Black Cutworm

Agrotis ipsilon Hufnagel

Appearance and Life History

Larvae Larvae
Photo by J. Obermeyer

Several cutworm species attack corn in the Midwest. One species, the black cutworm, poses the most serious threat. A single black cutworm larva is capable of cutting approximately four corn plants during its lifetime, depending on the size of the plants.

The black cutworm moth has a dark, dagger-shaped marking near the outer edge of each forewing. The inner two-thirds of the forewing is dark, while the outer one-third is much lighter in color. Its hind wings are light in color with a brown tint along the margins. Moths overwinter in Texas and Mexico and appear in the Midwest from late February through June. They are attracted to early spring vegetation, such as winter annual weeds, where mating and oviposition take place. The moths are active at night and prefer to deposit their eggs on low-growing, dense vegetation.

Adult moth with characteristic dark dagger-shaped marking near the outer edge of each forewing Adult moth with characteristic dark dagger-shaped marking near the outer edge of each forewing
Photo by J. Obermeyer

Black cutworm larvae vary in size from 1/8 inch (3 mm) when newly hatched to 2 inches (50 mm) when fully grown. They are light gray to almost black in color and have a greasy appearing texture and coarse granules of various sizes that can be seen on their skin.

Black cutworm problems most often develop in corn-planted and pre-existing vegetation (i.e., weedy fields, no till plantings, etc.). Fields adjacent to areas of permanent vegetation are also potential trouble spots. The following risk factors affect the probability of a black cutworm problem developing:

  • fall plowed - problem less likely
  • late spring plowed - problem more likely
  • corn following beans - problem more likely
  • late planted - problem more likely
  • minimum tillage - problem more likely
Black Cutworm Life Cycle

Damage

Young larva and leaf damage Young larva and leaf damage
Photo by S. Dlugosz

Because black cutworm moths are attracted to green vegetation for egg laying, infestations of larvae usually appear first in weedy areas of fields. Newly hatched larvae feed on weeds, and/or young corn plants if present, leaving small irregular holes in the leaves. Such early feeding is of little significance to plants. However, these signs of early feeding are good indicators of the potential for severe damage by later cutworm instars.

Larger larvae may notch the stems of seedlings immediately below the soil surface, which can cause plants to wilt and die. They may completely cut through stalks, which can result in severe stand reductions.

Black cutworms sometimes drag cut plants under dirt clods or into small holes in the soil to continue their feeding Black cutworms sometimes drag cut plants under dirt clods or into small holes in the soil to continue their feeding
Photo by C.R. Edwards

Black cutworms usually feed at night or during overcast days. They sometimes drag cut plants under dirt clods or into small holes in the soil to continue their feeding during the daylight hours.

Sampling Method

All corn fields can be damaged by this pest, but fields with pre-plant weeds are at highest risk and should be sampled for this pest. To help with scouting visits and prediction of cutworm damage, moth monitoring with pheromone traps and larval developmental predictions based on heat unit accumulations are provided by many state Cooperative Extension Services. When scouting, walk throughout fields shortly after plant emergence checking plants for signs of black cutworm feeding (leaf damage, wilted plants, or cut stalks). At the same time look for indications of other early season pests such as; wireworms, white grubs, corn flea beetle, etc. Continue to watch for cutworm damage during May and early June field visits.

Black Cutworm Delvelopment

If black cutworm damage is observed, sample the field immediately to make an accurate assessment of the threat posed by the larvae. The sampling procedure used must provide information on three variables:

  1. Percentage of cut or damaged plants.
  2. Stage of growth of the corn.
  3. Average stage of development of the cutworm larvae.

In each of 5 areas of a field, randomly select and inspect 20 consecutive plants, a total of 100 plants for the field. Count and record the number of plants cut or damaged by black cutworms within each sample set.

During the plant inspection process, collect at least 10 black cutworm larvae. Usually they can be found under dirt clods or plant residue or 1 to 2 inches (25 to 50 mm) deep in the soil around the base of freshly cut or damaged plants. However, early instars may be found feeding on young leaves. Carefully inspect damaged plants if cutworms can not be found in the soil. Drop collected cutworms into a small jar of alcohol. This will kill the larvae, making them easier to handle at a later time.

While checking for damaged plants, also determine the average stage of growth of the corn. Count and record the number of fully unrolled leaves on the 19th and 20th plant in each of the 5 sets of plants. The collar of the leaf is visible on a fully unrolled leaf.

To determine the percentage of infested plants, add together the number of cut or damaged plants from each set of 20 plants and divide by 100. To determine the average number of fully unrolled leaves per plant, add together the number recorded for each of the 10 plants and divide the sum by 10.

Management Guidelines

Primarily Leaf Feeding - Control may be needed if 3% to 5% of the plants show leaf feeding and 2 or more cutworms can be found per 100 plants. This management guideline is based on the assumption that for each 1% of plants with leaf feeding there is a good chance that 3 to 5% of the plants will eventually sustain damage due to cutting or notching. This does not mean that for each 1% of leaf feeding that one will lose 3% to 5% yield. Not all damaged plants will be killed and for those that are, adjacent plants may yield somewhat more due to less competition. Cutworm control is generally more effective at this time.

Stalk Damage - This table is based on cutworm feeding slightly above or below the soil surface, and may not work if the soil is dry and the cutworms are feeding 1/2 inch (13 mm) or more below soil surface.

Cutworm feeding and stalk damages

If control is necessary, contact your state Cooperative Extension Service for control materials and rates.

IPM Tip Black Cutworm