WORTHINGTON — A tiny orange fly with banded legs has advanced into southwest Minnesota, laying eggs near the base of soybean plants — and when they hatch, larvae devour the stem, leaving plants weakened, wilted or simply dead.

Meet the soybean gall midge, a nasty insect that made its way into southwest Minnesota in 2018.

“It’s a devastating little pest if it gets established and you have a high infestation rate,” said Bruce Potter, University of Minnesota Extension integrated pest management specialist at the Southwest Research and Outreach Center near Lamberton.

The soybean gall midge is also new, and that means researchers are still investigating exactly how it lives and how to fight it.

“We first discovered that it was actually a pest of soybeans in 2018,” said Potter, explaining that previously, researchers had believed it was coming in after plants were already injured. But when midge numbers blew up, scientists determined the insects themselves were causing the damage. “It was a new insect to science, so it had never been identified before or described before.”

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In 2018, the midges had infested fields in Rock County in southwest Minnesota and Lyon, O’Brien and Clay counties in northwest Iowa. In 2019 they moved into Pipestone, Murray, Cottonwood, Nobles and Jackson counties in Minnesota and in 2020 they were found in Osceola and Dickinson counties in Iowa too. This year has already seen them spreading east into Emmet County in Iowa, and Martin County, as well as other Minnesota counties to the north.

A mystery midge

Soybean gall midge larvae are believed to spend the winter in cocoons in the soil of infested fields, according to a University of Minnesota Extension webpage written by Potter and Extension entomologist Robert Koch, who is also working on the gall midge problem.

The insects pupate in the spring, start emerging in June, and as their life cycle takes about a month, there are usually three larval generations a year.

Because of the species’ newness, multiple teams of scientists are trying to learn all they can about the midges, a task made more difficult because they can’t yet raise them in the controlled environment of a lab. Some are examining what alternate hosts the midges prefer. Others are looking at various types of insecticides, and the relationship between the flies and tillage. Even the type of vegetation on the field edge could turn out to matter.

“Soybean aphids came in 2000 or 2001, I guess, but at least we could look to Chinese literature … a lot of that work on the biology had already been done,” Potter said. “This is brand new, so it’s all from scratch.”

Researchers aren’t even sure if the soybean gall midge is a native species that adapted to soybeans — entirely possible, since it can use sweet clover as a host without killing the plant, and has even been found in alfalfa — or a species introduced to the area from elsewhere.

Along with Potter and Koch, entomologists in South Dakota, Nebraska and Iowa are working on the problem.

The damage done

Different tactics for controlling or eliminating the midges are still being studied, but for farmers, a lot depends on how heavy the infestation is, and how far it has spread, Potter said.

“No single tactic looks like it’s providing total control,” he warned.

In some areas, people are trying insecticides to control the adult flies, made difficult because there are multiple flights of them each year. Researchers are also looking for varieties of soybeans that can better resist the midges, and studies on seed treatment and insecticides are on the way.

The damage the midges do depends on how large the plant is when it’s infested and how long the midges are in the plant, Potter said, and damage can range from barely noticeable to plants that die in July.

“If the symptoms are severe, you’ll notice it,” he said, because that first spring generation of midges will move from last year’s soybeans into the current year’s soybeans — meaning infestations are typically worst at the edges of a field.

Affected plants are often visibly stunted, and because of their weakened stems the wind can knock them down more easily than healthy plants.

“If you take those plants and peel back the outer layer of the stem you’ll see the larvae,” Potter said. “They start out white … the mature larvae are bright orange, so they’re pretty noticeable if they’re in the plant.”

How much the pests affect yield depends on how bad the infestation is, and because of how they spread, the damage is usually worst at the edge.

“People tend to ignore things until it affects them,” Potter said. “If you wait with this insect, and you’re not paying attention, the first year you find it can be pretty expensive.”

He encouraged farmers to keep an eye out this fall for stunted plants, and check the base of the soybean for symptoms of midge damage.

Anyone who finds the midges, especially in a county where they haven’t already been confirmed, should let the researchers know by contacting bpotter@umn.edu, koch0125@umn.edu or (507) 276-1184.

For more information, visit extension.umn.edu and search for “soybean gall midge,” or visit the Soybean Gall Midge Alert Network at soybeangallmidge.org.