Insect resistance to BT corn
By Carolyn King
After years of relying on this technology to control corn pests, some worrying situations are emerging.
We’ve had Bt corn to control European corn borer since about 1996 in North America, and there has never been a case like this until now,” says Jocelyn Smith, a research scientist at the University of Guelph’s Ridgetown Campus.
Smith is talking about the finding of European corn borers with resistance to a Bt protein in Nova Scotia in 2018. “This is the first documented case in North America of practical resistance – where the product actually does not work in the field – in European corn borer to any Bt corn trait.”
Bt corn expresses insecticidal proteins from the bacterium Bacillus thuringiensis. This technology is a key tool for targeted insect pest control, so strategies have been established to really reduce the risk of resistance to the Bt toxins in the target pests.
The Nova Scotia case of Bt resistance in European corn borer is particularly worrisome, but other corn insects have also developed field-evolved Bt resistance in recent years in North America.
These emerging problems underline the need for growers, seed companies and researchers to help stop or slow the spread of Bt resistance.
The Nova Scotia case
In 2018, a Bt corn hybrid with the Cry1F protein, which should control European corn borer, was found with damage caused by this pest in a few fields near Truro, N.S. Smith and Art Schaafsma, a professor at the University of Guelph-Ridgetown, carried out the work to check for Cry1F resistance in the local corn borers.
In 2018, Smith collected European corn borer populations from four fields in the Truro area. To look for the possible spread of resistance, she also collected a corn borer population in the Annapolis Valley, about 120 kilometres west of Truro. The Annapolis Valley is upwind from Truro, making it harder for the adult corn borer moths to fly in that direction. So if the resistance had spread, it would be less likely to spread in that direction.
“When we tested them in the lab, we found that all four populations from the Truro area were highly resistant to Cry1F,” Smith says. “And the population from the Annapolis Valley was also resistant to Cry1F, so there has already been some spread of resistance within Nova Scotia.”
In 2019, to find out if the resistance had spread any further, Smith and Schaafsma obtained collections of European corn borer from Prince Edward Island, Quebec and eastern Ontario, as well as more samples from Nova Scotia, from collaborators in these areas.
“We will be assessing all of those collections in 2020 to determine their susceptibility to Cry1F,” she says. This assessment of resistance in each population is a lengthy, rigorous process that includes rearing the insects, testing their response to the Bt protein and, if the insects show some reduced susceptibility to the protein, testing the insects on plants with the protein, and then replicating the assays at least three times to confirm the results.
Possible contributing factors
Smith emphasizes that Bt resistance in European corn borer is a serious concern for growers because larval damage to stalks and kernels can affect both corn yield and quality. “European corn borers can decrease the quality of grain in that their feeding wounds expose the plant to more fungal pathogens, so you get more mycotoxins and grain quality issues for livestock feed. Also, corn borers cause real problems when it comes to the crop’s standability,” she says.
“So it’s a significant pest and we don’t want to lose this technology.” And we don’t want Cry1F resistance to spread to other regions in North America.
The original strategy to prevent or delay Bt resistance in European corn borer is considered to be very effective. It involves combining high-dose Bt toxins with a non-Bt refuge. A high-dose Bt toxin is one that kills more than 99.9 per cent of a population of the target insect. A non-Bt refuge is a portion of a Bt field that is planted to a non-Bt hybrid.
The Nova Scotia resistance problem emerged only 12 years after Cry1F hybrids were first sold in the region. There could be various issues contributing to this rapid development of resistance.
Smith thinks one possible factor could be the use of Bt hybrids with only a single toxin for controlling European corn borer. “There are four Bt proteins that we consider high-dose toxins against European corn borer: Cry1F, Cry1Ab, Cry1A.105, and Cry2Ab2. In most of North America, we have corn hybrids that express more than one Bt toxin that will control this pest. With multiple modes of action, there is less risk of resistance,” she explains.
“However, in smaller markets with short-season hybrids, like Nova Scotia, some single-toxin Bt hybrids were still being sold.”
