Armyworms are a type of destructive caterpillar that damages crops and landscapes. The name comes from their specific habit of marching in large groups from one feeding area to another.
Fall armyworm is the most destructive species and the villain of the 2021 outbreak, but there are other types of armyworms to watch for. Learn what armyworms are and how to identify them if they land in your yard.
What Armyworms Look Like
The term armyworm refers to the larvae of the Mythimna unipuncta moth (known as the true armyworms) and the larvae of moth species from the genus Spodoptera. All these larvae have in common the habit of marching in colonies and damaging large areas of crops and turf in a matter of days.
They also look similar. Egg masses are covered in gray scales (tiny, flat structures that cover the wings and bodies of moths) and look like cottony, fuzzy patches. Each mass contains 100 to 200 eggs; a single moth can lay up to 1,000 eggs a night.
Armyworm larvae have a segmented body with eight pairs of tiny legs and range in color from light green to gray, brown, and black. Their backs and sides can have white, yellow, orange, or reddish-brown longitudinal stripes, depending on the species.
The body color gets darker with age. Older larvae also gain markings specific to their species. For example, the fall armyworm has a pale inverted Y on the head, while the true armyworm has a network of black lines.
Armyworm moths are about 1.5 inches across and have dark brownish-gray front wings with white and dark patterns. The hind wings are lighter in color, often pale gray-white. The moths are benign, and only the larvae are pests. They fly at night and are attracted by lights.
The Armyworm’s Life Cycle
The armyworm life cycle lasts 30 days in the summer to 60 days in spring and fall and includes four stages: egg, larvae, pupae, and moth.
During the larval stage, the caterpillar pests go through six instars (development stages) that can take 14 to 30 days. While they feed and grow the entire larval stage, armyworms do the most damage during their last instar, which lasts four to seven days.
In this last instar, they consume 80% of the food eaten as larvae. Their marching colonies can defoliate planted areas as large as a football field in two to three days.
Once the larval stage is over, they turn into pupae, then into moths, and the cycle starts again.
Armyworms overwinter in the south and migrate toward the north of the U.S. in the summer. States east of the Rocky Mountains deal with most species and infestations, while the western Coast is mostly affected by the western yellow-striped armyworm and the beet armyworm.
In the southern states, armyworms reproduce and feed from spring to late fall, while up north, they are most active during summer and early fall.
What Armyworms Eat and When
A foliar pest, armyworms primarily feed on leaves, but they also eat stems and buds. Though most species prefer grasses, armyworms are highly adaptable and can feed on hundreds of types of plants, including:
- Cereal crops (wheat, rice, maize, corn, oat, sorghum)
- Forage grasses and turfgrass (they prefer Bermuda but also feed on tall fescue, ryegrass, and other grass types)
- Vegetables (peas, beans, soybeans, cabbage, carrots, onions, cucumbers, beets)
Young larvae are more active in the early morning and late evening. During the day, they mostly hide in the thatch layer. They spend more time feeding when they get older, and if the sun is not too strong, you can see them on leaves during the day.
Common Types of Armyworms
Some of the common species of armyworms active in the U.S are:
- Beet armyworm (Spodoptera exigua). It invades the southern half of the United States, from Maryland to Colorado to northern California.
- Common armyworm or true armyworm (Mythimna unipuncta). Widely distributed east of the Rocky Mountains.
- Eastern yellow-stripped armyworm (Spodoptera ornithogalli). Common in the east of the United States as far west as the Rocky Mountains.
- Fall armyworm (Spodoptera frugiperda). Widely distributed east of the Rocky Mountains.
- Southern armyworm (Spodoptera eridania). Common in the south of U.S. up to Kansas.
- Western yellow-striped armyworm (Spodoptera praefica). Mostly present in California, Washington, and Oregon.
The fall armyworm and the true armyworm are the most active in turfgrasses, but the fall armyworm is the most destructive among the two species. In their “Fall Armyworm Alert!” from early September 2024, announcing there is the potential of a new outbreak, Dr. Dave Shetlar and Dr. Shaohui Wu from Ohio State University describe its fourth instar larvae as “mowing machines” that “will eat day and night unless extreme sunlight drives them down during the day.”
FAQ About Armyworms
How to Prevent Armyworms
To prevent armyworms from getting into your lawn, do the following:
- Avoid overgrowth by fertilizing only as needed
- Keep moisture under control with a correct watering schedule and periodic soil aeration
- Mow the grass at the right height
- Dethatch to eliminate hiding places
- Apply preventive insecticides
For more preventive pest control methods, read our guide, “How to Prevent an Armyworm Infestation.”
What Worms Are Often Confused with Armyworms?
Cutworms are often confused with armyworms. The most straightforward difference is armyworms have stripes and cutworms do not.
How Can You Tell You Have Armyworms in Your Lawn?
You can tell you might have armyworm in your lawn if you see:
- Areas with thinned turf
- Ragged or transparent grass blades
- Brown patches and bare spots
Learn more about armyworm damage and how to check for their presence in our guide, “Signs of Armyworm Damage.”
How to Get Rid of Armyworms?
To get rid of armyworms, you can use several methods:
- Handpicking – when caterpillars are in a small number
- Organic insecticides and natural predators such as Bacillus thuringiensis and ladybugs
- Synthetic insecticides – pyrethroids and chlorantraniliprole insecticides
Find out more from our complete guide, “How to Get Rid Of Armyworms in the Lawn.”
Bring in the Big Guns!
Armyworms can ravage even large lawns in just a couple of days. Don’t risk it! Call the professionals at the first sign of infestation. Find a lawn care pro with LawnStarter and keep your yard safe, green, and lush.
Sources:
- Dean, A., Anderson, M., Hodgson, E. (2022, June 7). Scout for True Armyworms this Spring. Iowa State University Extension.
https://crops.extension.iastate.edu/cropnews/2022/06/scout-true-armyworms-spring. - Joseph, S.V. (2024, August 18). Fall armyworm: Biology and Management in Georgia Turfgrass.
- Shetlar, D., Wu, S. (2024, September 4). Fall Armyworm Alert! Ohio State University. https://buckeyeturf.osu.edu/news/fall-armyworm-alert
- Reynolds, C., Merchant, M., Reynolds, D.S. (n.d.). Armyworm. Texas A&M AgriLife Extension. https://aggieturf.tamu.edu/turfgrass-insects/armyworm/
Main Image Credit: USGS Bee Inventory and Monitoring Lab / Flickr / Public Domain
