Armyworms march across lawns like a cavalry, eating and destroying all grass in their path. If there are enough of them, they can turn a green yard into a brown mess in a matter of days. You need to know how to get rid of armyworms before that happens.
What is an armyworm? It’s the larval form, or caterpillar, of an armyworm moth. Those very hungry caterpillars need lots of food to grow, and grass is one of their favorite things to eat. They’re one of the most common lawn pests in the United States.
If you want to take your lawn off the menu, we’ll cover everything you need to know to get rid of armyworms:
How to get rid of armyworms with chemicals
Chemical armyworm treatments are typically the most effective method for wiping out an infestation, but success is in the timing.
If you wait until you see significant damage in your lawn, the armyworm caterpillars will already be in the later stages of development, when they’re harder to exterminate with chemicals. The key to armyworm control is hitting them as early as possible.
Which pesticides to use for armyworms
Broad-spectrum pesticides do more harm than good in this case because they affect beneficial insects that prey on armyworms, such as certain species of ground beetles, wasps, and flies. Without those natural enemies, armyworm populations can grow and thrive.
Instead of broad-spectrum insecticides, use a targeted or “low-impact” pesticide that won’t hurt beneficial insects. Look for pesticides with one of these active ingredients for armyworms:
- Acetamiprid
- Azadirachtin
- Chlorantraniliprole
- Halofenozide
- Tebufenozide
WARNING: When using any pesticide, follow the instructions on the package carefully. Each pesticide is slightly different and comes with its own set of rules for application.
When to apply pesticides for armyworms
Best time of year to treat armyworms: Armyworms can have several generations per year. Apply pesticides around the time each new generation hatches, before the caterpillars have a chance to grow large.
Armyworms usually hatch in:
- Late April – Early May
- Late June – Early July
- Late August – Early September
Best time of day to treat armyworms: Armyworm treatments are most effective when you apply them in the late afternoon, just before armyworms come out to feed in the evening.
Treat after fall rains: Armyworm infestations are common after heavy rain, especially when it happens in fall. If your area is prone to armyworms, it’s a good idea to apply low-impact pesticides as soon as the ground dries after fall rains.
Remember, once you see brown patches in your lawn, it may be too late to control armyworms because they’re too large and mostly done feeding. Try to wipe them out while they’re still young to prevent damage.
How to get rid of armyworms naturally
Don’t want to use chemicals on your lawn? There are several organic pest control methods that have proven effective against armyworms.
Mow the lawn
Mow your lawn as short as possible for your grass type to reduce the number of armyworm eggs in your lawn. The adult moths find short grass less attractive as an egg-laying site. Plus, when you cut the grass, you can destroy some of the eggs that are already there.
Already have an armyworm infestation? Mowing the lawn can still help. Mowing exterminates some armyworm caterpillars in your lawn and cuts back the survivors’ food source.
Surviving armyworms from the mowed area might move to neighboring grass in search of food. To keep them from migrating to another part of the yard, treat the surrounding area with a natural pesticide, such as the following.
Beneficial nematodes
Beneficial nematodes are microscopic living organisms that feed on more than 200 species of lawn and garden pests, including armyworm larvae.
Follow the instructions on the package to release the nematodes into the part of your lawn with armyworms. The nematodes will find and essentially eat armyworms from the inside out.
There are many different species of nematodes. Make sure you purchase a species that preys on armyworms, such as:
- Steinernema carpocapsae
- Heterorhabditis bacteriophora
- Heterorhabditis indica
These nematodes can handle your armyworm problem in as little as 48 hours in some cases. They cause no harm to you, your pets, pollinators, or the environment. You’ll find them for sale at garden supply stores and online.
Pro tips for using beneficial nematodes:
- Know your soil type, as certain nematodes perform better in certain soils.
- Apply nematodes soon after you buy them.
- Water the treated area before and after applying nematodes unless the soil is already moist from rain. Nematodes love moisture.
- Apply nematodes in the early morning, late afternoon, or on an overcast day because nematodes are sensitive to sunlight.
- Re-apply every two weeks until your armyworm infestation ends.
Bacillus thuringiensis
Bacillus thuringiensis (Bt) is a bacterium that is toxic to armyworm larvae. Bt naturally occurs in soil, so it has no adverse effects on the environment. Bt targets specific pests and has very low toxicity to other organisms (such as humans, pets, or pollinators).
You can find Bt in more than a hundred pest control products, including sprays, granules, dusts, and pellets. Each strain of Bt works on a different type of pest, so make sure you purchase the Bacillus thuringiensis ssp. kurstaki strain that targets caterpillars.
Pro tips for using Bacillus thuringiensis:
- Put Bt on grass that armyworms haven’t eaten yet. Armyworms have to ingest the Bt bacterium for it to work.
- Apply Bt products in the early morning or evening, when the sun isn’t too intense.
- Re-apply Bt once a week until the armyworm infestation goes away. Bt will only stick to your grass for a week or less.
- Store Bt products in a cool, dry place and don’t plan to use them more than a year after you buy them.
- Be patient. Bt doesn’t exterminate armyworms on contact. It needs at least a few days to poison the larvae from inside.
Wait for the first frost
Even if you’ve tried everything and you can’t get rid of the armyworms in your lawn, there’s still an end in sight. The first frost of winter will kill the entire armyworm population (except in the very hot climates of South Texas and South Florida).
After the first frost, you and your lawn will get a break from armyworm torment before spring. You’ll have several months to regroup and work on preventing a new infestation next year…assuming your grass survives this one.
How to prevent armyworms in grass
Unfortunately, it’s impossible to guarantee you won’t have armyworms even if you take precautions to prevent them.
