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Fall Armyworm Identification and Control

Pest Control · September 2025 · 5 min read

Fall armyworms can devastate pastures, hay fields, and lawns in a matter of days during late summer and early fall. Unlike true armyworms that arrive earlier in the season, fall armyworms migrate northward in waves and feed aggressively. Early identification and quick action are essential to prevent complete forage loss.

Identification Features

Fall armyworm larvae are green, brown, or nearly black with a prominent inverted white Y on the head capsule. This Y marking is the key feature distinguishing them from true armyworms, which lack it. Larvae grow up to 1.5 inches long and have four dark spots arranged in a square on the upper surface of each abdominal segment.

Adults are gray-brown moths with a 1.5-inch wingspan and a distinctive white spot on each forewing.

Scouting Methods and Thresholds

Scout at dusk or early morning when larvae are actively feeding. During the heat of the day, they hide in thatch and soil crevices. Part the grass canopy and count larvae in multiple one-square-foot areas.

• Treatment threshold: 3 or more larvae per square foot in pastures and hay fields
• Lawn threshold: 3–5 larvae per square foot depending on turf value
• Check field edges first, as infestations often start along borders

Control Options and Timing

Pyrethroids such as lambda-cyhalothrin and bifenthrin provide fast knockdown and are the most common choice for pasture and hay fields. Apply when larvae are small—under 0.75 inches—for best results. Larger larvae are harder to kill and have already caused significant damage.

Spray in the late afternoon or evening when larvae are most active on the foliage. Observe pre-harvest intervals for hay fields and grazing restrictions on treated pastures.