How to Control Fall Armyworms in Lawns

What Are Fall Armyworms?

Fall armyworms (Spodoptera frugiperda) are the caterpillar stage of a gray-brown moth. They arrive in late summer and early fall, feeding voraciously on grass blades. A single larva can destroy its body weight in grass daily, and outbreaks can strip a lawn bare in days.

Identification: Signs of Armyworm Damage

• Brown patches: Irregular, spreading areas of dead or dying grass
• Notched leaf edges: Grass blades chewed down to the soil line
• Visible caterpillars: Green, brown, or black with stripes; up to 1.5 inches long
• Bird activity: Excessive birds feeding on your lawn signals grubs or worms
• Frass (droppings): Small green pellets on grass or soil

Life Cycle and Timing

Understanding the armyworm life cycle helps time treatments:

Stage	Duration	Damage Risk
Egg	3-5 days	None
Larva (caterpillar)	2-3 weeks	Highest — feeding stage
Pupa	1-2 weeks	None
Moth	1-2 weeks	None — lays eggs

Treat when larvae are small (under 1/2 inch). Larger caterpillars are harder to kill and have already caused significant damage.

Organic Control Methods

Beneficial Nematodes

Apply Steinernema carpocapsae nematodes to moist soil at dusk. They hunt and kill armyworm larvae underground. Effective when soil temperature is 60-85°F.

Bacillus thuringiensis (Bt)

Bt is a natural bacteria toxic to caterpillars but harmless to humans, pets, and beneficial insects. Spray in the evening when caterpillars are most active. Works best on young larvae.

Spinosad

A naturally derived insecticide from soil bacteria. Apply at label rates for fast knockdown of active infestations. Reapply every 7-10 days if needed.

Chemical Treatment Options

• Carbaryl (Sevin): Broad-spectrum; kills armyworms on contact
• Bifenthrin: Residual control lasting 2-4 weeks
• Chlorantraniliprole: Systemic, lower toxicity to beneficial insects

"Catch armyworms early and you'll need far less product. A lawn with half-inch caterpillars responds to Bt; one with inch-long larvae may need stronger chemistry." — Emily Watson, entomologist

Application Best Practices

• Treat in early morning or late evening when caterpillars feed
• Water lawn lightly before application to bring larvae to the surface
• Follow label rates precisely — more is not better
• Re-treat in 7-10 days if caterpillar activity continues
• Avoid spraying during bloom to protect pollinators

Preventing Future Outbreaks

• Mow at proper height (2.5-3.5 inches) to encourage deep roots
• Monitor with soapy water flush test: mix 2 tbsp dish soap per gallon of water, pour over 1 sq ft of grass, watch for caterpillars surfacing within 10 minutes
• Encourage beneficial insects (parasitic wasps, ground beetles) by reducing broad-spectrum pesticide use
• Keep lawn healthy — stressed grass attracts egg-laying moths

Early detection and timely treatment are the keys to controlling fall armyworms. Scout your lawn weekly during late summer and fall, and act at the first sign of damage.