She also notes, “We don’t know whether there was good compliance to the refuge requirement in Nova Scotia, so that may also have contributed to the problem.”
Bt-resistant western bean cutworm
In Canada, we have another corn pest with resistance to a Bt toxin: western bean cutworm. In corn, the larvae of this moth species feed on the ears, causing yield losses and increasing the risk of fungal disease and mycotoxins in the grain.
Smith and Schaafsma were the first researchers to document Cry1F resistance in western bean cutworm, through field and lab studies between 2011 and 2016 in Ontario. Since then, resistance to Cry1F has been confirmed in populations of this insect in the U.S. in the Great Lakes region and the Corn Belt. Smith notes, “Anywhere that western bean cutworm populations occur in North America they now have very low susceptibility to Cry1F.”
According to Smith, several things probably led to the very quick development of Cry1F resistance in western bean cutworm.
First of all, Cry1F is not a high-dose toxin against western bean cutworm. “From the beginning, western bean cutworm populations had some tolerance or weren’t highly susceptible to the Cry1F protein. It looks like there was a lot of variability in western bean cutworm populations when researchers first tested the insect on Cry1F.”
Another factor was that the pest expanded its range; beginning in about 1999, it spread eastward from the western Great Plains through the Corn Belt. “Cry1F has been ubiquitous in most Bt corn hybrids since about 2006 so the insects had more and more exposure to that protein,” she says.
A third factor was the reduced refuge requirements. She explains that back in the early days of Bt corn deployment, a single-
toxin hybrid was required to have 20 per cent of the field in a structured refuge, where the non-Bt corn is planted in blocks or strips. “However, because the rest of the industry has gone to pyramiding traits that control European corn borer in this part of North America, we’ve allowed the refuge size to decrease to five per cent and incorporated as refuge-in-a-bag [where the non-Bt seed is mixed with the Bt seed in the same bag],” she says.
“We now know that this really increases the risk of resistance in western bean cutworms because these insects move around so much.”
Since Cry1F is no longer effective against western bean cutworm, there is now only one Bt protein to control this pest: Vip3A.
“We have been working on Vip3A in our lab. It looks like it is a high-dose toxin against early instar western bean cutworm larvae – the really small larvae after they hatch out of the egg masses. However, when we give third and fifth instar western bean cutworm larvae Vip3A in the lab, it takes significantly more Vip protein to kill those older instars. And we’re not convinced that the Vip corn plants produce a high enough level of the protein to provide a high dose for these older instars,” Smith explains.
“This is a concern especially with refuge-in-a-bag. Because the larvae move around a lot, they might be able to grow to become a third instar or later on a refuge plant and then move to a Bt plant and be able to tolerate the Vip protein.”
So deploying Vip3A as refuge-in-a-bag over many acres could lead to resistance in western bean cutworm. And that would leave corn growers with no Bt proteins to control this pest.
One of the things that Smith would like to study is whether a larger refuge would help slow the development of Vip3A resistance in western bean cutworm. “Since we just have a single effective toxin for western bean cutworm, in theory we should be doing at least a 20 per cent structured refuge.”
Resistant earworm on our doorstep
In the U.S., several other corn pests have developed Bt resistance including corn earworm, fall armyworm, northern corn rootworm, southwestern corn borer and western corn rootworm.
Of these pests, Smith thinks corn earworm is probably the greatest Bt resistance risk for eastern Canadian corn crops. “We always get some migratory flights of corn earworm moths that come into eastern Ontario each year late in the season. They originate from the southern U.S. and even further south.”
Corn earworm attacks many different plant species. In corn, the larvae feed on the silks and kernels, where the feeding damage creates an opening for fungal infections that can affect grain quality.
This insect has always had some tolerance to Cry toxins so the high-dose approach to fighting resistance was not possible.
“There is a lot of research showing that corn earworms are developing resistance to all of the Bt toxins that are out there already,” Smith says. “The only one that they are still susceptible to is Vip3A. But some preliminary data is showing that there may already be some reduced susceptibility in corn earworm to Vip in the U.S.”
