Armyworms (Spodoptera Frugiperda)
Learn to identify armyworms in your garden and what to do about them.
Armyworms get their name from their habit of congregating and moving en masse when searching for food, like an invading army! While they are typically grass family pests and can do a number on your lawn, they are also capable of damaging corn, soybeans, and even broadleaf garden plants. Learn how to identify the caterpillars and what to do about them.
What Are Armyworms?
Several caterpillars go by the common name of armyworm or army worm. The fall armyworm (Spodoptera frugiperda) is responsible for turf damage in the South and is often a pest in yards, eating turfgrass. Seemingly overnight, your lawn will appear to be suffering from drought with brown-yellow patches.
True armyworms (Mythimna unipuncta) are an agricultural pest, often infesting fields of cereal grains, rice, corn, and other grasses, but can attack broadleaf plants (your garden) if large populations have depleted their primary food sources. See other pests of sweet corn.
In addition, there is a tent-forming armyworm (Malacosoma americanum), which is a forest caterpillar found in trees in spring and summer. They create a large webbed shelter with hundreds of caterpillars inside and feed on the leaves of broadleaf trees and shrubs. While tent caterpillars are a pest, we’ll talk about true armyworms and fall armyworms below.
Armyworm Life Cycle
Regardless of the specific species of armyworm plaguing you, their method of damage is the same. It’s the larval stage, not the adult moth, that does the damage. Caterpillars go through several instars, or stages, before pupating into adult moths. All instars cause defoliation, and the last stages are where most of the eating occurs. A late instar infestation of true armyworms can defoliate an entire cornfield practically overnight. A green lawn can become a victim to a wave of armyworms and turn into a blotchy, browned mess in short order.
Neither fall armyworms nor true armyworms can overwinter in northern areas with persistent freezing temperatures, but adults will migrate north in spring after spending the winter in milder climates. Armyworms generally prefer plants in the grass family (yes, corn is a grass) but will eat other plants when their preferred food source is unavailable. If you garden near an agricultural field, your garden could be a target, especially if the hay is baled or the corn is chopped, depriving them of food.
How to Identify Army Worms
Skinny, squishy caterpillars—the kind birds love to eat—are easy to find in the garden, but identifying an armyworm caterpillar from a different kind of larva can be challenging. You won’t likely notice them until you see leaf damage. In both fall armyworms and true armyworms, the younger instars look rather nondescript, small, and can be yellowish-green or brown-green. As they progress into later instars, distinct markings appear.
Armyworm caterpillars
True armyworm late-stage caterpillars have three sets of true legs near the head, four sets of prolegs (sort of fleshy, lumpy looking) near the middle, and one pair of prolegs at the tail end. They have a distinct head that is smooth and has a patterned or stippled appearance. The main body is striped lengthwise, with dark brown or black, creamy white, and rust-orange coloring.
Fall armyworm caterpillars are dark green with a dark head when younger. Later instars develop thin white stripes. Older fall armyworm larvae have a cream or white inverted V or Y shape on the head, with the point of the V facing up.
Armyworm eggs
True armyworm eggs are small, yellow-white, and laid in a compact mass of several rows. There can be only a few or up to 200 eggs in a batch.
Fall armyworm eggs are often laid on lighter-colored surfaces, such as fence posts, the undersides of leaves, or tree limbs. There are typically 100–200 eggs, and they appear fuzzy, like a small cotton ball, instead of well-defined. It’s the fuzzy look that gives it away.
Armyworm adult moths
True armyworm moths are nondescript (to us non-Lepidopterist folks) tan-colored moths. They’re a medium size, about an inch and a half across, with a fat furry body. The species name unipuncta comes from the single, very small white dot present in the center of the forewing (front wing).
Fall armyworm moths are grayish-brown medium-sized moths with patterned darker forewings, whitish and slightly transparent furry rear wings, and a furry tan body. Again, not colorful or very distinct to most observers.
Identifying Army Worm Damage
Armyworms often hide during the day, but anytime you notice chunks missing from your leaves, it’s time to go on a worm hunt. Look at morning and evening times when there is still enough light to see them, and they are active. You will find these little worms feeding on your turf grass.
Fall armyworms can even feed on ears of corn, leaving them inedible and ruined. Fall armyworms feeding on your lawn give it the appearance of drought damage, with browned, dead-looking spots.
Armyworms have chewing mouthparts and will eat holes in the leaves, eventually consuming all but the tougher midribs and stalks. You may see them crawling up a stem or actively munching a leaf. Look on the undersides of stems and leaves for eggs (and smush them with wild abandon).
Check carefully anytime you see damaged plants or plants that look drought-stressed or wilted when the rest of the garden is fine. If two plants are wilting but their neighbors in the same row are fine, that’s a clue it might not be drought stress–check for larvae.
Andy Wilcox
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