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20 years of Bt corn hybrids

Growers are armed with more powerful, more precise tools than ever in history — but the war on corn pests is far from over

In any discussion of agricultural biotechnology, Bt corn hybrids shine as a huge success. Launched in 1996 in the U.S. and Canada, their 20-plus years in eastern Canadian production are worth celebrating for their unparalleled control of European corn borer (ECB).

The technology has performed precisely as designed — without the development of the resistance that would prove a bane in so many other biotech applications.

According to figures from the United States Department of Agriculture (USDA), nearly 80 per cent of corn acres in the U.S. in 2017 were planted with Bt hybrids.

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“In Canada, we’re strictly a feed-corn market, so we don’t have the same drivers as they have in the U.S.… I would say it’s 90 per cent,” says Dave Harwood, technical services manager with Corteva Agriscience (the agriculture division of DowDuPont). “Most of that five per cent that’s not would be Roundup Ready corn grown primarily in Western Canada. In Eastern Canada, it would be north of that 90 per cent.”

As a result, growers have seen far less damage from ECB during the past 22 growing seasons — whether you’re talking “shot holes” in the leaves or late-season stalk breakage.

As well, with more corn acres planted to Bt, ECB populations became more prevalent on other host crops like peppers, potatoes and tomatoes.

“When Bt corn was rolled out, I would say the average yield loss due to corn borer would be in that 15 to 20 bu./ac. range,” says Harwood. He recalls early experiments into Bt hybrids versus non-Bt hybrids. “That’s with the native population as it was at that time. Today we have reduced the population through the deployment of Bt, so in non-GM corn fields in the Ottawa Valley (for Ingred­ion’s sourcing of non-GM hybrids) you would have lower risk of significant injury because the population is smaller.”

It’s not to say that 20-plus years after its inception, Bt technology is no longer needed, adds Harwood, likening it to the reliance on weed control products in corn. Return to planting any amount of non-Bt corn and the evidence suggests corn borer would quickly colonize such a field, eventually leaving growers open to the same levels of yield loss common in the mid-1990s.

Until Bt technology arrived, it hardly seemed worthwhile even scouting for the telltale “shot holes” that indicate the presence of European corn borer.
photo: Tracey Baute, OMAFRA

Tracey Baute agrees that growers are a long way from seeing high infestation numbers in ECB, at least unless something significant happens, like widespread cutbacks in the use of the technology.

Even if a grower were to opt out of Bt hybrids for one year, it’s unlikely to have much of an impact. All of the Bt traits that have been introduced in newer genetics have corn borer as one of their primary targets, so Bt corn acts as a shield for other neighbouring host crops, reducing ECB populations in the area to provide some protection to adjacent crops as well.

“It was high dose and was 99.9 per cent effective on controlling corn borer,” says Baute, field crops entomologist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), referring to the early days of Bt corn. “We also understood the behaviour and the genetics of the corn borer, so we knew where they mated and the refuge criteria that would work for them.”

It was this level of scientific understanding and industry input in breeding and development that helped ensure corn borer resistance would not become a factor in eastern Canada.

Not all pests are susceptible

When the discussion turns to other pests that Bt corn was used to control almost incidentally, that’s when the erosion of its efficacy becomes apparent.

Unlike ECB, the Bt hybrids had only low or moderate activity versus corn earworm or western bean cutworm. Plus, there wasn’t always the same understanding of the behaviour and genetics of those pests as there had been with ECB, and as a result, there wasn’t as much insight into when and how resistance might develop.

“We had to come up with a compromise on whether we should stick with what everybody knows or maybe sacrifice some of the protection against resistance,” says Baute. “Or do we completely change up what is required for refuge for each pest and make something of a headache for growers and likely not see them comply as much to resistance management requirements?”

The “Bt story” also has its detractors with some ecologists pointing to the impact on the food chain for predator species such as birds and other animals, some of which relied on higher populations for feeding. But by far, Baute says, this has proved a good news story from the standpoint that the technology has decreased reliance on insecticides while boosting yields and productivity for farmers.

To some extent, it’s also a help that the majority of farmers in Canada see only one generation of corn borer per year. It’s only in regions south and west of Woodstock, Ont., that farmers have to deal with two generations.

