The soybean gall midge
An emerging pest to be aware of in 2019 and beyond
AMES — Is it a fungal infection? Is it a pest? Is it both?
Oftentimes it can be a difficult diagnosis when something is attacking soybeans. To add injury to insult, there is a new pest that soybean producers in Iowa need to be aware of heading into the 2019 growing season.
The soybean gall midge was confirmed in 65 counties in 2018 throughout Minnesota, South Dakota, Nebraska and Iowa, with 16 of those counties located in the western part of our state.
Erin Hodgson, Iowa State University Extension entomologist, said there was significant yield loss in many of those reported midge-infested fields. In some reported cases, losses came in at 50 bushels to the acre.
According to ISU, the soybean gall midge was first noted in northeastern Nebraska in 2011 and eastern South Dakota in 2015. The midge infestations were isolated and spread slowly before they were eventually detected in Minnesota and northwestern Iowa in subsequent growing seasons.
“It’s likely extending more to the central part of the state in low lying levels,” said Hodgson. “We don’t know. That’s part of what we are going to do this summer is to survey counties for new detections.”
In several of those fields, the soybeans experience hail damage during the early half of the growing season and entomologists thought, at that time, the adult midges were attracted to damaged plants.
Similar reports of midge infestations were associated with plant diseases, such as stem canker, charcoal rot, pod and stem blight, cercospora leaf blight and brown spot.
“When I initially started to see plants last summer, sometimes they were associated with hail damage. Sometimes they were also associated and mistaken for fungal infections,” said Hodgson. “My heads up to you guys; if you are scouting, don’t assume that the enlargements or swollen areas of the stem near the base is something like stem canker, phytophthora or rhizoctonia. I think oftentimes people are overlooking soybean gall midge for a fungal pathogen because of some of the external features.”
She added there are more than 6,000 different kinds of gall midges in the world.
“They’re a very diverse group of insects,” she said. “We have 1,000 species of gall midges in North America.”
As the name suggests, the plant will develop a gall where the midge is feeding. This is an attempt to wall off the feeding from the larvae.
“Remember they are gall midges, so they’re going to cause plants to react and try to defend itself,” she said. “So there is going to be an enlargement that is discolored and if you split it open, the soybean stem is going to look pithy or corky — it changes its character compared to an uninfested plant.”
She added the enlargements from the gall midge feeding will be at the base of the soybean plant and, at that point, could be detrimental to the soybean plant.
“What happens is, those plants quickly die,” she said. “This is what caused a lot of excitement last year, especially in western Iowa. The areas of plant death showed up pretty quickly and some people would mistake that plant death for fungal pathogens, especially if sometimes they have had a problem with soybean cyst nematode or SDS (sudden death syndrome).”
“But it was way too early for those to show up.”
Hodgson said the soybean gall midge will almost always attack beans at the field’s edge, which is a first.
“We saw injury and death at the edges and as the summer progressed, the circle got bigger and bigger as it went to the field’s interior,” she said. “The patches are a mixture of living and dead plants, but got bigger as the summer progressed.”
Identifying the soybean gall midge
Hodgson said the soybean gall midge is about a quarter-inch in length and resembles long-legged flies with hairy wings and long antennae.
The most damaging life stage of the soybean gall midge is the larvae, which in flies are maggots. The soybean gall midge’s larvae resemble a bright orange color.
“They really stand out,” Hodgson said. “What is more difficult to see are the smaller ones.”
The first instars are smaller and are clear and translucent. The second instar is bigger and begins darkening in color.
“Those are going to be difficult to see in the fields so mostly people are looking at the third instars,” she said. “The orange ones.”
She added it’s not uncommon to see all stages of the soybean gall midge on the same plant.
“We almost always saw mixture of life stages on the same plant,” she said. “We saw them in June, July and August, over a three-month period. To me, that suggests there are about two to three overlapping generations over a three month period.”
At this time, Hodgson said they do not know how long it takes the soybean gall midge to grow from egg to adult, but in other similar species, it take about four weeks.
