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Armyworm in Forage Grasses

Updated: Aug 31, 2021

There have been some reports of true armyworm (also known as common armyworm) damage in forage stands occurring in Ontario this summer. Armyworm tends to prefer feeding on forage grasses (including alfalfa-grass mixtures), sorghums and sudangrass, cereals (wheat, oats, barley, triticale & rye) and corn. The name “armyworm” reflects the large-scale invasive and destructive behavior of the larvae, where they “march” into a field and feed on crops. Severe damage can occur quickly.

Fully grown true armyworm are 4 cm (1-1/2 inches) long. They are dull-green to brown, with white-bordered stripes running laterally along the body. (Figure 1) Adult moths emerge in early spring and lay their eggs in grassy vegetation. When larvae hatch they begin feeding. There are 2 generations per year, but the first generation tends to do the most damage. The most severe damage typically occurs during June and July. Outbreak years tend to occur when there are cool springs that are detrimental to parasites that usually keep them under control.

Figure 1 – True armyworm larvae (photo credit – OMAFRA)

True armyworm larvae feed at night. Young larvae skeletonize the surface of leaf blades and inner surface of the sheath. They later feed from the margins of the leaves, consuming large amounts of tissue. Armyworm do not usually feed on pure stands of alfalfa, but do feed on forage grasses and alfalfa-grass mixtures, as well as sorghums and sudangrass.

Scouting & Thresholds

The best time to scout for true armyworm is shortly after dusk when they begin active feeding. During sunny days, they will be down on the ground in soil debris or clods and difficult to find. On cloudy days, they might be visible in a leaf whorl or head. Brown frass might be visible on the plants and soil surface.

When scouting, note the larvae size, and whether there are any white parasite fly eggs that may be attached to their backs behind the head. (Figure 2) When these eggs hatch, the maggots feed on the armyworm larvae and kill them.

If there are 4 – 5 un-parisitized larvae per square foot and the larvae are smaller than 2.5 cm (1 inch), insecticide control may be warranted. If they are larger than 1 inch, they have already done most of the feeding damage, and insecticide will not provide adequate control. Refer to OMAFRA Publication 812 “Field Crop Protection Guide”, page 57, for insecticide information.

Figure 2 (left) – Look for these whitish, oval-shaped, parasitic fly eggs located behind the larvae head. (photo credit – OMAFRA)

Figure 3 (right) – Feeding damage to sudangrass.

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