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Soybean gall midge not only small orange fly larvae in Minnesota soybean fields: Another species associated with white mold

Bob Koch, Extension entomologist; James Kurle, plant pathologist; Dean Malvick, Extension plant pathologist; and Bruce Potter Potter, Extension IPM specialist

Part 1. The insect

Over the past several weeks, there have been numerous observations of tiny orange fly larvae on soybean plants infected with white mold (Sclerotinia sclerotiorum). It was recently determined that these orange larvae associated with white mold are not those of the recently described soybean pest, the soybean gall midge, Resseliella maxima. Dr. Raymond Gagne, an authority in this group of flies, examined adult and larval specimens from multiple locations in Minnesota and determined the orange larvae found in and on white-mold-infected soybeans to be Karshomyia caulicola. This species also belongs to the gall midge family (Cecidomyiidae) but is an entirely different species than the soybean gall midge.

Figure 1. Distribution of the soybean gall
midge in Minnesota as of 10/01/2019.
Karshomyia caulicola, [for the purpose of the article hereafter referred to as the white mold gall midge (WGM)], has previously been reported from several plant species in North America and northern Europe. The 2018 reports of soybean gall midge from white mold affected fields in central and southeast Minnesota were actually WGM. Therefore, the identification of WGM means that the known distribution of the pest species, soybean gall midge now appears to be limited to the southwest part of Minnesota (Figure 1).

This year, the severity of white mold across much of Minnesota has led to a large number of observations of WGM. Unfortunately, the similar appearance of the two species’ larvae can lead to confusion in identification and soybean pest management. The soybean gall midge is becoming a significant soybean pest. However, the WGM has been reported to be a fungus feeder, and likely not a pest of soybean. Additionally, we have seen no scientific reports of WGM spreading or promoting white mold infection.

WGM, might be found in all parts of the field, wherever white mold infected plants occur. It is most likely the gall midge species present when finding orange larvae both inside and outside stems, pods and other white mold infected tissues (Figure 2). Unlike soybean gall midge, WGM infestations have only been reported late in the season, after flowering and white mold infection.

On the other hand, soybean gall midge infestations are most abundant on field edges adjacent to the previous year’s soybean crop. Infested plants may be found as early as the third trifoliate stage. Symptomatic of soybean gall midge infestations is a dark discoloration at the stem base of stunted, dead or dying plants. Some plants with brittle lower stems break near the soil line. The white to deep orange soybean gall midge larvae are typically found under the stem epidermis at the base of the plant, only rarely in the stem pith, and never infesting the external soybean tissues (Figure 3).

Figure 2. Karshomyia caulicola (WGM) larvae and white mold mycelia inside a soybean stem. Photo: Joseph Moisan-De Serres. Click on photo for larger image.
Figure 3. Resselliela maxima (soybean gall midge) larvae and damage under the epidermis of a lower soybean stem. Note the dark discoloration and the lack of white mold mycelia. Click on photo for larger image.

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