An insect found across North America seems to be developing an appetite for an important cash crop: soybeans.
The leaf miner was previously known to feed on only two plants, but it has been found in soybeans in Minnesota, according to Bob Koch, associate professor in the Department of Entomology at the University of Minnesota.
Koch said he thought it might be some kind of fluke when the insect was found in soybeans near St. Paul and Rosemount, Minnesota in 2021.
“This year it really changed with it being found in a lot of southern Minnesota, and then some fields have pretty severe infestations,” Koch said. “It’s not like a serious field-wide infestation, but it does leave me wondering: is this just the tip of the iceberg for a serious new pest problem or is it just an anomaly where, whatever the conditions, in those particular areas, are for that pest?”
The worst infestation was in Sibley County, southwest of the Twin Cities, but Koch said the insect appeared in fields from Lamberton in the southwest to Morris in west-central Minnesota. .
Bruce Potter, an integrated pest management specialist at the University of Minnesota Extension in Lamberton, said there was also evidence of the insect in eastern South Dakota, but at a low level of infestation. .
The insect was documented in soybeans in Quebec, Canada in 2016, and Koch and a Canadian researcher wrote about the phenomenon last year.
The genus-species name for the insect is macrosaccus morrisella, and it has been called the hog-peanut leaf miner. It was only known to infest American groundnut and kidney bean, plants commonly found in much of North America, although Potter said it could feed on other plants, but this has not been documented.
Unlike soybeans, the insect is native to North America, with a range that extends from Manitoba, Canada, and as far south as Texas and North Carolina.
Soybean growers are encouraged to check the undersides of leaves for mines, a kind of white, windy area inside the leaves.
Adult insects are tiny and will be difficult to spot. The caterpillars are also tiny, actually living inside the leaf as it mines. “They can’t be very big to do that,” Koch said.
If growers find evidence of the leaf miner, Koch would appreciate receiving photos with locations by email. His email is [email protected]
The most likely areas of infestation would be along trees, where hog peanut often grows.
Because the evidence is on the undersides of leaves, mostly on the edges of fields, Potter said it may not be as novel as it seems.
“We may have been going through low-level infestations for years,” Potter said.
But he said it was evident in Lamberton when scouting for aphids.
Koch said the insect mines became apparent from mid-June to late June and continued through September.
Koch and Potter said there were many unanswered questions about the insect, the main one being why the infestation in Sibley County was so much more pronounced than others.
Others include how the appetite for soybeans could spread through the insect population and what the potential impact on yields could be.
“There’s a lot of science that needs to happen before we can answer these questions,” Potter said.
A fundamental question is what to call the bug. While macrosaccus morrisella, has been called the peanut hog leaf miner, Koch said an official common name for the insect is under review by the Entomological Society of America.
There is already a different insect called the soybean leafminer, which is why Koch suggested the tentiform leafminer.
Although it’s too late to spray the insects this growing season, Potter and Koch said farmers should resist the urge to press the panic button on the pests if they see evidence in 2023.
Koch said it appears some wasps feed on the caterpillars and an insecticide could do more harm than good.
But it emphasizes tracking and calculating damage thresholds.
Koch said the insect will likely be a topic of education at agricultural meetings and shows over the winter.
“It’s something new; I’m sure people will be interested in hearing about it,” Koch said.