Farmers have few options for soybean gall midge control

A new pest to the Midwest, soybean gall midge has proven difficult to manage for farmers. First noticed in 2011 and officially identified and named in 2019, the soybean gall midge has spread through 155 counties in Iowa, Nebraska, Missouri, Minnesota, and South Dakota. 

Thanks to overlapping generations and the ability to infest soybean fields throughout the growing season, tried and true methods for managing other pests have little effect on soybean gall midge. 

About the midge

Adult soybean gall midges are hard to see with the naked eye. About two millimeters long, the long-legged fly has an orange body and hairy, mottled wings. Like other flies, they go through complete metamorphosis with four distinct life cycles: egg, larvae, pupa, and adult. 

“Over the past few years, we have confirmed they have two to three overlapping generations,” says Erin Hodgson, an Iowa State University entomologist. “This is a challenge because you could be finding soybean gall midge for 60 to 100 days, and it's a kind of continuous production.”

Most concerning to farmers is the larvae, which develop through three instars and feed on tissues within the soybean stem. 

“At its most mature, the soybean gall midge is bright orange and it uses abdominal muscles to flick itself off the plant,” Hodgson says. 

The midges overwinter in the soil and emerge as temperatures warm, looking for new soybean fields to lay eggs. Soybeans at V2 or later are most susceptible to infestations. 

“After that point, they’re able to infest soybeans through the season,” Hodgson says. “You could have a clean field with clean plants and get infested right at the end of the season. That’s hard to see because a plant that’s heavily infested and basically dead can be held up by the other plants.” 

Scouting the pest

Due to the pests' relatively weak flight patterns, scouting should be done in fields that are closest to last year's soybean crop, and focused on the first eight to 12 rows. Scouting for larvae is genuinely more successful than scouting for adults, because they’ll be easier to see with the naked eye. 

“If I’ve never seen soybean gall midge on my farm, I would probably wait to scout until July, when there’s greater odds of seeing movement from the first generation,” Hodgson says. “If I was confident I had it last year, I would start looking at V2. Then I’m going to scout weekly because injury can happen very quickly. You can have a plant go from healthy to dead in 15 to 18 days.” 

Soybean gall midge feed just above the soil line, leaving the base of the soybean stem swollen or cracked. In severe infestations, a lesion or gall may form. 

“If you’re on the leading edge of infestations, I encourage farmers to split stems and look on the inside for those larvae,” Hodgson says. 

Management Obstacles

For most row crop pests, insecticides are a go-to management tactic, but they fall short for soybean gall midge. Adult soybean gall midges don’t feed on the soybean plant, making them difficult to target with chemical applications. 

“The adults are small, fragile, and they're flying for a long period of time, so they're not being impacted by sprays that we would apply to soybeans,” Hodgson says. “The larvae are somewhat protected by being inside of the stem. Most of the products that we use in field crops work through contact or ingestion. They’re not reaching the larvae so it’s not impacting their development.” 

The overlapping generations of soybean gall midge also create challenges. 

“There’s not a specific time point on the calendar or growth stage where we can focus our efforts,” Hodgson says. 

Controlling the pest

Experts at several universities have started the process of researching new management techniques, but it's too early to make steadfast recommendations, Hodgson says. 

Beyond insecticides, researchers have looked at opportunities to control the pest through genetic resistance potential, later planting dates, and increased tillage. 

• Genetic resistance: “It takes about 10 years to find host plant resistance genes,” Hodgson says. “It takes a long time to discover what genes the midges either don’t like or don’t do as well on. It’s an ongoing process that I’m really excited about.” 
• Planting dates: While some evidence shows a later planting date may help reduce plant injury, the yield reduction may counteract any benefit. “There are agronomic reasons why people don't plant past June 1,” Hodgson says. “You’d have less midges, but you also can’t reach that 100% yield potential.” 
• Tillage: Soybean gall midges are found in the top 1.5 inches of the soil, prompting several studies on the ability to control the pest through tillage. “I think it's too soon to tell, and I don't necessarily want to encourage more tillage in Iowa for several reasons,” Hodgson says. 

Today’s best practices

For now, farmers in highly susceptible areas should be diligent in their scouting efforts. Making harvest plans around infested fields can help make the most of that crop. 

“If you have heavy infestations, those areas should be harvested first because you're gonna have grain that is senesced or nearly senesced that’s held up,” Hodgson says. “Over time, you lose that grain. Even if you have to harvest the edges first to pick up what you can and then come back for the majority of the field, that can help pick up a bit of grain savings.”