Armyworm is a native species occurring throughout North America. It is most common, however, in the eastern United States and Canada west to the Rocky Mountains. It does not overwinter at northern latitudes such as Canada and the northern states. Rather, armyworm disperses northward each spring, principally along the Mississippi River Valley, and then disperses southward during the autumn, principally along the east coast (McNeil, 1987). It is often called “true armyworm” to distinguish it from other armyworms such as fall armyworm, Spodoptera frugiperda (J.E. Smith), and yellowstriped armyworm, Spodoptera ornithogalli (Guenée). Armyworm also occurs in Central and South America, southern Europe, central Africa, and western Asia.
Armyworm moths are brownish gray in color, and have a wing spread of about 37 mm. The distinguishing white-colored mark in the center of each forewing helps separate them from other moths. Caterpillars are the damaging stage of the armyworm in turfgrasses. They can grow to be as long as 37 mm and have characteristic white, orange, and brown stripes that run the length of the abdomen.
Armyworms feeding on plants may actually cut off the plants at crown level. Turfgrass stands may exhibit an overall ‘ragged’ appearance with brown patches. To be certain of an armyworm diagnosis, scout ragged turf areas from June to September, searching for the small caterpillars. Damage is most severe during hot, dry weather in midsummer.
Caterpillars are nocturnal and seek cracks, crevices and aeration holes in which to hide during the day. During the evening they emerge, to feed on the crown areas of the turfgrass grass plants, often completely severing them before moving on. Feeding on the grass plant growing point kills the plant. Heavily damaged turfgrass appears as if it has been scalped.
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.
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.
- Applications of Garden Dust (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.