This destructive garden pest gets its name because it travels in small insect armies and consumes just about everything in its path. There’s 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, sometime 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.
Markings on newly hatched caterpillars are usually hard to distinguish, older larvae have distinctive stripes that run the entire length of the body. Fall armyworms (Spodoptera frugiperda) are brown with yellow stripes, 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.
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 army worms 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.
Life Cycle Armyworms are prolific and responsive to favorable conditions. Their eggs are laid in fluffy masses on crowns of seedlings and on leaves of older plants. In 5-10 days tiny caterpillars hatch and feed for several weeks. They then 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. 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.
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. 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 reduction of graze-able pasture for feed animals and unsightly lawns for homeowners.
Control If you don’t suffer army worm outbreaks, thank its natural 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 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 on 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 mite and insect eggs.
Plant to attract birds and beneficial insects. Birds are especially fond of the 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 are murder on army worm eggs and pupae found in the soil.
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.