Army Worms: How to Identify and Control Them Effectively
Armyworms are destructive garden pests that get their name because they travel in small insect armies and consume just about everything in their path.
But what exactly are armyworms? And are there types of armyworms? This article answers all that and shares how to control them effectively.
There are a number of species of armyworm caterpillars, many with a distinct taste for a particular plant or vegetable. But some will eat anything green or red or yellow. They’re most active at night and hide in plants and under garden debris during the day.
In their larval stage, army worms attack a variety of crops as well as grasses, sometimes moving en masse to new areas in a way that brings to mind, as its name suggests, an army on the march.
The assault is mostly aerial, with the gray moths usually arriving under cover of darkness to lay eggs. The biggest invasion of army worms usually occurs after a cool, wet spring.
Read on to learn more about them including what the seven most common species of armyworms are, what their host plants are, and how to effectively control these destructive pests.
What are Army Worms?
Armyworms are the larvae of moths belonging to the Noctuidae family. These nocturnal fliers lay eggs that develop into tiny larvae that can reach lengths of 2 inches and curl up when startled.
Although they eventually develop into moths, the damage is primarily caused by the worm-like larvae stage. Armyworms have the appearance of an army battalion when they are in large numbers. Lawn grasses can be wiped out by severe infestations in a matter of days.
Army caterpillars come in a variety of species, but they all share a distinctive inverted Y mark on their heads.
Agricultural crops, the grass blades of pasture grasses, and cool-season grasses serve as nesting grounds. Rice-strain and corn-strain plants are more susceptible than others.
The common name ‘armyworm’ comes from the fact that these insects often appear in large numbers when foraging or moving to their next host during the early morning or late evening hours.
Life Cycle of an Army Worm
Armyworms are prolific and responsive to favorable conditions. Their eggs are laid in fluffy masses on the crowns of seedlings and on the leaves of older plants.
In 5-10 days tiny caterpillars hatch and feed for several weeks. During early development, there might not be much damage. However, over 93% of the foliage is consumed after the fourth larval stage.
The larvae will tunnel into the thatch and soil layer by the sixth stage of development, where they will pupate and emerge as adults 10 days later.
Three and more generations are commonly produced each season — just as you’re ridding worms from the leaves of your garden plants, another generation is preparing to leave the soil to replace them — but some species of army worms will lay up to six times.
Female moths lay eggs in masses in the hundreds and prefer to lay them on light-colored surfaces such as tree trunks, fence rails, the underside of tree limbs, or even grass.
These egg masses are also light-colored ranging from white to cream with a furry or moldy appearance. They get darker in color over time. Like most caterpillars, they live where they eat and are often born on their food source.
In places with milder winters such as the deep south, armyworms will overwinter as eggs and pupae beneath the soil. In warm climates, they may be active all year.
How to Identify Army Worms
Markings on newly hatched caterpillars are usually hard to distinguish, older larvae have distinctive stripes that run the entire length of the body with an upside-down Y on the front.
Fall armyworms (Spodoptera frugiperda) are brown with yellow stripes, and beet armyworms (Spodoptera exigua) are green with light stripes.
Adults are gray, mottled moths (1-1/2 inch wingspan) with a small white dot in the center of each forewing and dark margins on the hind wings.
To figure out how to identify the seven most common types of army worms, make sure you read the next section in this article.
Note: Many areas are too cold to support overwintering army worms. But they’re often pushed north into these areas by strong spring winds and storms.
Fall armyworms cause serious defoliation to pastures and turf grasses in the southeast. Some of these army worms, as moths, migrate from as far away as the Caribbean.
Most Common Types of Army Worms (And What They Feed On)
Several species of armyworms affect lawns and crops across the United States and the rest of the world. Let’s look at the seven most common ones:
1. Common Army Worm (True Armyworm)
The common armyworms are usually greyish green or greyish brown, and they have long, dark stripes running along their bodies.
The white-speck moth’s adult form, named for the whitish spots in the middle of its wings, has tiny black dots along the tips of its wings.
This species is widespread in North, South, and Central America, as well as southern Europe, central Africa, and western Asia.
