Common Armyworm

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Mature armyworm larva.
Armyworm adult.
Armyworm damage to cool-season lawn in July. Note treated lawn in background.
  • Scientific Name: Pseudaletia (=Mythimna) unipuncta (Haworth) [Phylum Arthropoda: Class Insecta: Order Lepidoptera: Family Noctuidae]


  • Common Names: armyworm and common armyworm


  • Climatic Zone: subtropical to temperate


  • Geographic Distribution: North America, Europe and Northern Asia into the Mediterranean Basin.


  • Damaging Stage: larvae


  • Hosts: Virtually all turfgrasses, both cool-season and warm-season species can serve has hosts.


  • Damage Symptoms: Armyworms get their name from the habit of the larvae occurring in large numbers over a given area. This mass of caterpillars will feed in one area, then migrate, in mass, to another during the night. Descriptions of armyworm damage often include the statement, “My turf literally disappeared over night!” In large numbers, the larvae will eat all the green leaves and stems, thereby exposing the thatch.


  • Description of Stages: Cutworms and armyworms are the larvae of moths and thus have complete life cycles with egg, larval (caterpillar), pupal and adult stages.


Eggs: The eggs are usually round, 3/64 inch (0.75 to 1.0 mm) in diameter, flat on the lower surface, bluntly pointed at the top and often have sculpturing, lines and ridges, on the surface. Armyworms lay clusters of eggs, 100 to 300, usually on weeds within the turf or on trees and shrubs overhanging turf areas.

Larvae: Armyworms are brown to greenish with yellow and brown stripes. The head capsule is yellowish-brown with a broad H-shaped pattern. Full grown armyworm larvae are 3/16 inch (6.0 mm) wide and 1-1/4 to 2 inches (32 to 50 mm) long.

Pupae: The pupae are brown, reddish brown or black and 5/8 to 1.0 inch (13 to 22 mm) long. The antennae, wing pads and legs are firmly joined together, but the abdomen is free to twist around if the pupa is disturbed.

Adults: Armyworms are buff brown with a small, diagnostic white dot on each wing. The adults are generally have wing spans of 1 3/8 to 1 3/4 inch (35 to 45 mm). At rest, the wings are folded flat over the abdomen.


  • Life Cycle and Habits: Armyworms remain in the soil under turf or field crops and are able to withstand most winter temperatures in the larval and pupal stages. Adults emerge in March to April and the first generation of larvae may be present in May to mid-June. The first generation seems to prefer field crops. The second generation in June and July is the one most commonly associated with turf damage.

The female moths are attracted to lights and may lay clusters of 500 eggs in rows, lined up on grass blades. Each female may lay several thousand eggs. The eggs hatch within a week and the tiny larvae remain together feeding on leaf surfaces. When leaves are eaten, the group of caterpillars move on to new leaves. On sunny days the larvae hide in the thatch, coming out at night to feed in mass.

A third generation occurs in August and September and the larvae of this generation dig down into the soil ro pupate or remain as partially mature larvae during the winter.


  • Control Approaches:

Cultural Control - Use Resistant Turfgrasses - Perennial ryegrasses and fescues that contain endophytes are known to slow development or are toxic to armyworm larvae.

Natural Control - Diseases, Predators and Parasites - There are numerous bacterial and fungal diseases that are known to attack armyworms. Common predators include ground beetles, rove beetles, ants and birds. Many parasitic wasps and flies attack the eggs, larvae and pupae. These natural controls appear to have a significant affect on armyworms in turf, but their action is often too late to prohibit visible damage.

Biological Control - Entomopathogenic Nematodes - Products containing the insect parasitic nematodes, Steinernema carpocapsae and S. feltiae, are available and effectively control armyworms. Close attention to label recommendations, especially pre- and posttreatment irrigation and soil moisture requirements, is essential for success. A regular program of applications, beginning when the first eggs begin hatching and continuing at a 14 to 21 day interval, thereafter, has been shown to be successful.

Biological Control - Use BT - Products containing the toxin derived from the bacterium, Bacillus thruingiensis (BT), are commercially available and adequately control caterpillars, especially armyworms, if enough material is ingested by young larvae. Use of these products has had mixed results in turfgrass management because of treatments being applied to a population consisting of large and small caterpillars. Use of this microbial toxin is best when the caterpillars are in the first to third instars. Irrigation should be omitted for at least 12 to 24 hours after application. Affected caterpillars rarely surface.

Chemical Control - Insecticides - The principle of controlling grass eating caterpillars with insecticides is to apply the treatment to the turf and omit irrigation for 12 to 24 hours until the insect consumes the foliage and makes contact with the treated surfaces. Both preventive and curative approaches can be successful. Monitoring to determine population level and size of larvae before treatment and treatment effectiveness after application is recommended. It is well to keep in mind that young larvae are far more susceptible to insecticides and other forms of control than are larger larvae.

Curative Treatments. Numerous organophosphate, carbamate and pyrethroid insecticides effectively control cutworms in turf. On golf course greens and tees, it is recommended that spraying of a band of turf surrounding the affected area (a minimum of one boom or spreader width) will help keep larvae from moving back onto the greens from surrounding turf.

Preventive Treatments. Field tests with long residual, systemic insecticides have shown that application to golf course greens or tees in early May, provided suppression of armyworms and cutworms until, at least, late June. Application at this time also provides season-long control of grubs and billbugs on the tee and green banks when treatment is extended to these areas.

Monitoring

Regular monitoring of turf areas for evidence of cutworm or armyworm infestations and applying treatment only when larvae are present and/or damage seems eminent, is part of the IPM process. Monitoring includes looking for larvae, damage and/or evidence of birds (starlings) probing the turf for larvae.

To determine if larvae are actually present, an effective method is to use a solution of liquid soap and water (two tablespoons of liquid Joy® dishwashing detergent in two gallons of water) spread over a one square yard area to flush larvae to the surface. When using preventive biological or insecticide strategies, sample every 14 to 21 days to detect early stage caterpillars, before significant damage is evident.

Thresholds

Determination of whether treatment is needed depends upon the type of turf area concerned and the standard of quality desired or required. For golf courses, damage in roughs is more tolerable than damage in fairways, and damage on tees and greens may not be tolerated at all. Likewise, home lawns rarely need treatment except when large populations of armyworms are found.

Many books and extension publications on turf insect management often mention “population thresholds.” Thresholds are the numbers of pest insects present in a given area (usually per square foot or yard) that warrant control. The concept behind using thresholds is to reinforce the IPM principle that application of a pesticide is not always warranted simply if pests are present. Pests must be present at populations high enough to eventually cause damage.

In high cut turf, 5 to 10 cutworms or armyworms per yd2 may just begin to show damage. However, two to three cutworm spots on a high quality golf green may be more than can be tolerated.