The ‘Billion-Dollar Pest’ Reappears in Canada, Connecticut

In 2018, farmers in Nova Scotia, Canada, noticed a pest they hadn’t seen in a long time — the European corn borer. The pest was introduced to the United States in 1917 and spread to most major corn-growing regions by the 1940s. 

Coined “the billion-dollar pest,” the European corn borer caused yield loss by burrowing into cornstalks and feeding on them until the introduction of Bt corn in 1996. The main indicator of the borer is a horizontal line of shot holes in the corn leaves.

The discovery in Nova Scotia led researchers to investigate throughout Canada, where they continued to find the same Cry1F resistance in the borer as they found in Nova Scotia. Cry1F is the crystalline protein used in Bt corn that keeps larvae at bay. 

“In the years from 2018–2023, researchers started to make collections all across Canada and they were finding European corn borer resistance to Cry1F toxins all across the board,” said Kelsey Fisher, agricultural entomologist for the Connecticut Agricultural Experiment Station. 

The Experiment

Because of the Cry1F resistance discoveries in Canada, Fisher said she wanted to see if there was any resistance occurring in the United States. Fisher, who holds a PhD in entomology, said Nova Scotia was “late to the party” of using double-toxin products and was still using a single-toxin product, something that has been taken off the market in the Midwest. In 2023, she began by planting five different varieties of sweet corn in a 15-acre field at Lockwood Farms in Hamden, just north of the agricultural experiment station’s location in New Haven. 

“There is no Cry1F in sweet corn, so we couldn’t test for Cry1F resistance,” Fisher said. “We also didn’t think I was going to find anything.” 

The first two varieties, Providence and Obsession, were non-Bt corn. The third variety, Obsession Two included Cry1A.105 and Cry2AB2 traits. The final two varieties, Attribute and Remedy, included the Cry1Ab trait. Fisher and the research team did a late planting in June and monitored the corn in September. 

“When I went out in September to do my monitoring, I immediately found corn borer,” Fisher said. 

Fisher said she found evidence throughout all varieties of European corn borer in about 17% of the 300 corn plants she observed. To confirm her findings, she recruited Galen Dively, a professor emeritus of entomology and consultant at the University of Maryland, and Jocelyn Smith, assistant professor of field crop entomology at the University of Guelph (Ontario)’s Ridgetown Campus, to do a crop survey. In the same field on Oct. 20, they found evidence of European corn borer again. 

“So that was like, ‘OK, I’m not crazy, this is really happening,’” Fisher said. 

Fisher expanded the search for European corn borer beyond Connecticut in 2024 by teaming up with the Sweet Corn Sentinel Monitoring Network through the University of Maryland Extension. The volunteer-based program typically collects only ears. Fisher asked volunteers across 26 states and five Canadian provinces to split stalks to look for European corn borer. She also expanded the network to Connecticut and planted the same standard mix of sweet corn varieties as she did in 2023 along with some Cry1F seed at three separate farms. 

“When you first see something, you don’t really want to ring the alarm bells until you know what’s really going on,” Fisher said. “We saw this once, so what’s happening next?” 

Fisher also visited two of the largest corn farms in Connecticut, Laurelbrook Farm and Oakridge Dairy. With permission, she searched for evidence of European corn borer and found it only in the refuge in a bag plants, a mixture of non-Bt and Bt corn. 

Results

In the 2024 Connecticut-planted fields, Fisher said they found European corn borer larvae in all Bt plants, but in “incredibly lower” numbers than in 2023. In the Cry1F plants, Fisher and her team said they found a very small amount of tunneling and no larvae, unlike the results seen in Nova Scotia. Throughout the rest of the Sweet Corn Sentinel Monitoring Network, the results were similar. 

“We only found European corn borer in Canada and Connecticut, and the people looking for corn borers barely even found them in non-Bt corn,” Fisher said. 

Hypotheses 

Why were the results so different in 2024 compared with 2023? Fisher said the population of European corn borer is cyclical and varies year to year. However, the pest wasn’t found anywhere else besides Connecticut, which left Fisher with another question.

“The question I keep getting is, ‘Why Connecticut?’” Fisher said. “And my gut instinct was, ‘Well, because I looked.’”

Fisher’s first hypothesis for why European corn borer was found in Connecticut: They came from Canada. According to Fisher, borers can travel 6.2–49.7 miles per generation, and with two generations a year, 12.4–99.4 miles. However, she said there was another sentinel site in 2024 in Massachusetts — between Canada and Connecticut — where corn borer was not found. 

The other hypothesis Fisher presented is that Canada and Connecticut have similar selection pressures.

“We have really low row crop production, and it’s in continuous corn for silage,” Fisher said. “Farmers are selecting hybrids with low lignin, and limiting hybrids even further than if they were growing corn for any other reason.” 

The final possible explanation Fisher gave for European corn borer in Connecticut was the amount of organic crop production. As of 2022, 1,185 acres out of the total 381,539 acres in production were certified organic, but Fisher said many farms in Connecticut use organic practices without being certified. She said agrotourism and you-pick operations in the area use DiPel, a biological insecticide in organic practice. 

“With DiPel, what’s interesting is that it has Cry1A, but it doesn’t have Cry1F,” Fisher said. “So, when I’m saying I’m finding resistance to the Cry1A proteins but I didn’t see anything to the Cry1F, it could be because of organic practices.”

Recommended Practices 

Fisher said she will continue to research the background genetics of the Bt-resistant corn borer, keep planting sentinel plots, setting pheromone traps, and monitoring the situation in Connecticut. She recommended that farmers rotate crops and Bt traits to reduce continuous pressure, consider monitoring with pheromone traps, and look for signs of European corn borer by splitting stalks and identifying shot holes. 

“I’m kind of the warning person that something may be coming,” Fisher said. “So let’s be prepared before it gets here.”