Soybean Gall Midge

Soybean gall midge (Resseliella maxima) is a new pest of soybean in the Midwest. Infestations begin at and are generally confined to field edges in most fields; however, smaller fields may see field-wide infestations. Research is ongoing to develop a colony for research purposes, understand soybean gall midge biology, and develop management recommendations.

Identification

Adult: Adult soybean gall midges are tiny (1/4 inch long), long-legged flies with slender bodies (Photo 1). They have an orange abdomen and mottled wings. Their antennae and long legs have alternating light and dark bands. It is unlikely that you will see adult soybean gall midge in the field.

Egg: Eggs are not typically seen in the field and have not been described.

Larva: Soybean gall midge larvae are small maggots (Photo 2). They do not have legs or any distinct features. Larvae go through three instars. First instars are small, clear, and difficult to see; second instars are milky-white or light orange; and mature, third instars are larger and bright orange.

Pupa: Pupae are not typically seen in the field and have not been described.

Beware of look-alikes! Previous reports of soybean gall midge in some areas have been confirmed as the white mold gall midge, which feeds on internal portions of stems and pods but is associated with white mold infected plants. This species feeds on the mycelium of white mold and is not a pest of soybean.

Biology

The history of soybean gall midge goes back to 2011 when farmers in Nebraska found orange larvae feeding in stems of hail-damaged or diseased plants. Because the incidence was low and always associated with unhealthy plants, entomologists wrote it off as another secondary, opportunistic pest. Isolated reports from Iowa and South Dakota were also reported in 2015. It was not until 2016 that larvae were collected from healthy plants and injury was noted in the field. Then in 2018, we saw widespread infestations and economic injury in 65 counties in those three states. As of 2021, soybean gall midge has been found in 140 counties in 5 states (Iowa, Nebraska, Missouri, Minnesota, and South Dakota).

In 2018, a gall midge systematist determined that the orange larvae infesting soybean plants were a new species that had not been documented in other soybean production areas. It is unknown whether soybean gall midge is native or invasive in the U.S., but these 140 counties represent the known worldwide distribution. You can keep up with soybean gall midge distribution and sign up to receive alerts at https://soybeangallmidge.org/.

Entomologists are still doing research to fully understand soybean gall midge biology. Soybean gall midge overwinters as mature larvae or pre-pupae in fields that were planted with soybean the previous growing season. Adults emerge from the soil in early June and seek out the nearest soybean plants to lay eggs. Midges are relatively weak fliers, so it is assumed they only go as far as needed to lay eggs. Because of this, infestations typically begin at the field edge and expand to the field interior over the summer. Female adults use their long ovipositor to lay eggs inside of soybean stems near the base of the plant. Once eggs hatch, they develop through three larval instars inside of the soybean stem. Once mature larvae are ready to pupate, they “flick” themselves out of the stem and into the soil. Recent research suggests there may be three overlapping generations of soybean gall midge in the Midwest.

Research suggests that soybean gall midge larvae infest plants that are V2 or later. This is because cracks or fissures begin to develop in soybean stems at V2 (Photo 3), which is likely the entry point for females to lay eggs. Plants V2 or later are considered susceptible to soybean gall midge infestation.

Soybean gall midge has been primarily noted on soybean, but larvae have been found on sweet clover (Photo 4) and alfalfa as well. Very few larvae have been observed on these other hosts, and it is unclear what the ecological role of these hosts are in soybean gall midge infestations.

Plant Injury

The larvae are the damaging stage for soybean gall midge. Larvae feed on the tissues within the soybean stem, disrupting nutrient and water movement within the plant. At first, the stem may become dark and discolored (lesions) near the soil line (Photo 5). Initial symptoms can be easily confused with fungal pathogens like Phytophthora and Rhizoctonia. A gall may form, which appears as a swelling, discoloration, or outgrowth of the stem. However, galls are not always observed. Infested plants quickly wilt and die or break off at the feeding site (Photo 6). Significant yield reductions may occur, especially at the field edge.

Risk Factors

Soybean fields adjacent to a field that was injured by soybean gall midge the previous year should be prioritized when scouting. Plants that are V2 or later when adults begin emerging are susceptible to infestation. Research is ongoing to understand any other risk factors for soybean gall midge infestations.

Scouting

Soybean gall midge can usually be found in the first few rows of soybean adjacent to a field that had an infestation the previous year (Photo 7). In the first few rows, look at the base of soybean plants for a dark discoloration (lesion) at or above the soil line. If lesions are found, peel back the layers of the stem at the lesion (Photo 8). You can use a knife or your fingernail to do this since the lesion is typically soft and spongy. Look for white or orange larvae feeding at the site. A hand lens may be helpful for seeing small larvae in the stem. Continue to peel back the outer layers surrounding the lesion to look for additional larvae.

Monitoring the distribution: If you suspect you have a soybean gall midge infestation, send us photos or contact your regional field agronomist to aid in confirmation. This is especially helpful if you live in a county that is currently not marked on the distribution map. We will continue to monitor the spread and distribution of this pest throughout Iowa and the Midwest.

We encourage people to confirm soybean gall midge in the field by scouting as advised above. However, sampling for larvae is destructive and time-consuming, which may be impractical for many people. Mitchell Helton, formerly a graduate student at Iowa State University, developed an injury rating system that is correlated to yield losses, which may be used as a non-destructive sampling method in research or on farms once larvae are confirmed.

The 0-4 Injury Rating Scale: Injury ratings are associated with an approximate percentage of total injured plants (Helton et al. 2022). Table 1 shows the relationship between injury ratings and injured plants. Ratings can also be given in increments of 0.25 to describe small differences or provide more accurate assessments.

Table 1. The Injury Rating Scale described in Helton et al. (2022).

Management

Unfortunately, there are no research-based, effective management strategies to suppress larvae at this time. Multiple universities are working to evaluate insecticides (seed treatments, soil-applied, and foliar-applied) and soybean genetics for possible solutions. We are also investigating several cultural tactics to reduce yield losses. Given the biology of this pest, it is likely that insecticide application timing is critical and needs to be fully explored.

References

Helton, Tinsley, McMechan, and Hodgson. 2022. Developing an injury severity to yield loss relationship for soybean gall midge (Diptera: Cecidomyiidae). Journal of Economic Entomology.

McMechan, Hodgson, Varenhorst, Hunt, Wright, and Potter. 2021. Soybean gall midge (Diptera: Cecidomyiidae), a new species causing injury to soybean in the United States. Journal of Integrated Pest Management.

Gagne, Yukawa, Elsayed, and McMeghan. 2019. A new pest species of Resseliella (Diptera: Cecidomyiidae) on soybean (Fabaceae) in North America, with a description of the genus. Proceedings of the Entomological Society of Washington.