Bright orange soybean gall midge larvae feed on a soybean stem in a field in western Iowa. (Photo: Joseph L. Murphy/Iowa Soybean Association)

Soybean gall midges, thistle caterpillars spotted in Iowa

June 26, 2020 | Bethany Baratta

Soybean gall midge and thistle caterpillar have been found in soybean fields in Iowa, and agronomists say farmers need to scout their fields and keep tabs on pest activity.

“We’re on the early side of these pests, but both have been found,” said Drew Clemmensen, Iowa Soybean Association (ISA) agronomist.

For much of the state, the instances of these two pests are low and farmers are in a scout-and-wait pattern before meeting the threshold to use insecticide. But continued scouting could determine the need to treat affected fields.

Soybean gall midge

After overwintering in Iowa, the first capture of an adult soybean gall midge this season was found June 12 in northwest Iowa. If previous years are any indication of the soybean gall midge life cycle, farmers can expect feeding to begin 7 to 10 days after identifying midge adults.

“I’ve started seeing the midges around here, unfortunately, they didn’t go away over the winter,” said Adam Bierbaum, an ISA farmer member near Griswold.

It’s the eggs the gall midge adults lay—eventually larvae—that do the damage inside the soybean plant stem. When the larvae hatch, they eat the tissue around the stalk, which essentially stops the movement of nutrients and water to the plant, according to Clemmensen.

Bierbaum and his dad Brent know too well the damage soybean gall midge can do to soybean fields and yields. In 2018, the pest ate its way about 60 feet into their soybean field and robbed about half the yield on that field.

Gall midge infestation is slow to spread as midges are weak fliers. However, it’s difficult to control adult midges before they lay their eggs.

The Bierbaums have set up emergence traps and have tested various seeds and insecticides to determine the best way to prevent and control the overwintering pests. Last season, the Bierbaums saw gall midge larvae in more fields, but less damage than the year before. Still, there are a lot of questions.

“I’m not sure we have a very good handle on what it takes to manage them yet,” Bierbaum says.

He hopes various trials this year through Iowa State University and ISA will provide more answers.

ISA Agronomist Drew Clemmensen says ongoing trials are helping farmers and researchers learn more about yield-robbing pests like the soybean gall midge and the thistle caterpillar. (Photo: Joseph L. Murphy/Iowa Soybean Association)

Meaghan Anderson, an Iowa State University Extension agronomist, says scouting is the best first step in determining if there is an issue, especially in areas where the soybean gall midge has not yet been identified.

“We need to be out in our fields looking along those edges for wilting, dead, dying, ill-looking plants and checking out the base of the stems right along the soil surface for injury consistent with soybean gall midge,” Anderson said.

Soybean gall midge was identified in more counties last year, signaling a spread from its typical presence in western Iowa.

Thistle caterpillar

Thistle caterpillars have arrived in Iowa after overwintering in the southern United States and Mexico. The caterpillars are similar to the gall midge in that they start eating from the edge of the field inward.

“They defoliate soybean leaves, folding them up like white webbing,” Clemmensen said.

Damage associated with the thistle caterpillar was prevalent last year, according to Anderson.

“Thistle caterpillars just voraciously ate soybeans last year,” she said.

The threshold for insecticide use is 30% defoliation in the vegetative stage (before flowering). The threshold is lower—20% defoliation—if in the reproductive stage (flowering to pod fill).

“Based on what we’re seeing right now, I don’t expect to see thousands upon thousands more of these in July, which is when most damage occurred last year, but certainly there is a chance of pockets of higher populations,” Anderson said.

Japanese beetles will likely be making their emergence within the next few weeks and will join thistle caterpillars to defoliate soybeans. If weather conditions remain relatively dry and warm, there’s a potential to see other pests and diseases like spider mites, soybean aphids, or soybean cyst nematode more than other years, Anderson says.  

She reminds farmers to consider all types of species when considering treatment options. Calling an ISA or ISU agronomist for assistance in scouting is also recommended.

“We want to keep an eye on all of the defoliators when we’re out there scouting for thistle caterpillars,” Anderson said. “Having a second or even a third set of eyes to help scout is really good.”

Contact Bethany Baratta at