The first case of European corn borer resistance to Bt corn underscores the need for Manitoba corn growers to take steps to protect the technology, says Manitoba Agriculture’s entomologist John Gavloski.
That development was recently confirmed in Nova Scotia.
“If corn growers are growing Bt (corn borer)-resistant corn they need to be taking the refuge very seriously,” Gavloski said in an interview June 13.
Why it matters: Bt corn has been an important tool to fight corn borer, but it’s susceptible to selection pressure just like pesticides are.
“The other message we need to get out is we need to get away from these single-traits hybrids for corn borer. That’s critical as well.”
The resistance found in Nova Scotia corn borers is specific to the Cry1F protein found in Bt corn (Herculex I), the Manitoba Corn Growers Association reports in its June/July newsletter.
Borer-resistant hybrids should contain at least two Bt traits to reduce the chances of killing almost all the borers susceptible to Bt control and leaving those that are naturally resistant, Gavloski said.
Planting a refuge of non-Bt corn with the Bt-protected hybrids also reduces selection pressure.
“Some hybrids are a blend (of Bt and non-Bt corn),” Gavloski said. “They call it refuge in a bag so there is no way they can’t plant the refuge. But anyone who isn’t using that strategy and is buying pure Bt seed, they need to buy and plant that refuge.”
He described following this protocol as “quite critic” and warned Manitoba growers could easily see resistance here if they’re not vigilant.
“It’s a good lesson in a way of what can go wrong potentially,” Gavloski said. “It’s not to say they didn’t follow the rules (in Nova Scotia). Maybe they were planting proper refuge and it still happened, but it shows why we have to take that refuge seriously.”
Corn growers must follow the refuge requirements of the hybrids they are planting, the newsletter says. Refuge planting options can be found at cornpest.ca.
Farmers should also consider whether they need to plant borer-resistant corn in the first place. Corn borer populations rise and fall over time.
“We don’t need all our corn acreage in Bt corn every year,” Gavloski said. “Years when we really haven’t had a lot of corn borer pressure, growers might want to consider planting a non-Bt hybrid. They’re cheaper for one, and it doesn’t put that selection pressure out there. When we start to see corn borer building it might be a cue — now it’s time to introduce a Bt hybrid into the rotation.”
One might expect Bt-resistant corn borers to first show up in the American Corn Belt where millions of acres of Bt corn are seeded annually. Gavloski speculates it happened first in Nova Scotia where, because of its shorter growing period, there are fewer hybrids with at least two different Bt traits.
Nova Scotia is a long way from Manitoba so it’s unlikely Cry1F-resistant corn borers from there will end up in here.
“But it’s a precaution for us. We don’t want the same thing to happen,” Gavloski said. “Right now the resistance is just in that population, which we hope is contained to Nova Scotia. There’s no evidence of it being outside of there right now. So right now it’s just a heads-up for us. We need to start thinking about how we do resistance management and to take resistance management quite seriously.”
The corn industry knew corn borers potentially could overcome resistance to Bt via selection. That’s why planting non-Bt corn refuge and hybrids with two or more Bt traits is critical.
The Canadian Biotechnology Action Network, which questions the safety and value of genetically modified crops, is concerned “seed costs will rise with an increasing reliance on stacked GM traits, while insects continue to evolve resistance,” spokesperson Lucy Sharratt said in a news release June 11. “These genetically engineered plants are starting to fail, as was predicted, and are part of a costly technology treadmill.”
The potential corn borers’ resistance to Bt-protected corn doesn’t mean the technology shouldn’t be used, Gavloski said.
“Insects are good at developing resistance to almost anything you throw at them,” he said. “We’ve had cases where insects develop resistance to crop rotation. So what do you do? Do you tell farmers they shouldn’t be doing that? Crop rotation is good. Insects are just so robust they can develop resistance to many of our technologies and resistant varieties, Bt or not.”
Gavloski said it’s also important to note resistance to a particular Bt protein is naturally present in some corn borers. Killing most of the susceptible borers creates bigger populations of insects that are not controlled by that trait.
This Bt trait table at cornpest.ca shows the number of traits that control each pest.