Written by Bruce Potter, UMN Extension IPM specialist
Don’t assume that wilting or dead plants are due to disease, compaction, or drought while checking soybean fields this year. A close look could show that the damage is from one of Minnesota’s newest insect pests, the soybean gall midge (Figure 1).
Before 2018, any tiny orange gall midge larvae found in soybean stems were thought to be secondary invaders of injured or diseased plants. That year, however, an outbreak of these larvae caused significant yield loss to soybeans in portions of NE, IA, SD, and MN and larvae were found in plants that did not have mechanical injury or plant disease. Since then, the “new to science” soybean gall midge (SGM) has continued to cause damage. Larvae have been observed in five states and 115 counties, including sixteen counties in Minnesota. (Figure 2). Up to 100 % yield losses on field edges are known with losses usually decreasing toward the field interior. Fortunately, most of the known infestations in Minnesota have not yet been yield limiting.
SGM damage is greater on field edges next to the previous year’s soybean crops (Figure 3). Symptoms appear after the third trifoliate stage. The lower stem becomes discolored and may be calloused (Figure 4). There is often a dark border between SGM-injured and healthy soybean stem tissue. Heavily infested plants wilt and die and stems can become brittle and may break near the soil line. Without closer inspection and looking for larvae, these symptoms can be mistaken for disease.
SGM adults emerge from previous year’s soybean fields for several weeks in June. Adults move to adjacent soybean fields where females lay eggs in cracks or wounds in stems. A generation takes close to a month and three adult flights occur during the growing season.
The SGM larvae can be found by peeling back the outer layer from the stem of symptomatic plants. The young larvae are pale and become bright orange during the 3rd and final larval stage. The mature larvae are less than 1/10 inch long and drop to the ground to pupate in the soil. The late summer generation overwinters in the soil.
However, orange larvae found associated with white mold are most likely the white-mold gall midge, a non-pest species that feeds on the fungus.
Soybean checkoff funded research has provided much of what we know about this insect. To understand how to effectively manage this insect, extensive research efforts to understand SGM midge distribution, host range, behavior, physiology, and identify effective chemical, biological and cultural control practices are underway or planned. Effective management of this pest will likely require an IPM approach because no single management tactic has been found to be adequately effective.
Please let Bruce Potter (firstname.lastname@example.org) or Bob Koch (email@example.com) if you think you have found soybean gall midge in your fields. A clear photo of plant symptoms and larvae can be used for diagnosis.