by Ashley Dean and Erin Hodgson

Soybean gall midge has been causing significant yield losses for farmers since 2018. Currently, the worldwide distribution includes 164 counties in 7 Midwest states (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota and South Dakota.)

There’s promise in the battle against this pest, though. Entomologists are researching ways to better manage soybean gall midge through cultural methods that includes planting dates, resistant varieties and biological control agents.

Feeding

Larvae have been found feeding on soybeans, alfalfa, sweet clover, dry beans and leadplant, although soybean seems to be preferred. The adults, which are small long-legged flies, emerge from the soil in late May to early June and lay eggs in fissures in soybean stems.

Once eggs hatch, the larvae (maggots) feed inside the stem at the plant’s base and develop through three instars. Once larvae complete development, they flick themselves out of the stem and into the soil to pupate. This cycle takes about one month to complete, and there are likely three overlapping generations each year.

Scouting tips

Soybean gall midge infestations tend to be worse at field edges, especially near where soybean was grown the previous year. Larvae feeding inside the stem results in corky stems, wilting and plant death. These symptoms develop rapidly (one to two weeks after egg laying). Our recommendation is to begin scouting at the V2 growth state and continue weekly throughout the growing season. Look at the base of the stems for a dark lesion; then, peel it open to expose soybean gall midge larvae.

Confirming larvae is important, because early symptoms look similar to seedling diseases. The orange third instars are obvious to the naked eye. If there is a history of soybean gall midge in an area, scout soybean fields along the edge that is near where soybean gall midge was found the previous year.

Black lesions can form as a result of larvae feeding, which makes stems brittle. (Ashley Dean)

Because infestations and timing of infestations vary, yield losses are difficult to predict. A few plants dying early in the season likely will not impact yield as much as plants that die after flowering. Additionally, yield losses are likely to be higher at the field edge, and small, irregularly shaped fields may be impacted differently than large rectangular fields.

Management research underway

Management has been elusive since soybean gall midge first came on the scene. That’s largely because several aspects of this pest’s biology make it challenging to contact them with insecticides They include:

Even so, research has been underway to learn more about soybean gall midge and discover tactics farmers can use.

Perhaps one of the most important discoveries was that fissures in the stem are important for infestation by soybean gall midge. If the fissures are covered, soybean gall midge will not enter the plant. Therefore, management likely relies on protecting the base of the stem after fissures develop at V2 or by altering planting dates. This helps ensure that fissures are not present when adults emerge and lay eggs. The University of Nebraska has had success shifting planting dates a few weeks later, but more studies are needed to confirm the best timing in a particular year.

Iowa State University has also helped with a germplasm study for several years to identify varieties that resist soybean gall midge. Although complete resistance doesn’t seem likely, we have identified genetics that tolerate soybean gall midge that are being advanced through more rigorous trials. However, they are not yet in commercial varieties.

Lastly, biological control agents — including parasitoid wasps and ground beetles — have been identified in Iowa and Minnesota that likely contribute to suppression of soybean gall midge.

Continued research into these areas, as well as other cultural tactics that include various forms of timing of tillage, will further our understanding of soybean gall midge and provide better future management recommendations.

You can stay up-to date on soybean gall midge research with annual seminar here.

Call to action

If you discover soybean gall midge larvae in a county that is not marked as infested on the distribution map, please reach to the author or your local Extension entomologist to help us track the distribution of this pest.

Dean is an ISU Extension specialist for field crop entomology. Hodgson is an ISU Extension entomologist.