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Will Bt-resistant corn rootworm cross the border into Canada?

Corn rootworm on a corn husk

Growers in the western U.S. Corn Belt have a real fight on their hands, and they know it. Corn rootworm is showing resistance to Bt hybrids. From Illinois across to South Dakota and down into Kansas, the Cry3Bb1 protein that is the foundation of some Bt genetics is under threat.

Researchers now say that pyramiding the Cry3Bb1 protein into the SmartStax system may be partly to blame. When introduced in 2010, SmartStax hybrids let growers switch away from refugia rules that required planting large blocks of susceptible hybrids. Instead, growers could adopt the convenient “refuge-in-a-bag” system, with susceptible seed interplanted with Bt seed at a rate of five per cent. 

Because most rootworm hybrids depend at least in part on Cry3Bb1, the new system may heighten the risk of resistance, especially in continuous corn.

But is it a concern for Ontario?

Not yet, say several industry reps. But vigilance here is essential. 

Growers need to get actively engaged in finding and reporting the pests, the reps say, and growers must also expect to play a leading role in developing the solutions. 

As with so many pest and weed issues, eastern Canadian growers get to be spectators to what strikes farms south of the border first. It’s happened in past with herbicide resistance and the spread of weeds such as Palmer amaranth. It also happened with Asian soybean rust.  

The corn rootworm situation, however, is different, largely because Ontario’s standard three-crop rotation does a good job of breaking the rootworm cycle. In much of the U.S. where resistance is showing up — even in first-year corn fields — continuous corn is more the rule than the exception.

For Tracey Baute, field crop entomologist with the Ontario Ministry of Agriculture and Food, the issue comes down to recognition and reporting. First, she says, farmers need to understand there are Western corn rootworm beetles in Ontario, and that some are showing up in first-year corn. As well, growers need to participate in learning how to handle the pest. That means when they recognize corn rootworm in their fields, they need to report it.

“They may see them in August or September, and say ‘Oh, this is definitely what it is,’” says Baute, who was one of three Ontario participants in a U.S./Canada report last year on managing Western corn rootworm resistance. “But they don’t remember to ask until September, October, November or even March, and that’s when we have lost our seasonal ability to go in and get those adults.”

As soon as growers find the insect, it’s time to contact someone — a company representative, a seed dealer, an agronomist or goverment extension. Baute stresses this isn’t to assess blame for possibly abusing the technology. It’s to collect adults, perform bioassays and then to develop go-forward strategies before resistance gets to U.S. levels.

“The point is to identify where those source-resistant fields are, and look beyond that, and to help work toward not allowing the continuation of those resistant populations,” says Baute. 

“We have that opportunity to at least mitigate those risks now,” Baute says. “Three-year rotations can definitely benefit us with this issue.” 

However, Baute sees it as an increasing concern in volunteer corn in soybeans. In a tough spring, she says, when a farmer can’t get out and spray those fields, that field is no longer “out of corn.” Even a small number of volunteers can help resident rootworm population build up.

Hard pest to beat

Five years ago Baute was vocal about stacked-trait technologies in corn. Those technologies were launched with the goal of adding more protection. But Baute’s concern was that more traits added more management variables, possibly aiding in the development of resistance based on a lack of understanding of the biology and corresponding technologies. 

“Especially with an organism like rootworm,” Baute adds, “because that pest has developed resistance to almost everything we’ve thrown at it.” 

An additional threat is there may be cross-resistance possibilities with Cry3Bb1, so you could quickly lose other genes, leaving you with no options.

“We’re playing a little bit with fire,” Baute says. “It’s being stacked with everything.” 

In more than a few cases, that means growers are using the gene where they don’t need it, contributing unnecessarily to selection pressure. Then there are growers who need the protection but don’t follow good integrated pest management (IPM) strategies, planting corn-on-corn instead.

The stewardship issue is top priority for Pat Lynch, independent certified crop adviser from Stratford, Ont. This whole situation shows what can happen when stewardship guidelines aren’t followed, Lynch says. 

“When a small number of growers ignore the stewardship proposals,” Lynch says, “all growers in that area suffer the consequences.” 

Lynch recalls when problem weeds, such as velvetleaf, spread across Ontario. Discoveries were mapped, purely for the purpose of identification and to inform neighbouring farmers. 

Similar vigilance could help on corn rootworm resistance. The current situation in the U.S. has been developing for four or five years, says Lynch, and it reminds everyone that stewardship guidelines are not being followed.

That’s especially important because of the difference between corn rootworm and the European corn borer — the first pest targeted by corn Bt genetics. It proves again, Lynch says, that growers need to know more about the technology they’re buying. 

With corn borer, the Bt technology has been around long enough that resistance should have developed by now, but control continues to be good. 

The same cannot be said of corn rootworm, and Lynch echoes some of Baute’s concerns. It’s important to understand the technology, but it’s also essential to understand the biology of the pest, including traits such as extended diapause and laying eggs in soybeans.

“Education is teaching and learning, and I think the teaching was done,” Lynch says, referring to the job the seed and chemical companies did in bringing the technology to market. “I don’t think the learning was done; there was too much of ‘It’s not a problem on my farm and I’m going to do this, so I don’t need to take the time to understand it.’ But it’s definitely another of these wake-up calls that continue to pop up, similar to glyphosate resistance.”

Maybe some optimism

As much as there’s a need for continued vigilance, there are three reasons why the corn rootworm situation is worse in the U.S. than in Ontario: the flight of the beetles, the lack of rotation in the U.S. Corn Belt and stewardship, says Doug Alderman, national sales manager for Pride Seeds, of Paincourt, Ont.  

Alderman says flight patterns aren’t funnelling the pests towards Ontario, at least so far. Ontario rotations are more robust, and U.S. growers typically don’t plant any wheat at all and only add a token soybean crop every third or fourth year. As for the stewardship issue, Alderman says U.S producers were clearly the authors of their own misfortune. 

“Right from the start there weren’t very many people being very responsible with the stewardship down there,” says Alderman. “That’s why we have refuge requirements, and farmers in the U.S. weren’t being compliant, and if they continue to not follow that stewardship and not be compliant, they’re going to continue to have issues. Technology’s a wonderful thing but we have to treat it with respect.”

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CG Production Editor

Ralph Pearce

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