By Pamela Smith
DTN Progressive Farmer Crops Technology Editor
DECATUR, Ill. (DTN) -- The mysterious orange maggots that drilled into soybean stems and killed plants in parts of Iowa, Nebraska, South Dakota and Minnesota last summer now have known adult relatives.
Entomologists were able to capture the adult version of the soybean gall midge in the field and confirm their identity by rearing a few larvae to adulthood in a laboratory environment this fall. University of Nebraska entomologist Justin McMechan said knowing what the adults look like is a big step in figuring out their life cycle and monitoring emergence next spring in areas where there was a problem the previous year.
The adult soybean gall midge that emerged was approximately 1/4 inch in length with an orange abdomen and legs with distinctive black-and-white bands. Entomology teams cooperated in surveys last summer to find soybean gall midge larvae present in 66 counties in the four states.
McMechan said adults raised from the maggots collected from infested fields were sent to Raymond Gagne, a collaborator in the USDA ARS Systematic Entomology Laboratory, and Junichi Yukawa, emeritus professor of Kyushu University, Japan, leading authorities in midge identification.
Scientists had assumed the pest to be part of the large midge family, and identifying the adult has allowed them to narrow it to the genus Resseliella, which encompasses 55 species worldwide, 15 of which have been identified in the United States. However, none of these species are known to occur on soybeans, and DNA and morphological comparisons by Gagne and Yukawa indicate that it is a new species, McMechan told DTN.
"That's something of a challenge because this genus has a really wide host range, showing up on all kinds of plants and the adults look very similar," he said.
A BREAKOUT YEAR
McMechan said soybean gall midge caused a few late-season issues in 2016 and 2017. Scientists are now trying to determine why it arrived as early as June in 2018 and was found over a larger geographical area.
The good news is the earlier arrival time and multiple generations allowed researchers additional time to make some important field and laboratory observations. The bad news for farmers was the pest had more time to munch away at yields while the farmers themselves could do little to stop the feeding.
Wayne Martin witnessed just how destructive soybean gall midge can be. The Shelby, Iowa, farmer watched with horror as the maggots, which start out as creamy white, turn pink and then orange-to-red, munched away inside soybean stems. Plants wilted and eventually died from the invasion.
"At first we thought it was just the outside field edges, and watching the yield monitor, the outside passes were ugly. But we found more damage further into the field during harvest than we expected. Yield maps really told the tale," he said.
McMechan said studies of management practices such as planting date and soybean maturity group were evaluated for soybean gall midge damage at the Crop Management Diagnostic Clinic plots at the Eastern Nebraska Research and Extension Center near Mead.
These plots are used for demonstration purposes and are not replicated, he noted. Soybeans were planted every three weeks beginning in late April through the end of June. Each planting date consisted of four maturity groups (1, 2, 3 and 4).
Dissections of random plants from each plot showed that all maturity groups within each planting date were infested, with the exception of a late-June planting date. This matches observations of later-planted soybean fields in Iowa and South Dakota having reduced visual symptoms and lower infestation rates, he reported in a news article.
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Maturity groups 1 and 2 showed visible signs of damage at or near the soil surface, whereas late-maturity groups 3 and 4 showed signs of plant damage in the axils of the trifoliate approximately 6 to 8 inches from the soil surface, he said.
"In heavily damaged fields, losses associated with soybean gall midge are inevitable due to the number of dead or dying soybean plants, McMechan wrote in a news release. "Damage to the phloem and xylem of the plant is likely to result in yield reductions for surviving soybean gall midge infested plants. Additional losses are also anticipated due to the lack of stem strength, predisposing plants to increased risk of lodging if harvest is delayed. Yield loss estimates on a small sample of plants from a heavily damaged field indicate nearly complete yield loss from the field edge up to 100 ft., with about a 20% yield loss 200 and 400 ft. from the field edge."
NOTES FOR NEXT YEAR
What Martin wants to know is if the pests arrived because of an environmental set of events or if they are here to stay.
McMechan and other entomologists are working to answer that question and others. What they believe now is that gall midge is capable of overwintering.
"Knowing what the adult looks like is a critical piece of the puzzle we were lacking," he said. Last summer, McMechan said, he searched high and low for the adult form -- laying in the canopy and setting traps for the elusive gnat-like pests.
He hypothesized then that they fall from the plant foliage, pupate and emerge from the soil during the summer. In fact, he said fields that were back-to-back soy did not show an increase in severity in fields that had issues the previous year.
"Everything right now is assumptions," he emphasized. "However, based on distribution, we believe vegetation on the edge of the field might be important, and we're not sure if that's a host relationship or if it is simply an environment for the adults to move into early in the spring prior to infesting soybean fields."
McMechan cautioned frantic farmers from making rash management decisions about spraying or changing rotations that might have negative side effects to other beneficial insects or create secondary pest issues.
He said a monitoring program is being developed that will include insecticide trials. They also will be looking at insect emergence curves, disease interactions and simulating hail events.
Farmers interested in participating in monitoring and trapping for the pest should contact their state Extension entomologist.
Models that wrap degree-days or other cultural practices are the goal. "We don't know if this is a shift in insect population or if they've adapted in some way.
"But we are excited that we have the next clue, and we will keep learning and we are happy to have farmer input on all of this," McMechan said.
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Pamela Smith can be reached at Pamela.email@example.com
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