Cutworms
Overview
There are several species of cutworms, all with a wide host range, including soybean. Most cutworms overwinter as pupae in the soil or as young larvae, however, some move into the Midwest as moths from southern latitudes. Late-planted and/or weedy fields are at higher risk for cutworm infestation. Young cutworm larvae feed above-ground at night or on cloudy days. Older, larger worms tend to remain below the soil surface, and often cut plants above, at, or below ground level.
Cutworm Identification
Cutworms are similar in general appearance. They are smooth with very few hairs and are about two inches long when fully grown. They typically curl into a tight ‘C’ shape when disturbed.
Different species can look different from one another and they can range in colors including brown, tan, pink, green, gray or black. Some cutworms are a uniform color while others are spotted or striped. Some larvae are dull and others appear glossy or shiny.
Black Cutworm (Agrostis ipsilon)
Photo Credit: Adam Sisson, Iowa State University, Bugwood.org
- Range in size between one sixth to one and one half inches
- Color varies from greasy gray to black. A broad light gray band runs down the middle of the back
- Unequally sized, paired tubercles (small rounded projections or protuberances) run along the upper side of the body
Claybacked Cutworm (Agrotis gladiaria)
Photo Credit: James Kalisch, University of Nebraska, Bugwood.org
- Larvae are pale-gray and translucent with a gray-brown head
- There are bars on the front of the face and a broad yellow-brown stripe on the back
Dingy Cutworm (Feltia jaculifera)
Photo Credit: Canadian National Collection, The Canadian National Collection (CNC) of Insects, Arachnids and Nematodes, Bugwood.org
- Larvae have a light gray back and have four black dots on each segment
- The underside of the larvae is a pale yellow
Variegated Cutworm (Peridroma saucia)
Photo Credit: James Kalisch, University of Nebraska, Bugwood.org
- Larvae are gray, mottled and have distinctive yellow dots at the top of the front abdominal segments
- Some larvae may have a yellow-orange stripe along the body
Scouting
Fields should be inspected for stand loss during seedling growth stages.
- Risk for infestation includes fields with crop debris, fields with weeds, and low areas within fields
- Cutworm scouting should start at plant emergence and continue one-two times/week until plants are past V5
- Look for the following:
- Leaf feeding by small larvae
- Wilted or cut plants caused by larger larvae
- If feeding is present:
- Dig around the base of plants and check under debris to find larvae
- Check 20 consecutive plants in five different areas of the field to determine percent of plants cut or girdled
- Concentrate sampling in areas that are more at risk for damage
Photo Credit: University of Minnesota
Management
Soybean is a resilient crop, able to tolerate relatively high levels of stand loss. Damage from all cutworm species should be considered together. If cutworms are threatening the stand to the level that plant population will be a limiting factor for good yields, treatment may be needed. The minimum number of plants per foot normally needed for good yields at different row spacing, if plants are not under stress, is as follows:
| Row Spacing (inches) | Minimum Number of Plants per Foot |
| 7.5 | 1.5 |
| 15 | 2.5 |
| 30 | 4 |
| 36 | 5 |
| 38 | 5 |
| 40 | 6 |
Source: Purdue University
Established thresholds for treatment vary widely among states, ranging from as little as 5% damage in Nebraska to 30% damage in Kentucky. For the most effective treatment, cutworms should be present and less than one inch in length. As cutworm larvae becomes mature, control with insecticides becomes more difficult. Numerous insecticides are registered for cutworm management. Always read the label and follow directions before use.
Resources
Black cutworm on soybean, University of Minnesota Extension, 2015
Cutworms, Purdue University Field Crops IPM
Cutworms and Armyworms, K-State Research and Extension MF3276 January 2016
Information on this pest provided by Douglas J. Jardine, Professor Emeritus, Kansas State University 6/2020
Images provided by Crop Protection Network and IPM images and University of Minnesota
