Soybean gall midge, a new insect pest that’s shown up in western Iowa, eastern Nebraska and eastern South Dakota in recent years, expanded its range eastward in 2019. In Iowa, it was found infesting some fields as far east as Madison County, closer to south-central Iowa. Previously, in 2018, it was confirmed in 16 counties in western Iowa. Midge was first observed in northwest Iowa in 2015.
Soybean gall midge is a slender, long-legged fly with hairy wings and long antennae. It lays eggs to produce larvae that burrow into soybean stems. Eventually, infested plants become brittle and break off, resulting in plant death. Larvae are observed feeding in plants from June through August, suggesting there are multiple, overlapping generations possible in Iowa. Infested plants have swollen stems just above the soil line.
In 2019 in Iowa, three generations with larval feeding from V3 to R6 soybean growth stages were detected. Fields adjacent to previously affected soybean fields and early-planted fields experienced more severe plant injury. Injury from gall midge is often in isolated locations in the field and expands with multiple generations.
Pest difficult to control
A real concern is the midge problem eventually spreading across the state. The challenge is once a population is established, no control measures are available. Most growers haven’t experienced a soybean midge problem at damaging thresholds like we have in western Iowa. An often-asked question is how can we control this pest? Does it do any good to try to spray insecticide once you see midge infesting plants in a field?
Working with Iowa State University Extension entomologist Erin Hodgson, we had research and demonstration trials in southwest Iowa in 2019. Unfortunately for soybean growers, none of the insecticides were effective at controlling midge. Once the larvae burrow into the plant, they attack tissue inside the stem and the plant shuts down. If the pest is infesting in high enough numbers, they kill the plant.
As for controlling the adult population, spraying an insecticide to kill the flies before they lay eggs doesn’t work very well. The flies lay eggs over a long period of time, and the eggs hatch and emerge over a long period of time. Trying to control the adult stage of the insect isn’t as successful as we would like it to be.
Watch soybeans for symptoms
Midge is a pest specific to soybeans. There are hundreds of species of plants out there. Why midge has chosen to infest soybeans, we don’t have an answer yet.
So, what do we recommend to control or manage midge? That’s a difficult question to answer. In areas with high infestations and where we’ve had a lot of soybean plant loss, using crop rotation and rotating away from soybeans for at least a couple of years may be an option for some producers.
We need more research. We really don’t know yet how to control this midge. We’re still in the learning phase together with farmers. The key thing farmers need to do is watch for symptoms of midge feeding damage on soybean plants in the outer rows around the edges of fields and learn how to identify this pest correctly.
For more information, read the ISU Extension’s Soybean gall midge: a new field crop pest, Crop 3157.
Saeugling is the ISU Extension field agronomist in southwest Iowa. Email email@example.com.