Tips for managing Bt resistance
Smith has some advice for growers to help prevent development of Bt resistance and to slow its spread.
“The number one thing is to use pyramided, multiple Bt traits for any of the pests that you are targeting to control. Whenever possible, stop growing corn
hybrids that produce only one Bt toxin.”
However, single-toxin hybrids may be the only option in some situations. “In Nova Scotia, the problematic hybrid where the Cry1F resistance developed in European corn borer has been replaced with a pyramid Bt hybrid. But the pyramid consists of Cry1F and Cry1Ab. So growers are effectively down to one trait again, being Cry1Ab. So we are putting more selection pressure on Cry1Ab,” Smith says.
In addition, she explains that there is also a risk of cross-resistance with the Cry1 proteins, so a pest with resistance to Cry1F might also be resistant to one or more other Cry1 proteins.
“So we would prefer to see the two other Bt toxins for corn borer – Cry1A.105 and Cry2Ab2 – deployed on a greater acreage in Nova Scotia, rather than a pyramid that has Cry1F and one other protein.”
For corn earworm and western bean cutworm, Vip3A is the only effective Bt toxin available. “In the case of a target insect like that, you need to think about an integrated pest management strategy rather than relying solely on those Bt hybrids year after year,” she says.
“Some years we don’t have high populations of those pests, so you might be able to scout and spray insecticide if your threshold is reached. That type of approach would be a better way to delay resistance to any technology used to control the insects, whether it be Bt toxins or insecticides.” And if growers are using insecticides, they should rotate the chemistries.
She also reminds growers to always follow the refuge requirements for their hybrids.
Another good practice is to scout for insect damage in Bt corn and non-Bt refuges. If growers find damage by their target pest in their Bt corn, they should report it to their seed company representative or provincial specialist so the insects can be tested for resistance.
“If you have Bt fields with European corn borer damage in them, definitely try to destroy the corn stalks in the fall to kill the overwintering population,” Smith notes. “These insects burrow down to the bottom of the corn stalk to overwinter. So if you do a really good job of chopping stalks [as close as possible to the soil surface] and then burying the chopped stalks with tillage, you could kill a lot of that resistant population.”
Smith explains that crop rotation is not very helpful in managing any of the moth pests because they move around so much. “However, for corn rootworm, crop rotation is your number one strategy to reduce populations and prevent resistance. If you have corn rootworm in your cornfield one year, the adult beetles will lay their eggs in that field. If corn is planted in the same field the next year, the larvae will survive on that. But if you plant a non-host crop like soybeans in the field, the larvae will all die.”
Research towards better management
Smith and Schaafsma plan to continue their Bt resistance research, working on European corn borer, western bean cutworm, and possibly corn earworm.
In particular, they have submitted a proposal for a major study on the European corn borer resistance case in Nova Scotia. “We want to determine how far the resistance has spread so we need to do a lot more monitoring of populations to find out the distribution of the resistance,” Smith says.
“We also want to assess the susceptibility of the Cry1F-resistant corn borers to the other Bt proteins for controlling this insect to see if the insects are still susceptible to those other proteins or if there is some cross-resistance.
“And we need to learn a lot more about European corn borer in Eastern Canada. Because corn borers have been well controlled for over 20 years, they have fallen off the radar in research in a lot of ways. We don’t really have a good understanding of the populations [in the Maritimes]. We don’t know exactly how many generations per year they get. We don’t know what host crops they are surviving on there – they can feed on over 200 plant species including potatoes, apples and peppers, which are all present there.”
The researchers also want to determine if a molecular method developed in the U.S. to detect Cry1F resistance in a laboratory-selected European corn borer colony would also be applicable to the corn borer population in Nova Scotia. If the method does apply, then it could really speed up the testing to confirm whether or not resistance is present in a population.
Ultimately, Smith and Schaafsma’s research will help in figuring out the best strategies for stopping the spread of the current Bt resistance problems in Canada and preventing development of new problems, so corn growers will continue to have access to effective Bt proteins.
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