They prefer eating any species that belongs to the Gramineae grass family, which includes wheat, barley, sugarcane, corn, sorghum, oats, rice, and rye.
In addition to this, they are able to consume other types of crops, including sweet potatoes, alfalfa, artichokes, onions, celery, beans, peppers, cabbage, lettuce, carrots, and cucumbers.
2. Fall Armyworm
The larvae of the fall armyworm are mainly brownish in color with two distinct, brighter yellow stripes running down their sides, which are occasionally bordered in white.
Compared to other species of armyworms, these appear to be more hairy. The moth has a darker forewing and a white hindwing, with patterns on the forewing. Compared to females, males have more elaborate patterns.
Early fall and late summer see heavy populations of the fall army worm across much of eastern and central North America and even into South America. The fall armyworm caterpillar poses a significant threat to food security in Africa, exacerbating the continent’s hunger and poverty problems.
More than sixty, and possibly as many as eighty, different plant varieties are consumed by the fall armyworm’s voracious appetite. These include most vegetable crops, as well as forage and crop grasses, corn, alfalfa, soybeans, cotton, and more.
It’s also this particular armyworm species that are associated with foliar damage on turfgrasses such as tall fescue and bermudagrass. Late summer and early fall see the arrival of fall armyworms.
3. Northern Army Worm
The larvae of Northern army worms are green, and their bodies are usually striped. However, they have two wider stripes down the back that are separated by a lighter line, and their heads are brown. The wings of the moth are grayish with a yellowish tint.
These are widespread throughout Asia, including Sri Lanka, China, Japan, Southeast Asia, New Zealand, Australia, and some Pacific island nations.
Rice, maize, and sorghum are the three grains that are consumed the most frequently. Other Gramineae species may be affected as well by these pests.
4. Southern Army Worm
Larvae of the Southern army worm species have a greyish-green or blackish-green body and a reddish-brown head.
The larvae grow into a yellow-striped caterpillar with extra white or off-white stripes as they get older. They get darker until they look greyish-black. The front wings are brown, and the back wings are off-white.
They are prevalent throughout much of South and Central America, as well as the Caribbean. In the United States, it is most frequently observed in the southern states.
Numerous plants, such as avocado, beet, cabbage, carrot, citrus, collards, eggplant, okra, peanut, pepper, potato, sunflower, sweet potato, tobacco, tomato, velvet bean, and watermelon are hosts for the southern armyworm. They consume weeds as well, but pigweed and pokeweed are their preferred varieties.
5. Lawn Army Worm
Initially appearing as light green larvae, lawn armyworm eventually develops into green caterpillars with white and brown stripes running along their sides.
Along the white stripe on their sides, you can see rows of half-circle black spots. The adult moths have dark patterns and are grayish-brown in color.
They can be found all over the islands of the Pacific, as well as in the area between the Red Sea and India, and from the Malay Peninsula to Australia.
Rice is the crop that is most susceptible to damage from this pest but it will also eat nutgrass, cabbage, kale, cauliflower, mustard, broccoli, turnip, sugarcane, and a wide variety of grasses, including lawn grasses.
6. Beet Army Worm
These caterpillars are a greenish brown color, and they have long, dark stripes across their backs. The back wings of the adult moth are ivory or beige, while the front wings are spotted with reddish-brown.
The beet armyworm is a widespread agricultural pest that originated in southeast Asia but has since spread globally.
Beet armyworms can attack a wide range of plants including sugar and table beets, beans, asparagus, celery, potatoes, cotton, tomatoes, lettuce, peas, tobacco, cereal grains like wheat and corn, oilseed plants like flax, many flowering plants, and a wide range of weeds.
7. African Army Worm
Intriguingly, the older African larvae have different colors depending on whether they are alone or in large groups.
Caterpillars that are in groups are black or dark greyish, while those that are alone are green. As with all other species, stripes run along the body. The adult has forewings that are a dull grey-brown color, while the hindwings are off-white in color with visible veining.
Africa and Asia are the most common regions where you can find them.
