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Leaf-miner insect is no minor threat to soybeans

Leaf-mining insect has been found in Canada, Minnesota and South Dakota. The worst infestation has been in Sibley County, southwest of the Twin Cities, but Bob Koch said the insect has shown up in fields at Lamberton in the southwest to Morris in west-central Minnesota. Unlike soybeans, the insect is native to North America, with a range that stretches from Manitoba, Canada, and as far south as Texas and North Carolina.

Soybean leaves with white patches
White blotches or veins on the underside of leaves are evidence of a leaf-mining insect in soybeans.
Robert Koch / University of Minnesota
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An insect with a broad range across North America seems to be developing an appetite for an important cash crop: soybeans.

The leaf-mining insect was previously known to only feed on two plants, but it has been found in soybeans in Minnesota, according to Bob Koch, associate professor, in the Department of Entomology, University of Minnesota.

Koch said he thought it might be some sort of fluke when the insect was found in soybeans near St. Paul and Rosemount, Minnesota, in 2021.

“This year that's really changed with finding it across much of southern Minnesota and then some fields have pretty severe infestations,” Koch said. “It's not like a whole fieldwide, severe infestation, but it leaves me wondering: Is this just the tip of the iceberg for a new severe pest problem or is it just some anomaly where whatever conditions lined up right, in these particular areas, for this pest?”

The worst infestation has been in Sibley County, southwest of the Twin Cities, but Koch said the insect has shown up in fields at Lamberton in the southwest to Morris in west-central Minnesota.

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Bruce Potter, an integrated pest management specialist with University of Minnesota Extension at Lamberton said there also has been evidence of the insect in eastern South Dakota, but at a low infestation level.

The insect was documented in soybeans in Quebec, Canada, in 2016, and Koch and a Canadian researcher wrote a paper on the phenomenon last year.

The genus-species name for the insect is the macrosaccus morrisella, and it has been called the hog-peanut leafminer. It was known to only infest the American hog-peanut and slickseed fuzzybean , plants that are commonly found in much of North America, though Potter said it may feed on other plants but hasn't been documented.

Unlike soybeans, the insect is native to North America, with a range that stretches from Manitoba, Canada, and as far south as Texas and North Carolina.

Soybean growers are encouraged to check the undersides of leaves for evidence of the mines, a kind of white, blustery looking area within the leaves.

A tiny insect overlaid on a coin.
The insect known as the hog-peanut leafminer is tiny and hard to detect in fields.
Robert Koch / University of Minnesota

The adult insects are tiny and will be hard to spot. The caterpillars also are tiny, actually living inside the leaf as it mines. “They can't be very big to do that,” Koch said.

If growers do find evidence of the leaf-mining insect, Koch would appreciate being email any pictures with locations. His email is koch0125@umn.edu .

The most likely areas of infestation would be along tree lines, where the hog peanut often can be found growing.

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Because the evidence is on the underside of leaves, mostly on field edges, Potter said it perhaps is not as new as it seems.

“We might have been walking by low level infestations for years,” Potter said.

But he said it was evident at Lamberton when scouting aphids.

Koch said mines from the insect became apparent in mid- to late-June and have continued into September.

Koch and Potter said there a lot of unanswered questions about the insect, with a big one being why the infestation in Sibley County was so much more pronounced than others.

Others include how the appetite for soybeans might be spreading among the insect population and what the potential impact on yields might be.

“There’s a lot of science that has to happen before we can answer those questions,” Potter said.

One basic question is what to call the bug. While the macrosaccus morrisella, has been referred to as the hog-peanut leafminer, Koch said an official common name for the insect is under review by the Entomological Society of America.

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There already is a different insect called the soybean leafminer, so Koch has suggested the soybean tentiform leafminer.

While it is too late to spray for insects this growing season, Potter and Koch said farmers should resist the urge to hit panic button on the pests if they see evidence in 2023.

Koch said it appears that certain wasps will feed on the caterpillars and an insecticide could do more harm than good.

But he emphasizes scouting and calculating damage thresholds.

Koch said the insect will likely be an education topic at meetings and farm shows over the winter.

“It's something new; I'm sure folks are going to be interested in hearing about it,” Koch said.

Related Topics: AGRICULTURECROPSSOYBEANSAGRICULTURE RESEARCH
Reach Jeff Beach at jbeach@agweek.com or call 701-451-5651 (work) or 859-420-1177.
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