Army Worms: Eliminating These Munching Menaces

If you’ve ever watched a row of plants get demolished by army worms, you’ll know in part how they get their name. These ravenous little caterpillars will devour an entire plant and then move on en-masse to the next, gobbling that one down with equal vigor.

But what exactly is an armyworm? Are there different kinds, and where are they found? What do they eat? How long do army worms last? And perhaps the most important, how to get rid of armyworms? We’ll discuss all of this and more today. I’ll give you everything you need to know about this persistent pest and how to wipe it out in your yard.

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Army Worm Overview

Common Name(s) Common armyworm, true armyworm, white-speck moth, northern armyworm, Oriental armyworm, rice ear-cutting caterpillar, fall armyworm, beet armyworm, asparagus fern armyworm, small mottled willow moth, lawn armyworm, paddy swarming caterpillar, African armyworm, okalombo, kommandowurm, nutgrass armyworm, southern armyworm
Scientific Name(s) Spodoptera exempta, Spodoptera exigua, Spodoptera eridania, Spodoptera frugiperda, Spodoptera mauritia, Mythimna unipuncta, Mythimna separata
Family Noctuidae
Origin Various regions internationally depending on species
Plants Affected Alfalfa, artichoke, asparagus, avocado, barley, bean, beets, broccoli, cabbage, carrot, cauliflower, celery, citrus, cole crops, collards, corn, cotton, cowpeas, cucumber, eggplant, flax, kale, lettuce, maize, millet, mustard, nutgrass, oats, okra, onion, parsley, peas, peanut, pepper, potato, rapeseed, rice, rye, sorghum, soybeans, sugarcane, sunflower, sweet potato, tobacco, tomato, turnip, velvet bean, watermelon, wheat, many flower species and wild grasses
Common Remedies Bacillus thuringiensis var. Kurstaki sprays or powders, azadirachtin sprays, spinosad sprays, horticultural oils such as neem oil, beneficial insects such as ladybugs, lacewings, predatory wasps, and ground beetles, birds. Floating row covers may prevent egg laying, and diatomaceous earth may help deter pests from moving in.

Types Of Army Worms

There are multiple types of army worms. I’m going to go over the seven most common army worm types and discuss some of the differences between them.

Mythimna unipuncta, ‘Common Armyworm’, ‘True Armyworm’, ‘White-Speck Moth’

Mythimna unipuncta, ‘Common Armyworm’
Mythimna unipuncta, ‘Common Armyworm’. Source: Bug Guide

Typically a greyish-green or greyish-brown in color, the common army worm has long, dark stripes along its body. Its moth form, the white-speck moth, has fine black dots along the outside of its wings, and the center of the wings has a number of whitish spots that give it its name. These are common throughout north, central, and south America, as well as southern Europe, central Africa, and western Asia.

Mythimna separata, ‘Northern Armyworm’, ‘Oriental Armyworm’, ‘Rice ear-cutting caterpillar’

Mythimna separata, 'Northern Armyworm'
Mythimna separata, ‘Northern Armyworm’. Source: IRRI Images

The greenish larvae of the northern armyworm are also characteristically striped along their body, although they have two wider stripes down the back separated by a lighter colored line, and a brownish head. The adult moth is greyish with a yellow tinge along the wings. These are common in China, Japan, southeastern Asia, Sri Lanka, New Zealand, Australia, and some of the Pacific islands.

Spodoptera eridania, ‘Southern Armyworm’

Spodoptera eridania, 'Southern Armyworm'
Spodoptera eridania, ‘Southern Armyworm’. Source: sassycrafter

Grey-green or blackish-green larvae with a reddish-brown head are a sign of the southern armyworm. As the larvae mature, they develop into a yellow striped armyworm with additional white or off-white stripes, and they darken to become a grey-black tone otherwise. Adult moths are brown in coloration with brown forewings and off-white rear wings. It’s widespread through central and south America and the Caribbean, and in the United States it’s typically found in the southern states.

