A new soybean pest is showing up in more Midwest fields, and researchers are urging farmers to keep a close eye on their crops.
The soybean tentiform leaf miner was first detected in Missouri in 2025 in Cooper County. Expanded monitoring this year has confirmed the pest in two additional locations: Macon County in northeast Missouri and Lawrence County in southwest Missouri.
The pest was first reported in Canadian soybeans in 2016 before being detected in Minnesota. It has since been confirmed in seven states across the Midwest, with Wisconsin among the states reporting detections in 2026.
Ivair Valmarbida, state extension entomologist at the University of Missouri, says the pest could be present in more fields and counties than current monitoring has identified.
“It’s possible that this pest is present in more fields and more counties across the state,” Valmarbida says. “Our monitoring doesn’t encompass the soybean acreage that we have in Missouri.”
The soybean tentiform leaf miner is small enough to go unnoticed, feeding within soybean foliage rather than on the outside of the leaf. As the caterpillar develops, it creates blotch-like areas that can lead to defoliation.
“My projects focus on integrated pest management strategies, and one of the pests that we detected for the first time in Missouri was the soybean tentiform leaf miner,” Valmarbida says.
A Pest Hiding Inside Soybean Leaves
Unlike many emerging agricultural pests, the soybean tentiform leaf miner is native to North America. Researchers believe it has moved from wild host plants into soybean fields.
The adult moth lays eggs on the undersides of soybean leaves. Once the eggs hatch, the tiny caterpillars feed within the leaf tissue, creating blotches that can eventually result in defoliation.
“It’s a very tiny caterpillar, so the adult moth is also very tiny,” Valmarbida says.
Most detections have occurred from July through September, particularly along field edges near wooded areas.
So far, infestations identified in Missouri have been light and have not reached economic thresholds. However, researchers are still learning how the pest could affect soybean yields if populations increase.
“At this time in Missouri, that’s a question mark because we just started detecting it,” Valmarbida says. “It could be that some fields, especially those close to the wood areas, could have significant defoliation and potential yield losses.”
Where Farmers Should Scout
Valmarbida recommends monitoring soybean fields for the pest, starting with field edges near trees and wooded areas. If farmers find signs of the leaf miner, the next step is to assess the extent of defoliation and determine whether it has reached established thresholds.
In Missouri, the recommended defoliation threshold is 30% before blooming and 20% after soybeans begin blooming. If defoliation reaches those levels, farmers may consider an insecticide application to help reduce pest populations.
Valmarbida emphasizes that finding the pest does not automatically mean an insecticide treatment is necessary. Farmers should first determine whether defoliation has reached the appropriate threshold.
“If they find it, the first thing is to determine whether those beans have defoliation that reaches threshold levels,” Valmarbida says.
For now, Valmarbida says the priority is monitoring fields and identifying where the pest is present as researchers learn more about its potential impact on soybean production.