Biologically, it would make sense to say that it’s less of a threat with only one generation and a lower probability of resistance development although Baute points to the development of resistance in western bean cutworm, which also typically sees just one generation.

“We can’t apply that (one-generation default) all the time, because a lot of factors are at play, and the number of generations is just one of those factors to consider when we look at resistance,” states Baute. “With corn borer, there are lots of trait options because none of these traits have developed resistance in the field. But for western bean cutworm, we only have one trait left that we are fully relying on because we’ve lost the other one to resistance, and there’s not another one coming down the pipeline that we know of. As we start to see more widespread use of one trait, we increase our risk of losing that trait too.”

That means farmers will come to rely on other technologies and options outside of transgenic means, and they will focus more attention on potential bio-control options.

Another facet of ECB has to do with its size. Whether they’re in a field near Des Moines, Iowa, or in Woodstock, Ont., corn borer larvae are always about the same size. That means the tunnels they make are the same size too.

Even so, the Canadian corn plants do get hit harder, mostly because they tend to be smaller than those found in the U.S. Midwest: their ear shanks are smaller, the stalks are thinner, and they have less biomass.

That means corn borer larvae can do more damage in Eastern Canada than in the Midwest or points farther south.

Species specific

The primary hurdle in applying one technology against incidental pests is the reality that transgenic applications are very species specific. Bt corn hybrids were bred to deal with corn borer, not corn earworm or western bean cutworm, and to expect them to perform outside of that target species is unrealistic.

“There are some that might have some activity across species but generally they’re fairly targeted,” says Harwood, noting there are new options being developed via Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) technology. “What’s really nice about CRISPR is the extent to which you can target the placement of genes in corn and create so-called site-specific insertions of a gene-of-interest into the plant.”

The challenge with developing any insect resistance approach is to find the target in the insect that the resistance is going to address. Discovering a critical process that is species-specific to create a gene that interferes with that process is what Harwood calls “heavy lifting.” Use of CRISPR technology might enable a faster deployment of the trait, he adds, but finding the target is a more technically demanding aspect than developing the genetic material that addresses it within the insect.

Baute also mentions RNAi — ribonucleic acid interruption — as another gen­etic option in developing resistance for various pests, although she’s quick to echo Harwood’s statement about the challenge of properly targeting the species. Implementation of a new technology can take more time than hoped for, especially when factoring in the cost of investment. Even with Bt technology, before newer hybrids came to market, it might have taken upwards of 10 years from sourcing the trait to its full availability.

“It does have us concerned that we’re relying on one trait for our major corn pest here in Ontario,” says Baute, noting that western bean cutworm is now resistant to the Cry 1F protein that once provided some control. “The other complication with something like ‘Refuge-in-a-Bag’ is that it’s very difficult to see and monitor for resistance until it’s full-blown and noticeable. You can no longer go in and scout the refuge area to see what the difference is between the refuge and the Bt area. Until we start to hear the complaints and do Bt tests and realize that ‘this’ wasn’t a Bt plant and ‘this’ is a western bean cutworm happily eating, it all takes time.”

Back to the block?

There has been some discussion that the integrated refuge approach (also known as “Refuge-in-a-Bag”) might not have been the way to go for long-term WBC control, but it was a measured response to its presence. In an attempt to lengthen the lifespan of Bt hybrids, seed and chemical companies, retailers and even the Canadian Food Inspection Agency (CFIA) tried to ensure — and then enforce — full compliance with block refuge practices.

Yet both Baute and Harwood concede those efforts led to incomplete deployment of the technology, leaving integrated refuge as an option that would lead to better control.

“Although no one was pleased with that scenario, it was one of those ‘win-win’ situations, where the grower’s life was made simpler,” says Harwood. “In doing that, we had complete deployment of the refuge where Bt corn was planted.”

There are differences of opinion, including among entomologists in the U.S. who have questioned whether incorporated refuge has been as effective as the early configurations, with opposing opinions on either side. Baute also points out that a research paper written by a group of entomologists titled “Seeds of Change” highlighted the concerns about the science and the practicality, with practicality winning but at the cost of the science.

“We knew that it was for convenience sake, and it was great,” Baute says. “But it’s going to increase the risk of resistance, especially for the ear-feeding insects like WBC or corn earworm.”

This article was originally published in the Sept. 2018 issue of the Corn Guide.

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CG Production Editor

Ralph Pearce

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