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Migration patterns bring armyworms to Midwest

Migration patterns bring armyworms to Midwest


Fall armyworm prefers green plant tissue, which they consume quickly.

A devastating pest has found its way into the region this fall, and fields left untreated may have seen damage already.

Fall armyworm has been reported in Iowa over the past several weeks after doing damage in Illinois and Missouri. Iowa State University entomologist Erin Hodgson said this is one of the worst years she’s seen for the pest.

“I’ve never seen it like this before,” Hodgson said.

She said armyworm typically overwinters in the southeastern U.S. and Texas, and there were high populations this season, leading to increased migration activity. She said the pests usually move on storm patterns or prevailing winds. While the recent hurricanes in the Gulf of Mexico may not be directly to blame, it certainly has helped movement.

“They usually move up north every year, so there must have been a storm to not only bring up fall armyworm but corn earworm and black cutworm,” Hodgson said. “It might not have been hurricanes specifically, but that combination of storms and population brought them to the Midwest.”

The further north they go, the more unfamiliar with armyworms farmers are, said Hodgson. They prefer green plant tissue, which they can consume quickly, and then they move on.

“They can show up in really large numbers and they are pretty mobile,” she said. “You could have a field or part of a field that is in good shape and then in 24 to 48 hours be consumed. That’s where the name comes from.”

Hodgson said fall armyworm has a “wide host range,” meaning they will feed on many different plants. They might attack and disrupt golf courses, athletic fields, home lawns or pastures. That makes singling out specific treatments difficult, but he said in some cases a mechanical approach might be the best bet.

“Cutting alfalfa or other forages used for animal feed is a really effective tool,” she said. “Then they don’t have anything to feed on or do. For corn or soybeans that’s not an option, so you have to think about the intent of the crop and what it will be used for.”

For those using applications to treat armyworm, she said pyrethroids are a good option, using a high volume and pressure to create small droplets “that ensure contact with the larvae.”

While armyworm has become an issue in the Midwest this year, Hodgson said it may not persist into next year. Armyworm isn’t able to survive the cold temperatures found in more northern regions.

Hodgson compared the 2021 presence of armyworms to the major thistle caterpillar outbreak from 2019. What pest comes into the region is simply based on migration patterns. It’s hard to determine how that will play out next season.

However, until those cold temperatures arrive, expect armyworms to remain active, meaning farmers need to continue scouting their fields. Hodgson suggests scouting early in the morning or late in the day, which is when larvae are most active, and to start in the most valuable crops first. If they are found in a field, treatment will need to be quick.

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