Perplexing pest rankles researchers

University of Nebraska-Lincoln research scientist Nikki Luhr (by easel with marker) fields questions from a group of soybean farmers during the UNL Extension Soybean Gall Midge Roundtable held at the Eastern Nebraska Research and Extension Center in Ithaca, Nebraska, March 2.

After years of concentrated effort, scientists have discovered some helpful information on the soybean gall midge, but the unknowns still far outweigh the knowns.

“The variation is tremendous,” said UNL entomologist Justin McMechan at a March 2 roundtable discussion at the Eastern Nebraska Research and Extension Center in Ithaca, Nebraska. “We are still learning about the gall midge.”

Of the things learned about the gall midge since its appearance around 2011, the first is that even though the adult is only a ¼-inch long, signs of infection are not hard to find. Farmers scouting their fields can find the swollen black stems and orange larvae of the gall midge simply by walking a couple of rows on the edges, McMechan said.

According to McMechan — who was joined by fellow researchers from Iowa State University, University of Minnesota, and South Dakota State University — key areas to check are fields with corn-soybean interface, areas next to dense vegetation and early planted soybeans.

“They are — right now — overwintering in fields they infected last year,” he said.

The experts have discovered that gall midge pupae can be found in soil from ¾ of an inch up to five inches deep in soil. The highest concentrations of pupae are found nearer to the ¾ mark. A very low number of pupae were found deeper than three inches.

Researchers have also learned that gall midge females prefer to lay their eggs in soybean plants at the V3 stage or after. The females seek out naturally occurring expansion fissures in the plant stalk. They have found that gall midges will also lay eggs in stalks with wounds from wind events or hail, McMechan said. The larvae generally infect two to three inches up the stalk.

Researching alternative hosts sought by the soybean gall midge, UNL Department of Entomology scientist Amy Hauver has found that the pest will also infect yellow sweet clover and alfalfa. Also in the Fabaceae family is the leadplant (aka downy indigo bush or prairie shoestring). UNL researchers are actively seeking people with this plant on their property, as it is the only native plant vulnerable.

UNL Department of Entomology scientist Nikki Luhr, an assistant to noted entomologist Tom Hunt, has been researching the effects of tillage and edge treatments on gall midge emergence. She has found that there is a slight difference between no-till and tilled soil and between mown versus unmown edges: There was lower emergence from tilled fields and lower infection rate from mown edges.

McMechan stated that the soybean gall midge keeps attacking the same plant with eggs. That has resulted in multiple larval infections. There is an average 30 days between generations.

“We don’t know if they prefer pre-infected stalks to healthy stalks,” he said. “We have observed them infect both in the same area.”

Another conundrum was discovered by entomologists at the University of Minnesota. After grinding up some soybean gall midge larvae gathered there, Extension educator Bruce Potter and his team tested the DNA, and discovered parasitoid DNA in that gall midge larvae. This is not consistent with samples examined in other states. It resulted in a lot of questions, McMechan said.

Also causing a “lot of questions” is herbicide interaction. Does the swelling caused by some herbicides —which can cause fracturing in stalks — lead to additional vulnerability? Inquiring producers wanted to know. This is one of the areas still under investigation, McMechan said.

Scientists will be checking on cracking difference between soybean varieties this season, as well as studying cover crops for overwintering implications. Some producers mentioned the efficacy of the older practice hilling beans to protect stalks. This idea was met with mixed emotions.

Still another idea bounced around was the notion of planting a “trap crop” of soybeans. This constitutes four rows of soybeans planted on the outside of a corn field or a field replanted that was infected last year. The gall midge tends to stay in fields that are already infected, McMechan said.

Planting times were also discussed as a possible mitigation technique. Early planting showed higher probabilities of infection. Producers who had planted on May 31 had shown only a 3% chance of infection. Growers who planted on June 15 or July 1 suffered negligible infection.

“Early planting dates gave no advantage,” McMechan said. “I’m not saying to plant late — but you may consider planting in higher-risk fields last.”

Three puzzles are still perplexing researchers: The adult capture date lacks a geographical trend; there is a wide variation in the total of overwintering adults captured per site; and the duration of overwinter emergence has been variable.

“We have only anecdotal evidence in many of these cases,” McMechan said. “We are still needing to study various scenarios.”

Producers are invited to join the UNL Soybean Gall Midge Alert Network. Those registered will receive text alerts on adult soybean gall midge emergence, current suggestions for management practices and links to other resources.

To join email: Justin.mcmechan@unl.edu your name, phone and email address with the subject line — SGM Alert Network.

Jon Burleson can be reached at jon.burleson@lee.net.

Jon Burleson is the Midwest Messenger reporter, based out of eastern Nebraska. Reach him at jon.burleson@lee.net.