How to Get Rid of Armyworms in Your Yard
Destructive and tenacious, armyworm invaders devour everything in their path — including your lawn. Here's how to mount a counterattack.
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Armyworms are so named because they invade in large numbers like a battalion of plant-eating soldiers, leaving a path of lawn death and destruction behind them. These plump pests feed on turfgrass and create large brown, dead patches scattered across your yard.
When you spot armyworm moths and their larvae in your lawn, you need to act fast to head off an infestation. “Armyworm moths can lay up to 2,000 eggs that hatch in two or three days and there are usually around four to five generations per year,” says Drew Wagner of Sod Solutions. This prolific reproduction, paired with the larvae’s ravenous appetite, makes armyworms one of the most destructive lawn pests in North America.
Armyworms destroy grass by nibbling the blades down to the nub. Once they have devoured your lawn, they will move on to ornamental plants and flowers and even your vegetable patch. “They can be found feeding on annual bedding plants, succulents, and even crops like home-grown tomatoes,” says Wagner.
The damage they cause resembles that of sod webworms, a similar moth larvae lawn pest. “It’s important to know the difference for proper treatment and prevention,” Wagner says.
Here’s a step-by-step guide on how to spot an armyworm invasion and get rid of these voracious troops before they conquer your lawn.
What Are Armyworms?
Armyworm caterpillars are the larvae of armyworm moths, and several species affect lawns across the U.S. Armyworm larvae typically measure 1-1/2- to two inches long; vary in color from green to brown or black; have green, yellow, red or brown stripes down their sides and backs; and sport a distinctive inverted “Y” mark on their heads.
Adult armyworm moths are about 1-1/2-inches across. Their forewings are dark gray with lighter and darker splotches, and their hindwings are pale gray or white.
Types of Armyworms
The most common types of armyworms that damage lawns in the continental U.S. are:
True or common armyworms: Found east of the Rocky Mountains, this species has a smooth, greenish-brown body, white stripes bordered with thin orange lines running along each side, four large dark spots on its underside and a yellowish-orange head.
Southern armyworms: Common in the Southern U.S., this armyworm has a dark green body, yellow or white stripe running along its length and a brownish-red head.
Fall armyworms: This species (pictured above) features a pale brown body with dark stripes running the length of both sides and a white upside-down “Y” mark on its head. “They are usually found throughout the months of July to October and outbreaks commonly occur after a heavy rainfall delivered by a tropical storm or hurricane,” says Wagner.
Beet worms: Originally from Asia, this species is found on all the continents except Antarctica. It is pale green with a dark head.
Signs of Armyworms
Since armyworm moths lay so many eggs, an invasion can quickly get out of hand. It’s important to check for signs of armyworms regularly and intervene immediately if you spot clusters of eggs, caterpillar frass —moist green or yellow fecal pellets at the base of grass or plant leaves — or live armyworm moths or larvae. The larvae are most active in the early morning or evening.
“You’ll know if you have an armyworm infestation because hundreds of larvae will be feeding in broad daylight,” Wagner says.
Armyworm grubs attract birds, skunks and rodents that feed on the larvae, so large numbers of scavenging birds or scratched areas of lawn may indicate a plentiful armyworm population. You can also switch on a strong outdoor light at night to check if any adult armyworm moths are attracted.
Small brown lawn patches are another early clue of an armyworm problem. “If you see ragged chewed edges or ‘skeletonizing’ of a grass blade that creates a transparent ‘windowpane look, this could be a sign of an armyworm infestation.” Wagner says. Armyworm damage often appears first near trees or buildings because adult moths usually lay eggs on tree bark, exterior building walls or near floodlights.
How to Get Rid of Armyworms
Armyworm larvae are most active in the early morning and late evening. Whether you use a chemical insecticide or more pollinator-friendly options, time your treatment to coincide with activity for maximum results.
“There are numerous effective insecticides you can apply to treat an armyworm invasion, including those containing bifenthrin, acephate, or chlorantraniliprole,” says Wagner. Non-toxic options that won’t harm other beneficial insects and pollinators are a better choice, however. Here are the top products and techniques:
Bacillus thuringiensis (BT): This targeted bacterium will paralyze the digestive system of an armyworm while leaving pollinators and birds untouched. In liquid form it can be sprayed over infected plants.
Pyrethrin: Pyrethroids are a type of natural pesticide made of crushed dried flowers and water that paralyzes armyworms on contact while leaving other insects alone.
Neem oil: Made from neem leaves extract, this natural pesticide controls armyworms without harmful chemicals.
Diatomaceous earth: The sharp particles in this dust are like crawling through tiny razor blades for armyworm grubs but are harmless to pollinators and wildlife.
For a more limited invasion, you can remove armyworm larvae from their host plants by hand and drop them into a bucket of soapy water to kill them. Remove any affected plants from your garden (do not compost) and wash gardening tools to reduce the spread of undetected eggs.
How to Prevent Armyworms From Coming Back
Lawn care best practices can keep armyworms from mounting a second attack. “Always maintain good cultural practices such as mowing, reducing thatch and lightly irrigating your lawn,” Wagner says. “Building a healthy lawn will help it withstand minor infestations of armyworms.”
Aerate your lawn annually. Add about 1/4-inch of organic materials such as peat moss to eliminate thatch buildup, which can harbor armyworm grubs.
Regularly water your lawn. Armyworms prefer turf that is dry and warm. “About one inch of water a week including rainfall is enough,” explains Wagner. “A well-watered lawn makes the soil surface cooler and is less attractive to armyworms.”
Cut your grass no shorter than two inches and keep weeds and wild grasses to a minimum.
Fertilize. “Although fertilizer doesn’t treat infestations, a proper fertilizer schedule will help your lawn remain strong and endure stress a little better,” Wagner says.
Check for armyworm moths and grubs regularly. The earlier you catch an infestation, the easier it is to treat.