Mythimna, Persectania, and Spodoptera species
If you’ve ever looked out on the horizon of your lawn and saw what looked like an army of creepy crawlers ascending, you know where this common caterpillar pest gets its name.
Armyworms are insect larvae that move together en masse and devour all kinds of garden plants, from grasses to veggies.
And since they move in large groups, they can do a lot of damage, rapidly.
We link to vendors to help you find relevant products. If you buy from one of our links, we may earn a commission.
While most species come from warm, tropical or subtropical regions, the cold weather won’t stop them. The adults are flying moths that travel long distances to find the food they want.
In this guide, we’ll cover how to identify and control an armyworm attack.
What Are Armyworms?
Armyworms are the larvae of night flying moths, mostly in the Mythimna, Persectania, and Spodoptera genera.
In the eastern parts of North America, fall armyworms (Spodoptera frugiperda) are a common pest of turfgrass and other grasses, cereals, corn, and rice.
They’ll feed on broadleaf vegetables, but it’s not what they prefer. Given a choice, they’ll always pick bermudagrass (Cynodon dactylon).
Mythimna unipuncta inhabits the same range as fall armyworms.
Western yellow striped armyworm (S. praefica) live in the Pacific Coast states and east to Utah, as well as western Canada.
The yellow striped armyworm (S. ornithogalli) inhabits eastern North America.
Beet armyworm (S. exigua) is an invasive species from Asia that thrives on the west coast and southern half of the US.
These pests exist outside of North America, of course. Mythimna unipuncta is found in warmer regions of Europe, Asia, South and Central America, and Africa.
Unique species have developed in other areas, as well. In Australia, for example, you can find the Australian common (M. convecta) southern (Persectania ewingii), and inland armyworms (P. dyscrita).
They all tend to thrive in warm, tropical or subtropical areas and are only considered serious pests in those ranges.
While they prefer grasses, they’ll eat nearly 100 species of plants.
Identification
All of these pests look fairly similar. The adults are moths and the larvae are chunky, caterpillar-like worms, usually with some kind of pattern on their bodies.
The adults are often tan with reddish-brown forewings, with a wingspan of about an inch and a half wide.
The larvae have three pairs of true legs at the front and five pairs of prolegs along the abdomen.
They are brown, sometimes with light or white stripes, and can grow up to an inch and a half long.
Fall armyworms (S. frugiperda) are about the same size, but the adults have gray forewings and white hindwings.
The yellow striped armyworm adults (S. praefica and S. ornithogalli) are just a bit larger with brown forewings and white hindwings.
The larvae are black with a narrow yellow stripe running along their bodies.
Beet armyworms can be pale green, yellow, or pink with black or pink stripes. The adults have mottled gray forewings and pale gray or white hindwings.
Don’t confuse tent caterpillars with armyworms. Tent caterpillars are sometimes called by the same name and look quite similar but they’re not the same.
Learn more about tent caterpillars here.
If you find large caterpillars in your turfgrass, it’s a pretty good bet that they are armyworms.
Lots of birds digging in your lawn or wasps flitting about are other signs that these pests are present.
When they’re feeding, you’ll have yellow or bare patches in turfgrass, ragged leaves on plants, or even missing or skeletonized foliage.
Biology and Life Cycle
Depending on the species and location, the females lay dome-shaped eggs in the spring. One female can lay nearly 2,000 eggs!
Within a few days, the eggs hatch and the larvae emerge. In tropical or frost-free regions, the females will lay eggs year-round.
There are about six instars, depending on the species, and it’s the final one that does the most feeding and causes the most damage.
After feeding voraciously for between 14 and 30 days, the larvae pupate in cocoons in the soil. These cocoons are made out of dirt or leaf debris held together by silk.
This period lasts about a week, and after that, the adult moths emerge to breed and lay eggs. They live about one to three weeks.
Many species can’t live in areas where the ground freezes but that doesn’t mean northern latitudes are safe. These pests travel in groups, and the adults can travel long distances.
The entire lifespan takes around 60 days, but this timeline depends on the climate.
In cooler regions or times of year, it’s more like 90 days and during the summer or in hot regions, it’s 30 days. There can be multiple generations in a single year.
Organic Control Methods
Populations vary from year to year, so one season you might see hardly any of these pests, the following year you may be overrun with invaders.
If there aren’t too many of them, you may not need to do anything as the natural predators will keep them under control.
In years where there are plenty, you’ll need to use preventative or control measures.
You can use lures to determine when the adults are present, but they won’t trap enough of the pests to control populations.
Don’t use chemical pesticides. These will also kill the beneficial insects that help control armyworms, further upsetting the balance in your garden.
Stick to the organic treatments below:
Cultural Control
Armyworms have become a bigger problem than they used to be as a result of monoculture and modern agricultural practices.
In their native range, the pests have plenty of natural predators that keep them under control.
But in a big yard filled with turfgrass, there is little biodiversity and not enough food and shelter to keep lots of birds, bats, beetles, possums, and other critters present and active.
So having a diverse garden with some native plants and various shrubs, trees, and ground covers to entice local animals and offer hiding places might be enough to help keep armyworms under control.
On top of that, rotate your crops as much as you’re able.
Physical Control
It can be hard to control pests that are hiding out underground. You need to bring them up to the surface so hungry birds can feed on them.
To do this, grab two tablespoons of liquid dishwashing soap and add it to a gallon of water. Dump this over the area.
This will flush the larvae to the surface within just a few minutes. Each gallon can treat about two square feet.
You can then treat them with an insecticide or just let the birds have a feast. If you have chickens, they’ll love it!
Biological Control
There are so many predators that want to eat armyworms that they really only become a problem in gardens that suffer from a lack of biodiversity.
All the birds, bats, possums, skunks, mice, wasps, and more will be happy to see those armyworm larvae and moths arrive.
Lacewings, minute pirate bugs, damsel bugs, assassin bugs, ladybugs, and other beneficial insects will eat the eggs and larvae.
Green Lacewing Larvae
You can purchase lacewing larvae in a variety of packet sizes at Arbico Organics.
There are also beneficial fungi that target softbodied insects, such as Beauveria bassiana.
Bacillus thuringiensis v. kurstaki (Btk) is another powerful option. Bonide’s Thuricide contains this powerful fungus.
Bonide Thuricide
You can find Thuricide available at Arbico Organics.
You can also treat the soil with beneficial nematodes, which will kill the larvae.
Organic Pesticides
To kill the adults, you can use pesticides. You can’t reach the larvae with insecticides unless you flush them out with the soapy water solution we talked about above.
If a lure indicates that adults are present, spray target crops with neem oil, insecticidal soap, or an insecticide that contains pyrethrins.
Monterey Bug Buster-O
I like Monterey’s Bug Buster-O, which you can find at Arbico Organics. It’s a handy all-purpose spray to keep around.
Fight Back!
It’s never a good time when you discover that your garden is under attack from an army of worms.
Dealing with pests is just part of gardening, if it’s not armyworms, it might be something else.
How did you realize you were facing these pests? Let us know in the comments section below!
And for more information about dealing with pests in your garden, have a read of these guides next: