Fall armyworm

What is fall armyworm?

The fall armyworm is a moth caterpillar native to the tropical Americas. It cannot survive northern winters, so each year moths ride north from southern Florida and southern Texas through spring and summer. UF/IFAS says it is a regular, serious pest mainly in the southeastern states, and NC State calls it a sporadic but potentially serious turf pest. Cornell notes it reinvades New York every spring.

Caterpillars prefer grasses, including bermudagrass, fescues, ryegrass and bluegrass, but will feed on more than 80 kinds of plants. Females lay masses of 100 to 200 eggs covered in gray scales on grass, posts, signs and buildings. The young caterpillars drop to the turf and feed in groups. When numbers are high they strip the grass and march on as an 'army,' leaving a clean line between eaten and uneaten turf. New sod is especially attractive.

The key with fall armyworm is size. NC State says insecticides are generally ineffective on large caterpillars, which are close to finishing feeding, and that this pest is harder to control than true armyworm or black cutworm. Confirm the caterpillars and check their size before acting.

Signs and damage

• Brown, chewed-down turf that shows up fast, with a sharp line between damaged and healthy grass where the caterpillars are moving as a group.
• Damage that starts near signposts, buildings or fences, where moths laid eggs, and spreads outward.
• Leaves scraped thin on one side by young caterpillars, then eaten from the edges by bigger ones.
• Fuzzy, gray-covered egg masses on posts, signs, walls and plants.
• Newly laid sod hit harder than established turf.

Fall armyworm identification: what to look for

• Larva: Six stages, from about 1.7 mm to about 34 mm (1 1/3 inch). Body green, brown or nearly black with white lines and a wide dark side stripe; raised dark spots on the back; light upside-down Y on an orange-brown head. Skin feels granular but not rough.
• Adult: Wingspan 32 to 40 mm. Forewings mottled gray-brown; males have white triangular spots near the tip and center. Hindwings silver-white with a narrow dark edge. Active at night.
• Eggs: Dome-shaped, laid in masses of 100 to 200 and covered with gray scales; a female can lay about 1,500. They hatch in 2 to 3 days in summer.
• Damage: Grass scraped, then chewed to the ground in patches that spread outward with a sharp front edge.
• Timing: A full life cycle takes about 30 days in summer. Larvae feed for about two weeks, then pupate in the soil for 8 to 9 days.

Life cycle

Moths fly north each spring and summer from overwintering areas in southern Florida and southern Texas. Females lay egg masses on plants and nearby surfaces such as posts, signs and buildings. Eggs hatch in 2 to 10 days, and the caterpillars feed for 2 to 3 weeks through six stages before digging into the soil to pupate. New moths emerge about one to two weeks later. In summer a full cycle takes about 30 days; it slows to 60 days in spring and fall. The number of generations falls from four in Louisiana to one in New York (UF/IFAS, NC State).

When to look: In the Southeast, watch from midsummer into fall. Farther north, outbreaks depend on how many moths migrate in; Cornell reports moths reinvade New York each spring. Check right away whenever turf browns suddenly with a sharp edge between eaten and healthy grass.

Fall armyworm look-alikes

A few things get mixed up with it:

• Black cutworm (Agrotis ipsilon): Gray to nearly black caterpillar with a rough, pebbly skin and no upside-down Y on the head. Older larvae feed at night from pencil-sized burrows, mostly on golf greens and tees.
• Sod webworms (Several species (Parapediasia teterrella, Pediasia trisecta and others)): Smaller caterpillars with dark spots and coarse hairs that hide in silk tunnels in the thatch and make small, scattered brown spots instead of a moving front.
• True armyworm (Mythimna unipuncta): Another migrant caterpillar that Cornell says causes sporadic outbreaks in New York every 3 to 8 years.

How to control fall armyworm

Cultural control

• Check the size of the caterpillars before doing anything; large ones are close to pupating and treatments work poorly on them (NC State).
• If treatment is planned, mow and lightly water the turf first (NC State).
• Keep an extra eye on newly laid sod, which draws egg-laying moths and suffers more damage.

Biological control

• Natural enemies often keep numbers in check (NC State). UF/IFAS lists parasitic wasps (Cotesia marginiventris, Chelonus texanus), a tachinid fly (Archytas marmoratus), ground beetles, the striped earwig, spined soldier bug, insidious flower bug, birds, skunks and rodents, plus a virus and several fungi.
• Lady beetles eat the eggs (NC State).
• UF/IFAS says Bacillus thuringiensis (Bt) is the one pathogen considered practical, but it must be on the leaves when caterpillars first appear.
• Cold, wet springs can weaken parasites, and natural enemies cannot stop very large outbreaks (NC State).

Chemical control

Treat only when caterpillars are small. NC State says pyrethroids, especially lambda-cyhalothrin, and carbaryl give fair control of smaller larvae, and chlorantraniliprole works if applied very early; it also lists acephate, indoxacarb and spinosad for turf. Insecticides are generally ineffective on large larvae. UF notes resistance has been seen in some regions. Use only products labeled for this use; the label is the law.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Keep a close eye on newly installed sod, which NC State says is more attractive to fall armyworms and more easily damaged.
• Watch for egg masses on posts, signs and walls near turf.

When to call a pro

Call a licensed pesticide applicator (see our pesticide applicator license guide) if many small armyworms are actively moving across a lawn or new sod, since timing on small larvae matters.

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Questions about fall armyworm

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