Glacier FarmMedia — Bt corn hybrids have been the story of success in transgenic innovation, a gold standard of biotech pest management, first with European corn borer (ECB) and then corn rootworm (CRW).
Now comes a case from Truro, N.S., where a resistant corn borer population confirmed in 2018 also appears resistant to another protein.
Before Ontario growers exhale in relief that it’s Nova Scotia and not southern Ontario, Tracey Baute, field crops entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs, says they should brace themselves.
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Nova Scotia is first to have pests with dual resistance, but resistant populations of ECB have also been confirmed in New Brunswick, south of Montreal and near Carman, Man.
Given the ability of ECB to overwinter, the wide range of hosts and its ability to fly up to 40 kilometres in a single generation, their arrival in Ontario — whether from eastern or northern regions, Baute suggests — is a matter of time.
For ECB hybrids, there are only four Bt proteins that work against the pest. Three of those are Cry1s: Cry1Ab, Cry1F and Cry1A.105 and the fourth is Cry2Ab.
Although there are three Cry1s, the Cry2Ab has always been partnered with the Cry1A.105 in attempts to reduce resistance.
“In 2018, when Cry1F showed up resistant in Nova Scotia, we were caught off guard,” says Baute, and finding the other three locations with resistance to Cry1F was worrisome.
“But the most recent detection in 2022 in Truro with Cry1Ab was more alarming because that showed us the Cry1s are starting to fall apart and in fact, when they tested that population against the Cry1A.105, it’s cross-resistant, leaving only the Cry2Ab working.”
The loss of one protein isn’t good news, but it was alarming to see resistance develop to others in less than five years. Once that population starts to spread, it seriously reduces the options that work against corn borer.
Further complicating matters is that Cry3 proteins impart resistance into hybrids for corn rootworm. Resistance in those pests is an issue for dairy producers, many of whom plant three- and four-year continuous corn crops. The resistance challenge can be minimized by eliminating that third or fourth year of corn to break the cycle.
That’s not the case for ECB.
Too many factors
As Baute notes, the reasons for concern are multi-faceted: the ability of ECB moths to overwinter and fly and the extensive list of hosts among them. Potatoes, peppers, apples, hops and wheat are susceptible, and could adversely affect their respective downstream markets and consumers.
Another wrinkle in this discovery is the Cry1s for corn borer are in every hybrid grown in Canada. Even if growers plant rootworm hybrids, those hybrids will contain a corn borer protein.
There’s no immediate help from plant breeders, either. It will be at least eight years before something completely different from Cry1s and Cry2s reaches commercial availability.
The use of Bt as a foliar application, primarily in the organic sector, may contribute slightly to resistance development, even though it is applied prescriptively. Corn borer pests also are not exposed to the products for the full growing season.
“I am more concerned what Bt resistance will do to the effectiveness of Bt foliars that share the same proteins, as it will affect all organically grown commodities that are ECB hosts,” says Baute.
“They already have limited options for organic production and losing Bt foliar insecticides could have a large impact on not just corn production.”
Where do growers turn?
There’s no easy solution and Baute concedes she’s not as optimistic about the ECB-Cry protein dilemma as she is with breaking the cycle of resistance to CRW-Cry3 hybrids.
“It’s more of a change of practice that we need,” she says. “Corn borer will need strong mitigation measures like shredding corn stubble, not leaving that intact over the winter and perhaps spraying multiple times or implementing some biocontrol, like trichogramma wasps, to suppress this resistant population.”
It also may take a “return to yesterday” approach — planting non-Bt hybrids, obeying refuge requirements and scouting for signs that many have forgotten since the late 1990s:
- frass (sawdust-like material)
- holes in the stalk at the leaf axils
- broken tassels
- ear feeding through the shank or at the ear tip
- ear drop
“If we try to add more insecticides, corn borer is that trickier pest to control, where timing has to be right for a spray application before they enter the stalk of the plant,” says Baute.
“As soon as they do that, you can’t get to them, so it’s limited and that adds to the complexity of monitoring with traps and looking for egg masses and timing those applications properly.”
