Erin Hodgson, Department of Entomology
In the last two weeks, I've heard reports of cutworm damage in soybean. Rich Pope visited a commercial soybean field in Harrison County and identified a mixture of black cutworm and dingy cutworm (Fig. 1). Matt O'Neal, ISU soybean entomologist, confirmed cutworm damage in soybean research plots at Curtiss Farm just south of Ames in Story County. The small cage plots were also infested with a combination of cutworm species (Fig. 2). It can be difficult to distinguish cutworm species without a hand lens, but a previous ICM article can help with identification.
Fig 1. A field in Harrison County, Iowa showing 50-60 percent stand loss due to cutworms. Photo by Rich Pope.
Fig 2. Cutworms found in Story County, Iowa.
Cutworm damage in corn is reported almost every year in Iowa. But infestations are patchy and sporadic because they have to migrate here every spring. On the other hand, cutworm damage in soybean is not typical. The last time cutworm damage was reported in ICM News was in 1999 by Marlin Rice. Entomologists don't fully understand why cutworms sometimes cause damage to soybean. However, there are a few field conditions that may make soybean fields attractive to female cutworm moths:
• Fields planted under reduced or no-tillage practices
• Fair to poorly drained fields
• Fields with winter annual weeds emerged prior to soybean planting
• More likely found in fields previously infested with cutworms
Economic thresholds have not been developed because infestations are unpredictable and infrequent. Treatment decisions must be based on the size of the cutworms and level of infestation. In some cases of very heavy cutworm density, fields may have to be replanted. Check 20 consecutive plants in five different areas of the field to determine percent cutworm damage. Young cutworms may feed on the stem or leaves, but older larvae can clip off the cotyledons. Also, look for discolored, wilted or dead plants. Cutworms will seek shelter during the day, so dig 2-3 inches down in the soil within a row. Consider an insecticide if larvae are less than 3/4 inch long and more than 20 percent of plants are damaged or missing.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at firstname.lastname@example.org or phone (515) 294-2847.