By Dr. Robert Koch, a checkoff-supported researcher and Associate Professor at the University of Minnesota

Last year, we documented for the first time a new insect pest in Minnesota soybeans. This pest, called Macrosaccus morrisella, is a tiny moth whose larvae live and feed inside leaves. It is a native insect, known to occur across much of the eastern US. Previously, it was known to feed only on two plants called American hog-peanut and slickseed fuzzybean. However, the list of plants fed upon by this insect has now expanded to include soybeans.

Description of the pest

When the larvae of the leafminer feed in leaves, they cause injury called mines. These mines occur on the underside of leaves and typically have a white blister-like appearance that is often outlined by the leaf veins (Figure 1). Individual mines from multiple larvae can coalesce to cover larger amounts of leaf area. The larvae which live in these leaf mines are typically pale green in color and reach about 4-5 mm in length (Figure 2). The adults, though tiny (6-7 mm in length) have a striking color pattern of orange, white and gray-black markings (Figure 2). In a recent article, we describe this insect and its injury to soybean in much more details. The mines from this new leaf-mining moth are distinctive from and should not be confused with those of a small beetle called the soybean leafminer.

Infestations in Minnesota

Last year, we found the leaf-mining moth at low levels in soybean in Minnesota in Rosemount and St. Paul. This, year, we have seen and heard reports of this insect in soybean across a large area of southern Minnesota. Infestations in most fields are relatively minor and likely not a threat to yield. However, we were alerted to a field in Sibley County with a significant infestation of this new leafminer.

Current research

Because this leafminer was so recently discovered feeding on soybean, we know very little about its biology and impacts to the soybean crop. Therefore, we’re using this field in Sibley County to try to learn as much as we can about this insect. Here’s an overview of some of the research we’re performing on leaf-mining moth…

• Within field distribution: We are sampling along transects going into the soybean field from different edges to learn about how the levels of infestation of the leafminer change across the field. Preliminary results suggest that infestations are highest on field edges near trees and generally lower on edges without trees and in the field interior.
• Within plant distribution: We are examining the leaves of whole soybean plants to learn about where the insect occurs on plants. Preliminary results show that the lower leaves of plants, which were fed upon by the previous generation of this insect, had the highest percent of leaf area affected with mines. However, the upper leaves are now being attacked by the current generation of the insect. We will continue to document how the infestation progresses throughout the plants.
• Parasitism of the leafminer: We placed soybean leaves containing mines of the leafminer into cages in the laboratory to see if the adult leafminer moths and/or parasitic wasps would emerge. The moths readily emerged in the laboratory, but we are also observing a surprisingly high number of parasitic wasps emerging from the leaf mines. It is very likely that these tiny parasitic wasps develop in and kill the leafminer, which would make them beneficial insects. We need to identify these wasps and learn about their biology, so we can promote them in the fields to help naturally suppress this pest.

Results of this research and related projects will be shared with soybean growers and agricultural professionals through University of Minnesota Extension.