Research Highlights

Getting to Know – and Manage – Soybean Gall Midge

By Laura Temple

In 2017, Erin Hodgson, professor and Extension entomologist for Iowa State University, started hearing about a new type of insect damage in Iowa soybeans. Soybeans along field edges were wilting and dying or breaking off, causing significant yield losses. 

The culprit? Soybean gall midge. 

In 2018, a large swath of soybeans in northwest Iowa was impacted by soybean gall midge. In 2019, researchers in Iowa, Minnesota, Nebraska, and South Dakota began coordinating efforts to understand and control the new pest. 2022 is the fourth year Hodgson has focused on learning more about the insect and options to control it.

“Soybean gall midge is a frustrating pest,” Hodgson says. “Where did it come from? Is it native or an invasive species? While we haven’t answered these questions, we do know that it has adapted well to commercial soybean production. It has now spread to 35 counties in the western third of Iowa, as well as about 95 other counties in western Minnesota and Missouri and eastern Nebraska and South Dakota.”

She expects soybean gall midge to slowly spread further into the Midwest soybean-growing region over time. 

To better understand the insect’s biology and potential control options, her team is evaluating 20 to 30 replicated treatments in two locations. This research is funded in part by an Iowa Soybean Association checkoff investment. 

“We are exploring every option we can think of to see what helps suppress it,” Hodgson continues. “The more we understand the soybean gall midge life cycle and biology, the better we will be able to disrupt it to protect soybean yield.”

Life Cycle Applies Consistent Pressure

The soybean gall midge is a small fly. According to Hodgson, it overwinters in the soil. Beginning in June, adults emerge and lay eggs inside the base of soybean stems. Once hatched, the larvae feed on that stem from the inside, weakening or killing the entire plant. She has observed three overlapping generations of soybean gall midge from June to September.

“That means that adults are continuously emerging and laying eggs that hatch inside soybean stems from vegetative stages of soybean growth through pod fill,” Hodgson explains. “Because the eggs and larvae are inside stems, they aren’t visible. They aren’t noticed until soybeans start to lodge and die.”

Soybean gall midges impact field edges the most. However, just a few pests can cause severe economic injury. This research is laying the foundation for developing treatment thresholds, which outline how damaged a plant can get before it should be treated.

“We created a 0 to 4 plant injury scale for soybean gall midge similar to the scale used with corn rootworm,” she says. “We also are developing a model for yield loss prediction. These tools will help farmers and crop consultants better estimate larval infestations and expected yield losses in the future.”

Hodgson is also observing the influence of other factors on pest pressure. For example, fields damaged by hail or under disease pressure may be more susceptible to soybean gall midges depositing eggs into the plants’ stems.

“We are trying to understand why some fields are hit hard by this insect, while other nearby fields are not affected,” she adds.

Exploring Control Options and Recommendations

The Iowa State treatment trials include a wide variety of seed treatments and foliar insecticides. They are testing different active ingredients and timings to determine the potential for control.

“Most foliar insecticides used in soybean are not systemic, meaning they don’t get taken up by the plant,” Hodgson explains. “They work through direct contact with pests. Because soybean gall midge larvae feed inside stems, they aren’t impacted by most active ingredients. In 2021, our more than 25 treatments did not appear to delay or effectively suppress larval feeding.”

Thus, insecticide trials are targeting adult soybean gall midges, attempting to control them before they lay eggs. Hodgson’s team is collecting adult midges with soil emergence cages to improve understanding of their life cycle and emergence patterns. 

“Soybean breeders have also been screening for natural host plant resistance to this pest for the past four years,” she says. “Based on what we’ve learned about midge species in other crops around the world, genetic resistance is likely going to be the most effective way to mitigate soybean gall midge pressure.”

Researchers are also investigating biological control of soybean gall midge through predators like parasitic wasps, along with cultural control options like different planting dates, row spacing, tillage and more.

Although effective management tactics for soybean gall midge are limited, Hodgson offers a few recommendations to reduce the severity of plant injury based on research done so far.

• Plant soybeans last, in late May or early June, in fields that have a recent history of heavy soybean gall midge pressure.
• Scout field edges carefully for initial detection of infested plants.
• Harvest infested fields/areas early to minimize grain loss from lodged plants.

Published: Aug 15, 2022