Spodoptera frugiperda, ‘Fall Armyworm’

Spodoptera frugiperda, ‘Fall Armyworm’
Spodoptera frugiperda, ‘Fall Armyworm’. Source:

The yellow striped armyworm known as the fall armyworm is mostly brownish, with two distinctly brighter yellow stripes along its sides. Sometimes the yellow stripes are bordered in white. These look hairier than other army worm species. The adult moth has a darker forewing and a white rear wing, and there are patterns along the forewing. Males tend to be more heavily patterned than females. The fall army worm is common in the autumn in eastern and central North America and into South America.

It’s also become a major threat to food security in Africa, which compounds the already hunger and poverty issues on the continent.

Spodoptera exigua, ‘Beet Armyworm’, ‘Asparagus Fern Armyworm’, ‘Small Mottled Willow Moth’

Spodoptera exigua, 'Beet Armyworm'
Spodoptera exigua, ‘Beet Armyworm’. Source: Must_ray

These greenish-brown caterpillars have long, dark stripes along their upper sides. The adult moth is a dingy brown in color with reddish-brown speckled forewings and ivory or beige rear wings. Known widely as an agricultural pest, the beet armyworm originated in southeastern Asia, but has spread worldwide.

Spodoptera mauritia, ‘Lawn Armyworm’, ‘Paddy Swarming Caterpillar’

Spodoptera mauritia, ‘Lawn Armyworm’
Spodoptera mauritia, ‘Lawn Armyworm’. Source:
Butterfly House

The lawn armyworm starts as a pale green larvae, but develops a green back with white and brown stripes along its sides as it matures. Rows of half-circle black spots are visible along the whitish stripe along their sides. The adult moths are a greyish-brown with dark patterns. It’s widespread throughout the Pacific islands, as well as from the Red Sea to India and along the Malayan peninsula down to Australia.

Spodoptera exempta, ‘African Armyworm’, ‘Okalombo’, ‘Kommandowurm’, ‘Nutgrass Armyworm’

Spodoptera exempta, ‘African Armyworm’
Spodoptera exempta, ‘African Armyworm’. Source:

Interestingly, the older African armyworm larvae have different colors depending on whether they are in an area alone or in large numbers. Those which are alone tend to be green in coloration, where those who are in a large population are black or dark greyish in color. Striping occurs along the bodies as in all other army worm species. The adult has a dull grey-brown colored forewing with an off-white hindwing and visible veining. It’s commonly found in Africa and Asia.

Life Cycle of Army Worms

Army Worm Life Cycle (African)
Army Worm Life Cycle (African)

The adult moth will lay her eggs. These are typically in oblong shape and hidden on the underside of leaves or grasses. While the location of the eggs and the number will vary by species, typically an adult will place between 30-80 eggs.

When the eggs hatch, the young larvae are almost all grey-greenish in coloration, and will develop through six different larval stages, called instars, to change color to a darker and visibly-striped color depending on their species. True armyworms may have up to nine instars, but most other army worms have six instars. This larval stage is when the pests are at their most detrimental to crops.

When the instars have concluded, the larvae will burrow into soil and form a pupa. While the pupation period will range in length by species, typically 12-14 days later most army worm types will hatch into moths.

The complete life cycle also varies by species, but ranges between 30-90 days from egg to adult moth, most of which is spent in the larval stages. The adult moth typically only lives for between 9-14 days, and during that time can lay between 1000-1500 eggs, making it a rapidly-spreading and destructive pest to encounter.

While most of this cycle will take place in the spring and summer, for the fall armyworm, it also continues into the fall months. The life cycle will take a bit longer in the autumn.

Common Habitats For Army Worms

Like the majority of caterpillars, army worms live where they eat. They are typically born on some of their preferred foodstuff, and will consume that and move on to other plants. In large larval populations, they really do look like a small army, creeping forward to the next series of food plants from their last host location in an almost organized fashion.

