The challenge with Bt

Insects are developing resistance to the formerly bulletproof building block of corn hybrid traits

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The United States Environmental Protection Agency (EPA) is considering a ban on some Bacillus thuringiensis (Bt) corn and cotton hybrid traits in an effort to slow the proliferation of resistant pests. In Canada, where the number of control options is limited, this move might be a cause for concern.

Why it matters: Tools to control western bean cutworm and rootworm are limited. Narrowing those available south of the border could actually enhance resistance dangers.

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Corn hybrids with Bt traits resist several types of insect pests in corn, depending on the trait.

Two years ago, the EPA commissioned an advisory panel to recommend how best to control increasing rates of Bt resistance in corn and cotton pests – specifically regarding corn earworm, though western bean cutworm is also a player. Reviews indicated hybrids containing low-dose toxins were a problem.

“A lot resistance issues have emerged, especially with secondary or non-high-dose pests,” says Jocelyn Smith, field crop pest management research scientist with the University of Guelph, Ridgetown. As a result, the EPA is considering eliminating single-trait, low-toxicity corn hybrids from the market over the next three to five years.

This isn’t the concern for Canada, as few single-trait, low-toxicity hybrids exist on the market.

“We would be in favour of phasing out single toxin hybrids. We actually thought this was the case in Canada already, but the Nova Scotia case showed some of them hung on,” says Smith, referring to an Atlantic case of Bt resistant European corn borer identified several years ago.

“The EPA proposal is mostly talking about lepidopteran pests, so corn earworm, western bean cutworm and European corn borer,” she says.

“In Canada, there are only high dose traits against European corn borer when it comes to Bt toxins. For rootworm there are four Bt proteins available in the market, but none are high dose. They are also heavily cross-licensed and are often stacked with above ground insect traits, so they’re probably planted much more often than they should be.”

At this point the agency has yet to make a final decision. Public consultations were to continue until Nov. 9.

The status of Vip3A

But there’s a catch. As part of its plan, the EPA phase-out includes corn hybrids and cotton varieties that do not have the Vip3A protein pyramided — layered with at least one other Bt trait targeting above ground pests — within.

“We don’t think they should phase out everything that doesn’t have Vip3A in it. In most corn growing regions in North America, the Bt toxins that are proposed to be phased out are still effective against European corn borer,” said Smith.

Pyramid Bt hybrid damage in Blyth, Ont. this year.
photo: Jocelyn Smith

Vip3A is the only high-dose Bt protein currently available to Canadian growers in their battle against western bean cutworm, now considered the most economically important insect pest in Ontario corn.

Canadian suppliers of corn seed are moving quickly to get more Vip3A into their corn hybrids, but it takes time to insert traits into the most desirable genetics, test those genetics and get them to market.

Syngenta, the company that originated the Vip3A trait is the only company with the trait in a wide range of hybrids.

The fear, says Smith, is a vast increase in Vip3A use will encourage faster resistance pressure in the pest, leaving Canadian farmers in an even tougher position.

“Plus we’re worried that using Vip3A with integrated refuge will create resistance more quickly.”

Integrated refuge compounds the problem

Indeed, refuge strategies themselves have evolved in problematic ways.

Designed to keep resistant pests from breeding with one another, farmers employing Bt technologies were required to keep portions of the field planted with non-Bt hybrids. This increased the likelihood that insects surviving toxicity will mate with susceptible insects, diluting resistance genes within the population.

But refuge size requirements shrank over time. From a high of 20 per cent, refuge area now mainly stands at five per cent. In most cases, the five per cent is deployed as integrated refuge — a system where the quota of non-Bt seed is pre-mixed in the bag.

As described by OMAFRA Field Crop Entomologist Tracey Baute in an article from October 2019, the transition came at a cost.

“Although it does ensure that every grower has planted a refuge, the integrated refuge is not always effective at reducing the risk of resistance, especially for the later season ear feeding pests (i.e. western bean cutworm and corn earworm). It may have also caused growers to have assumed that all hybrids have integrated refuge now, even though their hybrid of choice might only contain one effective trait against their target pest.”

Smith says diluted toxicity is a reality in integrated refuge systems. All factors combined, the risk of resistance development is substantially increased, especially for pests feeding on corn ears.

Prevention

Smith adds 2019 and 2020 brought many reports of emerging resistance in corn rootworm in Ontario, generally in fields where continuous corn production using Bt hybrid is the norm.

“Don’t use rootworm hybrids in first year corn. Avoid continuous corn as much as possible, and don’t go longer than three years continuous corn with the Bt rootworm traits,” she says.

While not a significant pest in 2020, western bean cutworm can be effectively controlled using non-Vip3A hybrids. Crops can be scouted and sprayed when populations surpass tolerable thresholds. This has the added benefit of reducing the risk of Vip3A resistance.

Research also suggests employing the older block refuge system for Vip3A will decelerate resistance evolution.

About the author

Contributor

Matt McIntosh

Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.

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