Research HighlightsGall midge war continues
Research ongoing to determine treatment options
By Bethany Baratta, Iowa Soybean Association senior writer
Iowa Soybean Association (ISA) member Adam Bierbaum and his dad Brent suspected a problem in their soybean fields in early July last year.
“We started seeing soybeans die on the edge of the soybean fields,” Adam says. “On the outside borders of the fields were strips of wilted soybean plants.”
They didn’t know how bad the problem was until they consulted with a variety of agronomists searching for answers.
The Griswold, Iowa farmers called agronomists at Iowa State University (ISU), which ruled out herbicide injury and sudden death syndrome. Splitting open stems, ISU agronomists confirmed the plant injury was the result of soybean gall midge.
“It was a pest we never heard of before, so that was something new for us,” Adam says. “It was easier to spot damage when we knew what we were looking for.”
The pest ate its way about 60 feet into the field and took out half the yield of that entire field.
This year, the war on soybean gall midge continues in the state and on the Bierbaum farm.
Confirmations of soybean gall midge were reported in 2011 from Nebraska and in 2015 from South Dakota. In 2016 and 2017, there were isolated reports of soybean injury by soybean gall midge in northwest Iowa. There was significant field edge injury and economic loss in at least 65 counties throughout Iowa, Nebraska, Minnesota and South Dakota last year.
Since finding the pest in Iowa fields, not much is yet known on the life cycle of the pest or how to control its emergence and spread.
“Despite everybody’s efforts to come up with a strategy to control them, there doesn’t seem to be a silver bullet yet,” says Drew Clemmensen, regional agronomist for ISA. Clemmensen has been working with farmers in western Iowa, an apparent hot spot for soybean gall midge infestations.
Soybean gall midge adults are identifiable by their distinct black and white banding pattern around their legs. But it’s the larvae that are doing damage inside the soybean plant stem. Soybean gall midge larvae are clear and turn bright orange as they mature.
“When the eggs hatch, larvae feed inside the stem, which essentially stops the movement of nutrients and water to the plant,” Clemmensen says.
Midges are weak fliers, which means gall midge infestation is slow to spread. However, it’s been difficult to control adult midges before they lay their eggs.
Erin Hodgson, associate professor at Iowa State University (ISU) and ISU Extension entomologist, has been researching the pest, trying to understand its life cycle.
Hodgson and her team set up adult emergence cages in fields planted to corn last year, suspecting the midges were overwintering in the soil.
“Our findings in the cages validated what we suspected,” Hodgson says.
The team discovered that adult emergence was over a two-week period, which makes targeting the adults before they lay eggs more difficult.
“In some of the research plots we set up, soybean plants went from looking really good to dead and dying within 10 days,” Hodgson says.
ISA field trials show that wilting plants are a sign that the midges are moving through to the next row, Clemmensen says.
“By the time you see the symptoms, midges are definitely present,” he says. “If your first row is starting to wilt, midges have moved on to the next row and are already feeding.”
Attempts to control pest
The Bierbaum family skipped their crop rotation this year, keeping corn on some acres in an attempt to suppress the pest. But the midge flew to nearby soybean fields to feed.
“It started out as a small area, and then we saw it more throughout the season. We’re seeing it in fields this year where we didn’t see it last year. Some of that, though, is that we’re better at identifying it,” Bierbaum says.
ISA’s On-Farm Network® team has been working with farmers, including the Bierbaums, to see if seed treatments or insecticides are effective in controlling the pest. So far, neither seem to be effective.
“Insecticides are usually sprayed over the top of plants, so the pest has to be somewhere out in the open where it comes in contact with insecticide. When the adults emerge out of the ground and lay eggs on the stem, there’s not a whole lot you can do to get a product in contact with them,” Clemmensen says.
He notes that no insecticides have been labeled for use in controlling soybean gall midge. ISA has been using the insecticides within a research/emergency declaration to research their efficacy.
Working with both ISA and ISU, the Bierbaums have tested a variety of seed treatments and insecticides to investigate responses.
“The trials have been inconclusive so far. It’s difficult to determine the response because plants are still dying,” Bierbaum says.
Hodgson and her team continue to check emergence traps to monitor the spread of the pest. Efficacy evaluations are also ongoing to see how seed treatments and insecticides could prevent injury from the soybean gall midge.
“We haven’t found the magic yet,” Hodgson says. She hopes information the team gathers this year will help to determine a mode of action for future growing seasons.
For more information:
Soybean Gall Midge in the SRIN Resource Library