Most army worms do not overwinter in soil, and instead choose to migrate to warmer climates for the winter months. For North American species, they typically survive the winter in areas such as Florida or the southeast, or in other frost-free locations.  However, for those few species which do, they typically stay in a pupal state over the winter.

What Do Army Worms Eat?

Due to the wide variety of foodstuff that army worms eat, we’re going to break this segment down by type of army worm.

Common Army Worm: Many Gramineae grass species including wheat, barley, sugarcane, corn, sorghum, oats, rice, and rye. Can also feed on other crops such as sweet potato, alfalfa, pepper, artichoke, parsley, celery, bean, onion, cabbage, lettuce, carrot, and cucumber.

Northern Army Worm:  Most commonly consumes maize, sorghum and rice. May also affect other Gramineae species.

Southern Army Worm:  The southern armyworm has a wide range of host plants, including avocado, beet, cabbage, carrot, citrus, collards, cowpeas, eggplant, okra, peanut, pepper, potato, sunflower, sweet potato, tobacco, tomato, velvet bean, and watermelon. They also eat weeds, although pigweed and pokeweed are the most popular.

Fall Army Worm: More than 60 and reported up to 80 different varieties of plants. Among these are forage and crop grasses, corn, alfalfa, soybeans, cotton, and most vegetable crops.

Beet Army Worm: An extremely large list of plants including sugar and table beets, beans, asparagus, lettuce, peas, celery, potatoes, cotton, tomatoes, tobacco, cereal grains such as wheat and corn, oilseed plants such as flax, multiple flowering plants, and a wide variety of weeds.

Lawn Army Worm: While rice is the preferred crop for this pest, it’s also known to feed on nutgrass, cabbage, kale, cauliflower, rapeseed, mustard, broccoli, turnip, sugarcane, and a wide variety of grasses including lawn grasses.

African Army Worm: Almost exclusively, the African armyworm feeds on cereal, pasture, and grassland grasses. Most cereal crops are targeted, including maize, sorghum, millet, rice, wheat, and oat seedlings. A study has shown that two larvae can eat an entire 10-day-old maize plant on their own, which makes these pests particularly dangerous to corn crops.

Armyworm damage is extensive in almost all cases on their preferred food. They act like cutworms on grasses, and on food crops, they often skeletonize larger leaves. On some crops such as corn or fruits/melons, they can bore into the cob or fruit to eat that as well. This pest is extremely destructive!

How To Get Rid Of Army Worms

The first line of defense for getting rid of army worms is to hand-pick them off of plants and drop them into a bucket of warm soapy water to drown. But you might be asking for other ways how to kill armyworms. Let’s go over some options which you can use to not only kill them off, but prevent them from returning.

Organic Armyworm Control

An excellent choice for army worms control is Bacillus thuringiensis var. kurstaki, also referred to as BT. Available in both a liquid formula in sprays such as Monterey BT and in a powdered version as Garden Dust, this bacteria will poison caterpillars. It is effective not only on army worms, but on other caterpillars such as cabbage loopers, cabbage worms, tomato hornworms, and cutworms as well as others.

Azadirachtin sprays such as Azatrol EC are also quite useful as an choice on how to kill armyworms. Azadirachtin, the naturally-occuring active ingredient that makes neem oil so effective, will smother the eggs of army worms and will kill the larvae.

Other horticultural oils such as neem oil are also very effective against army worms, but have lesser amounts of azadirachtin than a pure form such as Azatrol EC.

A spinosad spray such as Monterey Garden Insect Spray is just as effective as the other recommendations I’ve listed, and like BT and azadirachtin, it works against multiple pests including the entire caterpillar family, asparagus beetles, and potato bugs.

Environmental Armyworm Control

There are a surprising number of predators of army worms. Cultivating these natural predators should always be your first line of defense, even more than using the organic methods mentioned above.

