Beet Armyworms

“Go, Navy! Beat Army!” 

No, wait. Sorry, that’s not what I meant.

Beet armyworms are insidious little creatures that may be burrowing into your young beets as you read.

​Originally from Southeast Asia, beet armyworms were first seen in the U.S. in 1876. Today, they are considered major pests of many agricultural crops in areas without freezing winters. Like other armyworms, beet armyworms (Spodoptera exigua) are the larval form of a moth. In this case, it is a moth that flies around, laying eggs on some of your favorite garden plants.

​​Host plants

Beet armyworm larva (needpix.com)

While it might be easier to list the plants not susceptible to beet armyworms, you need to know where to look for these pests. In addition to beets, the list of potential beet armyworm hosts includes beans, celery, cilantro, citrus, cole crops, cucurbits, lettuces, parsley, peppers, strawberries, and tomatoes. Beet armyworms also attack alfalfa and cotton.

Beet armyworm lifecycle

Female moths lay pale, pinkish or greenish striated eggs in clusters of more than 100 eggs, often on the upper sides of leaves. These clusters look fuzzy, due to hairlike scales left behind by the moth. After they hatch, larvae begin feeding on nearby leaves, slowly dispersing throughout the plant. As larvae get older, they also feed on fruit. After defoliating your plant, the mature larva drops to the ground, where it pupates in a shallow depression in the soil, or in a pocket excavated just below the soil surface. An adult moth emerges, and the whole process begins again. This cycle is completed in one month, so there can be multiple generations each year.

Beet armyworm moth (Dumi) CC BY-SA 3.0

Beet armyworm description

Larvae are smooth, pale green caterpillars, with several pale, wavy lines down the back and a broad stripe down either side. You may also see a dark spot above the second pair of legs. Other color variations can occur, depending on the food source and developmental stage. After 2 or 3 weeks of feeding, caterpillars will reach 1.25 inches in length. Adult moths are mottled brown and gray, with a 1-inch wingspan.

​Damage caused by beet armyworms

Beet armyworms can destroy seedlings in only minutes. When feeding begins, the damage appears as clusters of circular or irregularly shaped holes in leaves. It can also cause flagging, a condition that slows or halts growth on one side of a plant. Larvae will feed on the crown of lettuce plants, killing them. As caterpillars get bigger, they can skeletonize all the leaves on a plant. Most fruit feeding occurs on or near the surface, and can be cut away, assuming other pathogens haven’t entered the fruit, causing disease or decay. Of course, you will want to wash the fruit thoroughly, to get rid of caterpillar feces. If beet armyworms feed on floral buds, the buds will abort.

How to control beet armyworms

In the home garden, natural predators are your plants’ best defense against beet armyworms. Predatory wasps will parasitize beet armyworm larvae, while big-eyed bugs, and minute pirate bugs will feed on the eggs. Spiders, damsel bugs, assassin bugs, tachinid flies, and lacewings will also feed on beet armyworms, so avoid using broad spectrum pesticides. In severe cases, you can apply spinosad or a specific type of Bacillus thuringiensis (ssp. aizawai).

Prevent beet armyworm invasions by monitoring nearby weeds, especially lambsquarters, goosefoot, and pigweeds for signs of egg clusters.

Harvesting your crops as soon as they are ready can also interrupt the lifecycle of these pests.

Beet armyworms have been known to travel as far as 10 feet during a night, putting most of your garden plants at risk. Monitoring for signs of beet armyworm infestation can help you prevent the problem from spreading.