The Armyworm and the Army Cutworm
(E830, Revised Aug. 2018)
This publication summarizes pest management of Armyworms and Army cutworms in field crops (alfalfa, canola, corn, small grains, sugarbeets) grown in North Dakota. topics covered include: identification, life cycle, crop damage, trapping, field scouting, and economic thresholds in different field crops.
Revised by: Janet J. Knodel, Professor and Extension Entomologist
Travis J. Prochaska, Area Extension Crop Protection Specialist
Armyworm and army cutworm feed on a wide variety of crops in North Dakota. Although the names are similar, these two insects are distinct, feeding at different times during the growing season. Identifying and finding these insects, and recognizing when they become an economic threat will aid in successful pest management.
Mythimna unipuncta (Haworth)
The adult armyworm is a light brownish gray moth or “miller” (Figure 1) with a conspicuous white spot about the size of a pinhead on each front wing. When expanded, the wings are about 1½ inches across.
Figure 1. Armyworm moth. (G. Fauske, NDSU)
Armyworm larvae (Figure 2) have five pairs of prolegs. Their color varies from pale green to tan in the early growth stage to dark green to black in later stages. The head capsule is brown with netlike patterns.
Figure 2. Armyworm larva. (G. Fauske, NDSU)
Full-grown larvae are smooth, striped and almost hairless. They grow to a length of 1½ to 2 inches. A series of longitudinal stripes on the body are arranged as follows:
- Thin, white, broken line down the middle of the black
- Wide, dark, mottled stripe halfway down the side
- Pale orange stripe with white border
- Brownish mottled stripe
- Another brownish mottled stripe slightly above the legs
The armyworm does not survive North Dakota winters. Armyworm infestations are due to moth migrations from the South. Heavy infestations in southern states produce large moth numbers that fly or are blown northward on southerly winds.
Moth migrations that produce significant infestations typically occur during early June and July. If weather is cool and wet during egg laying and food is abundant, outbreaks are common. However, natural enemies of armyworms often increase rapidly and help mitigate economic damage to field crops (see Biological Control section).
Moths lay eggs at night in folded leaves or under leaf sheaths of small grain plants and other grasses. They prefer to lay eggs in moist, shady areas of lodged, or hail- or wind-damaged grains or grasses.
Armyworm eggs look like small, white beads laid in masses or rows resembling miniature pearls. In eight to 10 days, eggs hatch into larvae (or caterpillars). Larvae pass through six instars (or stages) and complete feeding in three to four weeks.
Larvae stay in the area where they hatched until fully grown or until they run out of food. If all food is consumed, larvae often move in hordes or “armies,” eating and destroying vegetation as they move.
When feeding is complete, larvae move under litter and soil clods, or burrow 2 to 3 inches into the soil, where they make small cells and pupate (resting, nonfeeding stage) (Figure 3). About two weeks later, moths emerge from pupal cases, mate and lay eggs for the next generation. Typically, one generation is produced in North Dakota during most seasons.
Figure 3. Noctuid pupa, a nonfeeding and resting stage. (W. Cranshaw, Colorado State University)