The African armyworm relies almost entirely on grasses for its diet, including those found in cereal crops, pastures, and grasslands. It also attacks the majority of cereal crops, including maize, sorghum, millet, rice, and wheat seedlings, as well as oat seedlings.
According to research, just two larvae can devour an entire maize plant that is 10 days old, making this pest a serious threat to corn fields.
Armyworm Damage
In spring, larvae stay close to the ground, feeding on grasses and other low-growing plants. Later in the season, they move up to feed on plant leaves and fruit.
Early signs of the presence of armyworms include their characteristic damage where they chew the leaves leaving scalloping along the leaf margins.
Army worms ‘skeletonize’ leaves of lettuce, cabbage, beans, and corn. In tomatoes, they make shallow gouges in fruit. Corn is their favorite target. They feed on leaf whorls and burrow into the ears.
Sometimes pulling back the husk from an ear in an infested field will reveal several worms drilling through kernels. Young, early-season corn is especially vulnerable to worm attacks.
The damage from grass-loving fall army worms includes the reduction of graze-able pasture for feed animals and unsightly lawns for homeowners.
Fall armyworm damage is most likely to occur from August to October when populations are at their peak. It is not unusual for the first reports of damage to come as early as July during periods of drought.
How to Get Rid of Army Worms
If you don’t suffer army worm outbreaks, thank its natural enemies and predators, including birds, beneficial insects, and other larvae predators.
If pest numbers are high, it suggests these natural predators have been done in by the very pesticides applied to kill the army worms. The absence of predators gives the re-generating pest a decided edge in your garden. So, to manage armyworms…
- Avoid using harmful pesticides or practices that would inadvertently destroy beneficial insects, your first line of natural defense.
- Use pheromone traps to monitor the arrival of moths. When you first notice them — look for the distinctive white dot on their forewings — it’s time to start a closer inspection of your plants.
- Look for larvae and signs of damage beginning in early spring. Caterpillars will often be found feeding on the undersides of leaves and new growth. Handpick the worms you discover and don’t be tempted to crush them between your thumbs. Instead, drop them in a bucket of soapy water.
- Release trichogramma wasps to parasitize any newly laid eggs. These tiny beneficial insects — 1mm or less — insert their eggs inside of pest eggs, killing them before they enter the plant-eating larval stage.
- Other beneficial insects, such as lacewing, ladybugs, and minute pirate bugs feed on armyworm eggs as well as the young larval stage. Remember: beneficial insects help control other harmful pests, including aphids, earworms, cutworms, cabbage loopers, a variety of mites, and insect eggs.
- Plant to attract birds and beneficial insects. Birds are especially fond of moths and will pull larvae from lawns and plants. In the fall, uncover and turn your soil before putting it to bed, giving birds a chance to pick off the exposed pupae.
- If you’ve had an infestation or are otherwise worried that conditions, including a cool, wet spring, will encourage the worms, release beneficial nematodes into your soil. These microscopic soil creatures feed on the eggs, pupae, and larvae of some 200 pests. They will not harm vertebrates, whether human or amphibians, will not harm plants, honey bees, or earthworms, and won’t threaten beneficial insects who, like the trichogramma wasp, lay eggs in something, not just anywhere in the dirt. Yet beneficial nematodes murder army worm eggs and pupae found in the soil.
- Applications of Garden Dust aka Bacillus thuringiensis (Bt-kurstaki) or OMRI-listed Monterey Garden Insect Spray (spinosad) will kill caterpillars.
- After the season has advanced, natural horticultural oil sprays can be used on plants showing signs of worm infestations. Multi-purpose neem oil spray is effective on various stages of the larvae as well as mites. It also prevents fungus growth. Complete coverage, including undersides of leaves and junctions with stems, is critical.
- Use fast-acting organic insecticides if pest levels become intolerable.
- If infestations are large, you can use insecticides containing active ingredients such as spinosad, bifenthrin, cyfluthrin, and cypermethrin. Avoid irrigating the area for 24 hours to allow maximum impact on armyworm. For this reason, don’t apply insecticide when you’re expecting rain within 24 hours. Read the insecticide label to learn about application rates.
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