Living on the Edge with Soybean Gall Midge

• Soybean gall midges are characterized by the bright orange color of third instar larvae and rather unremarkable adults with striped legs.
• In soybeans, the pests are tough to find and difficult to manage since adults spend much of their time near the soil surface.
• The economic impacts are largely unknown, widely variable, but highly devastating when populations are high in a given geography.

A new species of miniscule fly in the family Cecidomyiidae is creeping along the edges of Midwestern soybean fields. Soybean gall midge begins its life cycle as a small, legless, clear- to white-colored maggot that turns bright orange as it matures. Adults are characterized by mottled wings, an orange abdomen and long-legged, slender bodies.

Orange larvae suspected to be soybean gall midge were found in 2011 by Nebraska farmers; they noticed the larvae at the base of a soybean plant’s stem. At the time, it was considered a secondary pest. That changed in 2018 when infestations were observed earlier, in higher numbers and without injury or disease present. As of April 2023, soybean gall midge had been found in 155 counties throughout Nebraska, South Dakota, Minnesota, Iowa and the northeast edge of Missouri.

“I first saw what we think was soybean gall midge in 2016,” says Justin McMechan, Ph.D., crop protection and cropping systems specialist and assistant professor at the University of Nebraska Extension. “I was called to a field that had orange larvae at the plant base and no signs of death or wilting. The extension educator asked me if there was a concern, and I didn’t think so because we only saw them on mechanically damaged or diseased plants. It didn’t cause economic yield loss, so no need to worry.”

In 2018, McMechan was told that the orange larvae were back in high numbers at the base of soybean plants — this time accompanied by dying plants. “It appeared the larvae were girdling the plants, so it was now a very different scenario,” McMechan says. “We got on a call with several entomologists and learned it was also being observed in South Dakota and Iowa.”

After sending samples off to Maryland and Japan for morphological characters and genetic testing, the pest was confirmed as a new species by Raymond Gagne and Junichi Yukawa. In 2018, it was classified as soybean gall midge, or Resseliella maxima. However, according to McMechan, along with an identification came numerous questions about the pest.

At the edge of a field, the yield impact can be severe – up to 100% because they can completely kill plants – but then as you go further into the field, it’s typically less severe. It really depends on the year and the field size/shape.

Soybean Gall Midge Scouting

Dean Grossnickle, technical development lead at Syngenta, says while soybean gall midges are not known to be aggressive flyers, powerful Midwestern winds could potentially blow this small fly into new fields. According to the Soybean Gall Midge Alert Network, 15 new infested counties were identified in 2022. Continued tracking and scouting is key to understanding the spread and impact of this pest.

To scout for soybean gall midge, Ashley Dean, education extension specialist at Iowa State University, recommends:

• Start looking at the adjacent edges to where soybeans were planted the previous year for plants that are wilting or dead
• Look at the base of the stem for a lesion that can be jet black, soft and mushy
• Peel the lesion open and look for bright orange larvae or other evidence of midges

Yield Impact

Soybean gall midge larvae feed in a tiny zone at the base of the plant – 1.5 to 2 inches from the soil – where nutrients and water flow into the plants. As they feed, they cut off water supply and stop nutrient flow, causing plants to slowly wilt and die. McMechan says it happens as soon as 21 days from infestation, outright killing soybean plants within three weeks.

However, there are still questions around exactly how much economic yield impact the pest has.

“The yield impact varies annually, but it can also be variable within a field because it tends to be an edge pest,” Dean says. “At the edge of a field, the yield impact can be severe – up to 100% because they can completely kill plants – but then as you go further into the field, it’s typically less severe. It really depends on the year and the field size/shape.”

“A couple of years ago, I saw an 80-acre field in Nebraska that pulled off just three bushels, so it was completely decimated,” Deane Jorgenson, Ph.D., marketing lead at Syngenta says. “But I’ve also seen an 80-acre field that had 30 bushels. The farmer didn’t even realize he had gall midge populations present because he saw green plants with lots of leaves – just no pods.”

Jorgenson says that when farmers don’t realize there’s an issue until the damage is done, that’s a real problem.

Unanswered Questions

Mitigating risk near previously infested areas is a key consideration. Management is difficult because there are still many unknowns with soybean gall midge, such as how many seasons they can survive in fields not planted in soybeans.

Researchers are investigating biology, ecology and potential management solutions. Management may include moving back planting dates, crop rotations, hilling at the base of the soybean plants to cover V2 fissures, resistant varieties and chemical management. Studies will be ongoing for the near future.

“We’re trying to find a solution, but there is a lot of stuff we don’t know yet,” Grossnickle says. “All we can do is put our best foot forward and implement some solutions, even if it sometimes feels like spitting in the wind. It’s going to take that enterprising farmer and that enterprising researcher to find the answer.”

From a chemical pest management perspective, Jorgenson says a multi-pronged approach seems like a potential solution, but unknown population dynamics are impacting studies.

“A multi-pronged approach that overlaps systemic seed treatments, like CruiserMaxx APX seed treatment, and foliar insecticides, like Warrior II with Zeon Technology, Besiege and Endigo ZCX, seems like it would work to manage both the larvae and the adults. We are still investigating and evaluating efficacy,” Jorgenson says. “However, that approach can be expensive to a farmer, so it’s really important to understand how the pest moves, where the pest may show up, and how far into the field the pest might penetrate so that a farmer can be very thoughtful and ensure they are using the best products.”

Jorgenson says that Syngenta values its partnerships with university extension. “We at Syngenta understand the value for the American farmer and invest in research and development to support the local communities where soybean gall midge is present,” she says.

What does that mean for the future?

“We need to really understand what makes soybean gall midge tick to know what areas are at risk each year,” McMechan says. “As long as we keep collecting data, we’ll start narrowing down on what the risk is. It’ll take a long time. Soybean gall midge may dial back as a problem, but it’s highly likely we’re going to find it in soybean fields for the rest of my career.”