Among these predators are birds, especially crows, starlings, and bobolinks. Some varieties of frogs and toads will also happily consume army worms and the adult moths. If you’ve been asking how to get rid of armyworms in grass, especially for your lawn or grain crops, attracting these natural predators is one way. However, some forms of birds may also pick at grain crops, so be prepared to place bird netting over your plants if they become more of a problem than a benefit.

Beneficial nematodes will help from within the soil. These microscopic soil-dwellers will quite happily feast on larvae in their pupal form. While they’re not as effective on the soil’s surface unless it’s moist, those which are on top of the soil will also attack larvae that may be burrowing down to pupate.

Don’t forget to encourage parasitic wasps to live in your yard. These tiny wasps are not harmful to humans or larger animals such as our pets, but they do prey on insects regularly, laying their eggs inside of or on top of larvae. As the eggs hatch, the juvenile wasps feast on the larvae and kill them.

Also, ladybugs and lacewings are quite useful as they will eat the eggs of army worms when they find them. They do double-duty as pollinators, although they are nowhere near as good at pollinating your plants as bees are.

A less commonly-known predator of army worms is the ground beetle from the Calosoma genus, which is generally considered to be a beneficial insect as they eat most types of caterpillar. Unlike parasitic wasps, ladybugs, and lacewings, ground beetles are not usually available commercially, but they naturally migrate to areas where army worms and other caterpillar-type larvae are common.

Preventing Army Worms

In prevention, ensuring that your yard is full of beneficial predators is a great step. Cultivate plants that attract lacewings and ladybugs such as dill, caraway, coriander, yarrow, marigold, and fennel to lure those wonderful insects to your yard. Most of these will also draw some predatory wasps as well, but other choices which can lure predatory wasps include tansy, lemon balm, and parsley. While the goal is to have your beneficial predators devour your eggs and occasionally the pests themselves (like aphids), ensuring that they have a constant food supply on hand will encourage them to stay year-round.

If you can keep the adult army worm moths away from your plants, they can’t lay their eggs on them. While this doesn’t work as effectively on grasses, using a floating row cover like a Harvest-Guard over your other food plants can help protect them from a number of pests including army worms.

Diatomaceous earth is a good cabbage looper preventer

While this is mostly an irritant and discouragement to army worms, spreading some diatomaceous earth over the leaves and stems of your plants may encourage the army worms to find another source of food. Diatomaceous earth is harmless to people and pets, but is like crawling over razor blades to the super-sensitive skin of army worm larvae.

Frequently Asked Questions

Q: My lawn has brown patches. Is it army worms?

A: It could be. Not all browning is caused by army worms, and it could be heat-related or animal-related. However, if you are seeing large patches of browning in your lawn virtually overnight, go outside early in the morning or just after dusk and look for armyworm larvae or moths. If you see signs of either, it would be wise to spray the lawn with some BT and see if it improves. Most lawns will come back relatively quickly after an army worm invasion, but only if you catch it quickly.

Incidentally, keeping your grass mowed regularly and mowing it a little shorter than you normally would is also an easy way to kill army worms. Watering regularly will also encourage them to move on.

Q: Where do army worms come from?

A: It really depends on the kind of army worms you’re trying to identify. However, there are variations internationally. They seem to be most prevalent in areas where their natural food sources are, so if you live in an area where rice is grown, you’re more likely to see rice-feeding armyworms in that area.

Q: Are army worm moths out during the daytime?

A: No, most moth species are nocturnal. Unfortunately, so are the army worms themselves. While you may find some on plants during the daytime, most larvae feed from dusk until dawn or on heavily-shaded areas of the plants.


While army worms are a dangerous pest to encounter in the garden, and can truly wreak havoc if they’re not controlled, hopefully this has given you plenty of information to destroy this menace. Have you ever battled armyworms, and if so, what kind? Are there any tricks that you’ve discovered to wiping them out? Feel free to share your stories!

 

Army worms can march on your garden rows and decimate your plants. There are many types, so learn how to ID them, prevent them, and get rid of them!

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