Experts from Nebraska, Iowa, South Dakota, Minnesota and Missouri have been dealing with a most troubling insect in recent years. The soybean gall midge has been disseminating since last year, and efforts to curtail its spread have yet to prove effective.
“Midges have been confirmed in 95 U.S. counties so far, spread over five states,” University of Nebraska-Lincoln Extension entomologist Dr. Thomas Hunt said. “The third generation is happening now in western Iowa.”
The initial pattern of diffusion in 2018 seemed to follow the Missouri River corridor from Minnesota and South Dakota through eastern Nebraska and western Iowa into northwestern Missouri, said Dr. Erin Hodgson, associate professor and extension entomologist at Iowa State University.
“This year, it is more spread out and not in a particularly logical pattern,” she said. “The adults are flies and mobile, so I assume they are flying short distances to new fields.”
Soybean gall midge has been confirmed in 19 new counties this year, said Adrian Duehl, agronomic solutions manager and early development insecticides expert for Bayer CropScience.
“This is a newly described species,” said Robert Wright of UNL Extension. “We know relatively little about its biology, so it is hard to speculate about its distribution.”
Detection seems to be the key to trying to manage these pests. Gall midge infestations are usually concentrated along field borders, said UNL Extension entomologist Justin McMechan. While plant death is most noticeable at field edges, thorough scouting of crops is highly recommended.
“Be sure to scout all of your soybean fields regardless of whether you see any damage from the road,” said Duehl. “We’ve observed a number of fields that appear healthy from the field edge but are infested with soybean gall midge.”
In many cases, the dead or dying soybean plants in these fields have been covered over by healthy plants, he said. Be sure to examine plants that are still green for soybean gall midge larvae. The midge larvae range in color from white to bright orange.
Infested plants have larvae aggregated at the base of the plant – they feed on the inside on the stem, said Hunt.
“The soybean gall midge doesn’t produce as much of a swelling gall as some other insects,” said Dr. Kevin Rice, University Missouri Extension entomologist. “The primary symptoms are a black fungus look-alike coloration at the base of soybeans near the soil.”
To accurately assess if there are indeed soybean gall midge, peel back the outer layer of skin and look inside the stem for bright orange nymphs, he said. Unfortunately, the soybean gall midge is hard to treat. Once larvae are hidden inside the stem, there is no way to reach them and the adult flies had a prolonged emergence period this year, which can outlast insecticide applications.
“Insecticide applications are difficult at best because you have to control the adults before they lay eggs and it’s hard to predict the timing because of a long emergence period,” said John Wilson, UNL Extension educator for Burt County. “You can’t control them all at once or with one application.”
Extended emergence will greatly reduce the likelihood that insecticide will have enough residual activity to control soybean gall midge.
Bayer is testing a number of current and developing products on the insect. It is a hard to reach the pest because it feeds in the stem and it is also part of a genus that is not a major pesticide target, so there are not many products able to control the insect.
How does the gall midge compare to soybean aphids, bean leaf beetles, Japanese beetles or spider mites?
“That is a tough question to answer,” said Dr. Adam J. Varenhorst, assistant professor and extension field crop entomologist for South Dakota State University. “It is definitely going to be more difficult to manage because we can’t treat the infestation once it occurs.”
It also appears to cause very uniform damage that can completely kill the plant. In this regard, it has the potential to be a more economically damaging pest than any of those other insects, he said.
“However, we still have much to learn about it,” Varenhorst said.
Soybean gall midge is a very new pest, therefore we know little about it, agrees Dr. Robert Koch, associate professor and extension entomologist for the University of Minnesota.
“As for why it is attracted to soybean – soybean is a food source for the larvae which develop within the stems,” he said.
Most insects are attracted to plant volatiles to distinguish suitable food and egg-laying sites said Dr. Hodgson. But, scientists don’t know what attracts egg-laying females to soybean.
“We haven’t determined that yet for soybean gall midge,” she said. “But, I don’t think it is nitrogen.”
They are not known to infest plants other than soybean at this time, Hodgson said. But, it might be logical to guess they could infest other legumes.
“More questions than answers at this point,” she said.
Research is being conducted to address questions like this and others, said Dr. Koch.
Jon Burleson can be reached at email@example.com.