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Soybean gall midge confirmed in states; no management protocol



LINCOLN, Neb. — Results from field surveys have confirmed the presence of soybean gall midge (adult stage) in eastern Iowa, Minnesota, Nebraska and South Dakota, with infestations found in 66 counties in the four states, according to state entomologists.

“Field surveys were initiated in these states and in neighboring Minnesota to determine the distribution and the extent of the damage,” said Justin McMechan, University of Nebraska-Lincoln crop protection and cropping systems specialist, Thomas Hunt, extension entomologist, and Robert Wright, extension entomologist, in UNL’s Nov. 7 Crop Watch.

“A portion of the fields surveyed had significant levels of damage, with a high frequency of dead plants at the field edge with decreasing damage from the edge into the center of the field,” the report stated.

In late June, entomologists in Iowa, Nebraska and South Dakota began receiving reports of soybean fields with visible signs of dead or dying plants associated with soybean gall midge infestations. The researchers said live plants in damaged areas of the field had dark discolorations at the soil surface that extended up to the unifoliate node.

“These plants easily snapped off at the soil surface, revealing white to orange larvae that appeared to be feeding on the darkened areas of the plant,” the report said.

Clarke McGrath, Iowa State University’s Iowa Soybean Assoc. on-farm research and extension coordinator, said he could sum up his thoughts on soybean gall midge in two words: surprise and frustration.

“Surprise – and not in the good way – because this thing really came at us from out of nowhere,” he explained. “The first reports I know of were from way back in 2011 out of Nebraska and South Dakota. They noted a few fields with some gall midge, mostly in areas with soybeans that had some type of damage earlier in the season, from diseases or hail primarily, if I recall right.”

He said a handful of reports were called in during subsequent years – and those were also connected with a pathogen or damage. “They didn’t seem to impact yield since, in those cases, any yield loss was considered to be related to the previous damage from the hail, disease or other injury.

“While we were vaguely aware of the gall midge, it wasn’t considered a primary pest of soybeans,” he added. “2016 and 2017 brought a few more reports, and in hindsight, maybe a small hint of them being more of an issue than anyone expected. But still they were pretty isolated in nature, and the infestations and damage showed up late in the season.

“So when we started getting calls in June (2018), it was an unpleasant surprise.”

That’s when frustration set in – “right after the surprise wore off,” McGrath said.

“We figure out that there are these gall midges in plant stems, then read the literature and talk to the experts, and nothing adds up since they aren’t supposed to be a pest in soybeans. With little known about them, we had no options for dealing with them.

“All we could do was gather as much information as possible – and when your clients have areas of the fields dying, that is obviously very frustrating to them and to their service providers,” he added. “The calls kept coming in as they were found in more fields, and as far as I know, they were confirmed in 16 counties in western Iowa.”

According to Nebraska entomologists, adult soybean gall midges were collected from emergence cages at the Eastern Nebraska Research and Extension Center near Mead, Neb., on August 2.

“Adults of soybean gall midge had not been observed,” the report stated. “In Nebraska, emergence cages were placed over midge-infested soybean plants on August 1 and adults later identified to the genus Resseliella were collected. Emergence of these adults continued for the next 18 days, with the last observation August 19.”

The report said the specimens were sent to Raymond Gagne, collaborator in the USDA Agricultural Research Service’s Systematic Entomology Laboratory, and Junichi Yukawa, emeritus professor of Kyushu University, Japan, leading authorities in midge identification.

“Raymond and Junichi were able to connect these adults to the maggots that we had been observing in soybean fields,” the report said. “The identification of the adults will be critical for monitoring their emergence next spring in fields where there was a problem the previous year.”

The report said yield loss estimates on a small sample of plants from a heavily-damaged field indicated nearly complete yield loss from the field edge up to 100 feet, with about a 20 percent yield loss 200 and 400 feet from the edge.

Currently, McGrath said no one knows enough about gall midge to make any management recommendations. “We have some ideas, but nothing has been tested and confirmed,” he said.

“Insecticide seed treatments didn’t appear to be effective on gall midge – at the rates currently used. Some researchers believe that higher rates could offer some suppression, but these high rates of seed treatment are not currently offered and there is no scientific data to support their effectiveness.”