solutions
“Traits are important for controlling big insect problems, and growers need to be aware that these pests are alive and well, even if you haven’t seen them in your fields yet,” Groen says.

The company’s portfolio has evolved over the years to include stacked biotech traits effective against several different pests, like Trecepta^® RIB Complete^® for above-ground protection of Western bean cutworm and black cutworm and SmartStax^® RIB Complete^® for above-ground protection against European corn borer, fall armyworm and black cutworm, and below-ground protection against corn rootworm.

New for 2024, growers can access SmartStax^® PRO with RNAi technology, the first product with three modes of action to defend roots against above- and below-ground pests like corn rootworm. SmartStax PRO with RNAi technology will have broad-spectrum control of above- and below-ground pests, as well as tolerance to glyphosate and glufosinate herbicides for weed control. And, looking ahead to 2025, Bayer will add VT4PRO™ to its portfolio, which is built on proven Trecepta technology to control above-ground pests such as Western bean cutworm combined with RNAi technology and Cry3Bb1 for below-ground protection against corn rootworm.

Dr. Smith says this new mode of action will aid growers dealing with Bt-resistant corn rootworm. “RNAi works differently in that, rather than binding to the gut of the rootworm, it interferes with their actual DNA, disrupting critical protein production that they need to survive.”

However, while RNAi technology is promising, proper management is crucial to preserve its use, she continues.

“What we’re really recommending is avoiding continuous corn, especially transgenic corn for corn rootworm, for more than three consecutive years,” she says. “We know that in the early days of Bt proteins, if rootworms were exposed to them for three consecutive years in the field, they would develop resistance and the same thing will happen with RNAi varieties.”

Brock Smith agrees that proper trait use is important, reminding growers that RNAi is not a silver bullet.

“It doesn’t replace proper trait stewardship and the need to rotate to a non-host crop,” he concurs. “Relying on RNAi solely, especially where Bt resistance is starting to increase, puts pressure on the RNAi trait, which could quickly lead to the development of resistant populations against the RNAi mode of action.”

The same advice applies with all corn pest management, Dr. Smith concludes.

“We don’t want to just rely on transgenic corn in the long run. When we get complacent and use the same thing over and over again, the pest will eventually adapt.”

As always, the experts remind growers to consult their agronomist or seed supplier to determine the best fit for their farm. Dr. Smith also advises getting in touch with the OMAFRA corn specialists, an agronomist and/or seed dealer if growers experience unexpected injury to transgenic corn, and visiting cornpest.ca for more information.

Watch a recording of the full webinar here, and visit traits.bayer.ca/en/corn to learn more about Bayer’s corn trait portfolio.

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ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Roundup Ready® 2 Technology contains genes that confer tolerance to glyphosate. Glyphosate will kill crops that are not tolerant to glyphosate. Insect control technology provided by Vip3A is utilized under license from Syngenta Crop Protection AG. DEKALB®, RIB Complete and Design®, RIB Complete®, Roundup Ready 2 Technology and Design®, Roundup Ready®, Roundup®, SmartStax®, SmartStax® Pro RIB Complete® Trecepta® and VT4PRO^TM are registered trademarks of Bayer Group. Used under license. Agrisure Viptera® is a registered trademark of a Syngenta group company. Used under license. LibertyLink® and the LibertyLink® logo are registered trademarks of BASF. Used under license. Herculex® is a registered trademark of Dow AgroSciences LLC. Used under license. SmartStax® multi-event technology developed by Bayer and Dow AgroSciences. Bayer CropScience Inc. is a member of CropLife Canada. ©2024 Bayer Group. All rights reserved.