Western Bean Cutworm An Emerging Pest In Ontario

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Edible bean growers are being warned the western bean cutworm is headed their way.

“You’ve never heard of it, and I hope you never do,” agronomist Chris Gillard told attendees at the recent Manitoba Special Crops Symposium. “But I think it’s coming.”

The cutworm, an ongoing pest problem in the southeast corner of Nebraska for the past six decades, began moving north in 1999, reaching Michigan in 2006 and Ontario in 2008.

“It’s a corn and dry bean pest, and it moved very quickly,” said Gillard, a professor at the University of Guelph.

The pest overwinters in cavities in the soil, pupates, emerges as an adult moth, lays eggs, hatches and then begins feasting on flowers and pods. The crop damage window runs from August to early September.

In beans, the damage was strikingly obvious when Gillard put up a slide.

“As you can see, they chew big holes in the seed which can then become a site for fungal infection,” he said. “In corn the damage is similar to what you’d see from corn borer.”

Finding the pests is a challenge. Gillard described scouting for eggs and larvae as “nearly impossible.” The established technique is to use milk-jug traps with pheremone baits to capture adult moths – which don’t cause damage – and count them until the number of moths reaches its peak and begins to recede.

“You want to spray 10 to 20 days past the moth peak and kill the larvae,” Gillard said.

Control thresholds

Another wrinkle is the fact there aren’t well-established threshholds for control applications. The original Nebraska threshold says risk remains low when there are less than 700 moths a week caught in traps and no action is recommended. When levels are between 700 and 1,000, it is deemed moderate and applications are considered on a case-by-case basis. Only when levels climb over 1,000 moths is the risk considered high.

However, Michigan growers are finding they’re getting damage as high as five per cent in bean crops with trap counts between 44 and 125 moths.

“We think those threshholds are way too high, but we don’t know for sure,” Gillard said.

He said the highest infestation levels were found at sandy sites, and he thinks that’s because the cutworm can more easily dig into the soil and overwinter. He also cautioned Manitoba producers not to assume the pest wouldn’t survive our cold winters.

“You’re right, this isn’t Nebraska,” he said in response to a grower query. “But Nebraska does get very cold and they don’t get a lot of snow to act as insulation – if they can survive there, they can probably survive here, too.”

He also told growers the Prairies is at risk for another reason – the availability of peas, another preferred host crop.

“They love peas,” he said. “We harvest ours as fresh market peas before they emerge. But you don’t harvest them early and you grow a lot of peas out here – hundreds of thousands of acres to the west, correct?”

Gillard said the time to begin monitoring is now.

“Put up traps in the bean and corn areas next year,” he urged. “They’re moving and you need to know if they start coming in.”

Gillard said he’s been taken aback at how quickly the problem has arisen.

“I’ve been doing this kind of work for 20 years, and they scare me more than anything I’ve ever worked with,” he said.





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