“It’s my assumption, at this point, the female deposits eggs and is targeting the egg lying at the base of the plants,” she said. “Flies go through complete metamorphosis: egg, larvae, pupa and adult. I always saw the eggs and maggots at the soil line — about zero to 6 inches right about the soil line at the main stem.”
The bright orange third instars drop to the soil, where they pupate and then emerge as adults later on.
“That is my assumption, just because we never saw any pupae, but we are going to try to figure that out,” she said.
Sometimes, Hodgson said, you can see the orange maggots on the outside of the soybean stem.
“That’s an indication they have been feeding for a long time” she said “That whole stem is being consumed. That gall is formed. The tissue really starts to break away.”
An unknown pest
Identification of the soybean gall madge was a collaborative effort between entomologists from the United States and worldwide.
“We needed expert help to identify the species,” she said. “It’s not found anywhere else in the world. It’s unknown. Is it a species that has been in North America for a long time and all of a sudden had a host shift to soybeans? We don’t know. There are so many questions that go along with that.”
With little known about the soybean gall midge, that means there is even less known about what can be done to control the pest.
“I’m trying to make some connections about cultural control,” she said. “What are farmers doing? What’s the weather? Why was 2018 such a big year?”
The wide array of farming practices performed make making those connections hard.
“You all do a lot of different things when you farm, so trying to make connections for trends and why some of these fields were infested and why some fields were not infested, it is hard,” she said. “It was all over the place.”
Hodgson saw fields that were planted in April that had infestation, as well as fields that were not planted until June. There were fields that were infested that had all different row spacings, soybean genetics and different traits.
She added the architecture of the soybean plant didn’t seem to matter either, as some plants that were affected were small and bushy whereas others were tall. The presence or lack of cover crops didn’t seem to affect the presence of the pest either.
“We also saw so many different types of tillage and fertility regimes,” she said. “Everybody is doing something different. I never saw a consistent pattern. Does a no-till field attract more soybean gall midge? I never saw anything like that. Weed control in and around fields was also challenging for a lot of people, so that was highly variable as well.”
“The bottom line is, it is too early for me to say if there is anything we are doing in the landscape or culturally to favor or suppress these midges so far.”
Hodgson added she was surprised at the lack of control of the soybean gall midge insecticidal seed treatments had.
“They really didn’t have an effect,” she said. “This is really unfortunate because that is what an insecticidal seed treatment would have the purpose — to target a stem-boring insect. Especially ones that are feeding early in the season.”
Hodgson said controlling stem-boring pests can be a challenge, adding she would expect those areas that had the soybean gall midge in 2018 to have them again in 2019.
“I would also expect to find new areas of detection in central Iowa this year,” she said.
However, if plans have already been made for the 2019 growing season, Hodgson recommends sticking with those plans.
“I would not do anything drastic right now,” she said.
There is nothing labeled, at this time, to combat the soybean gall midge, with the exception of a few products that may include emergency labels.
“I do not recommend foliar applications for the midge,” she said. “We only have one insecticide that is labeled as an emergency label for gall midge. Anything else people are applying for soybean gall midge would technically be off-label.”
In general, the products that are available for field crop insect control are not systemic insecticides.
“So if you are trying to target the larvae that are feeding on the inside of the plant, I’m not sure a foliar application would make it,” Hodgson said. “It would never target those feeding larvae, but if the foliar application is targeting the adults, that may work, but timing is going to be something I couldn’t help you with at this point.”
“I want to have a plan for you guys, so when you do spray, you are very likely to make a profit,” she added. “I just don’t have any data to support that right now.”
In the meantime, Hodgson said any soybean gall midge that is spotted in fields should be reported.
“If you just see them on a few plants, if you split them, open and see the maggots, I would love to know,” she said. “I am trying to confirm detections as it spreads east in our state. I would appreciate any observations on pods. Any cultural control, the history of genetics or any types of insecticides used. I am looking for your help in the future.”