Why Are Armyworms So Bad?

Armyworms are a threat to any ecosystem where they’re found. Yet, why are armyworms so bad? These insects travel in massive groups, destroying crops, lawns, trees, and nearly any other organic matter in their path. They move in a regimental pattern that’s reminiscent of a military march, hence the name. Armyworms are so destructive that they cause significant economic damage each year. Find out how to identify them, what they do, and how to control an infestation.

How to Identify Them

Infestations are most frequently seen east of the Rocky Mountains, although different species can be found across the U.S. The most common types of armyworms are the fall armyworm, the true armyworm, the yellowstriped armyworm, and the beet armyworm. Unfortunately, specific armyworms can be difficult to identify, as the species look similar.

The fall armyworm is light green to dark brown when young, changing to a pink color to dark brown with white stripes on its sides. The true armyworm is greenish-brown to black with orange stripes along its sides. The yellowstriped armyworm is mostly black with two prominent yellow stripes, plus additional narrow yellow stripes on each side of its body. The beet armyworm is pale green to yellow, developing pale stripes along its body as it grows.

A Seasonal Pest

There are typically three or four generations of armyworms each year. The generations generally take 30 to 60 days to complete, with some variations between species. Unfortunately, the next generation can produce just as you’re getting rid of the current generation. Fall armyworm and beet armyworm infestations are generally more numerous in the late summer and early fall. True armyworms are typically active in the spring and early summer. Yellowstriped armyworms emerge from March through November. This will also vary depending on the region and the weather conditions.

Regions with mild winters, mostly in the deep south, allow migrating armyworms to overwinter, keeping pupae and eggs beneath the soil. If the climate is warm year-round, armyworms can be active the entire year. Fortunately, most species cannot adapt to the cold. For example, even in regions with mild winters, fall armyworms will typically die if the temperature drops below 50° Fahrenheit. Other species will usually die when temperatures remain at or below freezing.

Armyworm Damage

In the larval stage, armyworms are a threat to a range of crops and grasses, depending on the species. They feed primarily on turf grasses and forage grasses, sorghum, wheat, oats, rye, corn, barley, and rice, among other plants. They also feed on some vegetable crops, including, but not limited to, tomatoes, beans, cabbage, and peppers.

Armyworms chew holes in foliage, shred leaves, make gauges in vegetables, and gnaw on plant tissue, weakening the plants and making them more prone to drought and disease. The damage varies in severity depending on the type of field it is. Foliage may also look as if it’s thinning, and the surfaces will look brown or burned. It may appear as if you’re looking at drought damage, rather than dehydration from armyworm larvae eating and destroying young plants.

The Armyworm Attack

In the beginning, the attack is largely aerial, with moths arriving in the dark to lay eggs on host plants. Once the eggs hatch, armyworms tend to be active in the early morning, again in the late afternoon, and at night. However, on fertile ground, such as unmowed, tall grass, they may feed all day. If they find well-kept hayfields or other pastures, the armyworm larvae may settle into the sod to escape the heat of the day.

As they don’t eat much at first, young armyworms don’t cause the bulk of the destruction. The cause of the vast majority of damage can be attributed to the older insects. Experts say that in four days of feeding, the oldest caterpillars are capable of eating more than all other ages combined.

Controlling Infestations

The best way to control armyworm outbreaks is to keep their natural enemies around. These include birds, beneficial insects like ladybugs, and larval predators such as spiders and wasps. The absence of armyworm predators gives the fast, and constantly re-generating, armies ample opportunity to keep the life cycle going. Ironically, in many cases, the armyworm predator population is scarce because of the very measures, such as insecticides, used to kill off the armyworms.

Try to avoid harmful insecticides and practices that might inadvertently destroy the predators. You can also minimize a possible infestation by controlling or eliminating grassy weeds in your fields. Carefully plant to attract beneficial insects and birds. Birds love the moths and happily snatch larvae from plants and lawns. Come the fall, uncover and flip the soil before putting it to bed. You then give birds a good chance at getting to exposed pupae.

Once in season, use a natural horticultural oil spray. Apply it to plants where you’re seeing indications of larvae and mites. The solution also minimizes the growth of fungus. Use it for complete coverage, especially on the undersides of leaves and on junctions with stems. Some situations are going to require the use of a professional armyworm exterminator experienced in dealing with an infestation. To manage armyworms while maintaining your ecosystem, we suggest you only work with contractors using organic methods that don’t damage the other elements of your environment.