These practices reduce your yard’s risk of armyworm infestation, but you should still keep an eye out for warning signs.
Preventive pesticides
If armyworms are common in your area, you might consider ongoing preventive pesticide treatments to protect your yard. That means applying pesticides every few months before you ever suspect an infestation.
Look for a residual insecticide, which will actively destroy pests in your lawn two to three months after you apply it. Check the label to make sure the one you choose works on caterpillars, the category that armyworms fall under.
Pesticides with these active ingredients should prevent armyworms, as long as you apply them to the lawn regularly:
- Methoxyfenozide
- Diflubenzuron
- Chlorantraniliprole
Follow label instructions: How to apply each pesticide, how much of the chemicals to use, and how often you should apply them all depends on the specific product you choose. You should find all the necessary information on the product label.
Remove weeds
Adult armyworm moths lay their eggs at the base of host plants, and weeds in your lawn make perfect egg-laying sites. One moth lays 50 or more eggs at a time, which adds up to a major armyworm infestation when those eggs hatch.
Remove weeds on a regular basis to make your lawn less appealing. If you give the adult moths fewer places to lay their eggs, you’ll have fewer armyworms (or none at all) in your lawn in a few weeks.
Keep the soil moist and cool
Armyworms want a hot, dry habitat, so make sure your lawn provides the opposite conditions. Water the lawn at least once a week to keep the soil surface moist and cool. Regular watering is especially important in summer when the sun evaporates more water from the ground’s surface.
Beware of overwatering, which can attract other pests and lead to fungal lawn diseases. One inch of water per week (including rain) should be plenty.
Dethatch at least once a year
Thatch is a layer of grass clippings and other debris that builds up between your grass and the soil over time. A little thatch is good for lawn health, but too much debris can host and protect armyworm eggs and larvae (plus a bunch of other pests).
Dethatching is the process of removing thatch from your yard. Dethatch your lawn at least once a year to make your yard less inviting to armyworms and other lawn-damaging insects.
How to identify armyworms
How can you be sure armyworms are the root of your lawn’s problems? Here are some signs to look out for, plus information on what armyworms look like and when you might expect to see them.
Signs of armyworms
Armyworms move quickly, especially when there’s a lot of them. Don’t wait until they’ve sheared down your entire lawn to do something about them.
Early warning signs (when armyworm larvae are still small and easy to exterminate):
- Unusually large numbers of birds feeding on your lawn
- Moths flying around your property
- Transparent tips of grass blades from young armyworms feeding
- Sightings of translucent or light green caterpillars only a few centimeters long
If you notice any of these signs, it’s a good idea to treat your lawn with a caterpillar insecticide or a natural remedy to exterminate the larvae before they grow any larger.
Once armyworms are old enough to destroy your grass, they’ll definitely make their presence known. Pest control methods might be less effective at this point, but you can still try to reduce the population and minimize damage to your lawn.
Signs of a more advanced infestation:
- Brown patches in the lawn
- Completely bare spots
- Grass blades sheared all the way to the ground
- Affected areas of the lawn growing larger every day as armyworms advance
What armyworms look like
Different types of armyworms are common in the United States, and they all look a little bit different.
Fall armyworm:
Fall armyworms are the most common type found in lawns across the country. Full-grown larvae are about 1 to 1.5 inches long. They’re pale brown with dark stripes on their sides and a white inverted Y-shaped marking on their heads.
True armyworm:
Also known as the common armyworm, these are found east of the Rocky Mountains. True armyworms are greenish-brown with white and orange stripes. Their heads are yellow-orange, and there are four dark spots on their undersides.
Southern armyworm:
You’ll only find these in the Southern U.S. They have dark green bodies with reddish-brown heads and a white or yellowish stripe along their sides.
Beet armyworm:
Also known simply as the beet worm, this species is an import from Asia and now lives across the United States. Beet worms are pale green with a darker head.
How to test for armyworms
Armyworm larvae live in the soil, and some are only active in the early morning or late evening. You may not actually see armyworms in your lawn unless you test for them.
Here’s a simple soapy water flush test that will show if you have armyworms in your lawn:
- Step 1: Combine 3 tablespoons of dish soap with 1 gallon of water.
- Step 2: Slowly pour the mixture onto the areas of your lawn that show signs of armyworms.
- Step 3: Look for small worms coming to the surface. Pick up a few to visually confirm if they’re armyworms or something else.
FAQ about armyworms
Whether or not your grass can grow back after an armyworm infestation depends on the type of grass.
Warm-season grasses such as Bermudagrass, Zoysiagrass, and St. Augustine grass can usually recover from an armyworm attack.
Cool-season grasses such as Kentucky bluegrass, fescues, and perennial ryegrass are more likely to die out completely as a result of armyworm damage.
Armyworms can be found in all 48 contiguous states, Hawaii, and Puerto Rico. If you don’t want to worry about armyworms, move to Alaska!
You can find armyworms in your lawn at almost any time of year except winter. The spring generation reaches maturity and lays eggs, which hatch and grow into a new generation that lays its own eggs, and the life cycle repeats again and again until winter.
Yes. Armyworms attack many crops and other plants, including but not limited to corn, barley, wheat, beans, and cabbage.
How a professional can help with armyworms
Have you tried everything to stop the devastating march of armyworms across your lawn to no avail? It might be time to hire a pest control professional with more expertise on armyworms than you can gain from an article online.
If armyworms have already left your lawn brown and dead, you can bring it back to life by seeding new grass. Find a local lawn care pro who can help.
Main Photo Credit: Jeanne Menjoulet | Flickr | CC BY 2.0
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