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Extension > Agriculture > Crops > Soybean Production > Insects & mites > Black cutworm in Minnesota soybean

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Black cutworm in Minnesota soybean

Robert Koch, Extension entomologist and Suzanne Wold–Burkness, Research assistant
2015

black-cutworm-adult black-cutworm-larva

Photos: Adam Sisson, Iowa State University, Bugwood.org


Figure 1. Black cutworm adult (top) and larva (bottom).

Name

Black cutworm (Agrostis ipsilon)

Identification

Adults

Eggs

Eggs are small, round and white, eventually turning brown. Laid singly or in clusters.

Larvae

Pupae

Pupae are dark brown, 3/4 inch long and rarely seen.

Natural history

Black cutworm moths migrate to Minnesota from overwintering sites in the southern U.S. and Mexico in early spring (April). Females seek out low–growing plants and plant residue for egg laying. After 5–10 days, eggs hatch. Young larvae feed on weeds or soybean leaves and later move to feeding on seedling stems. After completing six larval instars, they pupate in the soil, with adults emerging in 2–3 weeks. There are 2–3 generations per year.

Impacts

black-cutworm-and-seedling

Photo: Bruce Potter, University of Minnesota

Figure 2. Large black cutworm larva next to soybean seedling.

Small larvae feed mainly on leaf tissue and larger larvae can cut cotyledons and stems. The larger larvae feed on plant parts just above the soil surface and occasionally below the surface, if the soil is loose and dry near the base of the plant. Larvae can completely cut off the soybean stem. Even if the stem is not completely cut, enough plant tissue may be removed to cause the plant to fall or wilt.

Scouting and management

Unlike other stand reducing insects, black cutworm infestations can be controlled with rescue treatments of insecticides, if promptly detected.

Adult moth catch is monitored each year in southern Minnesota using pheromone traps. Using this trap data and a biofix of 8 or more moths in a 2 night period, degree days can be calculated to estimate when larvae will be present in your fields. More information on monitoring black cutworm can be found on the Cutworm Network page.

Scouting

Black cutworm scouting should start at plant emergence and continue 1–2 times/week until plants are past V5.

Look for the following:

If any feeding is present:

Risk for infestation by black cutworm is greater in areas that are attractive for egg laying and include the following:

Treatment thresholds

Labeled rates of insecticides can be used to manage this pest. Follow directions on the product label.

Estimation of stand loss and replant decisions

Fields should be inspected for stand loss during seedling growth stages. Soybean is a resilient crop, able to tolerate relatively high levels of stand loss. However, when significant stand loss occurs, replanting may be required, but this option should be considered carefully. Information is available on how to estimate plant stand and guide replant decisions in soybean (U of M Extension soybean replant guide). This emphasizes the importance of knowing the field history, previous infestations, and current degree days.

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