Armyworms Are Chomping Through Lawns Across the U.S.

By: Jesslyn Shields  | 

fall armyworm
Fall armyworms (Spodoptera frugiperda) are gray and tan caterpillars that top out at around 1.5 inches (3.8 centimeters) long. They can turn a green lawn brown in no time flat. SIMON MAINA/Getty Images

Animals are just animals, but humans get along with some of them better than others. If you've got a lawn, you might not love gophers, fire ants or the neighborhood dog that escapes his yard and digs up yours. You also probably hate the fall armyworm (Spodoptera frugiperda), because what they love is grass.

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Armyworms Are Super Spreaders

The fall armyworm is a moth native to the tropical and subtropical regions of the United States and Mexico. Neither the larvae nor moth can survive winter temperatures in the latitudes of the U.S. above Florida, Texas and Mexico, but each year when they start to hatch in the southern states, they begin their migration north.

"They continue to go through several generations in southern Florida in the east and in Texas and Mexico in the central and west," says Shimat V. Joseph, a turf scientist and entomologist in the College of Agricultural and Environmental Sciences at the University of Georgia, in an email. "They are dispersed incrementally in the spring and early summer to the entire northern U.S. via air currents. In Georgia, we usually start seeing them as a problem in turfgrass beginning late July, but this year, we have seen high populations beginning late June."

The fall armyworm moth
The fall armyworm can go from egg to moth within a month, and eggs hatch within 48 hours. The larvae, or worms, feed on host plants at night.
Wikimedia Commons (CC BY 2.0)

Some years they get farther than others. In the late summer of 2021, the moths traveled on storm airflows all the way to Ohio, Pennsylvania and even Canada — some accounts clocked them traveling 500 miles (805 kilometers) in 24 hours.

According to Joseph, it's difficult to pinpoint a single reason for why some years produce huge fall armyworm booms. He says it's likely the result of a combination of factors, such as availability of food resources, early hatching and just the right weather conditions.

"They like warm temperatures and humid conditions," says Joseph. "[In 2021], we have all that. Some growers have complained that some common insecticides used against fall armyworms are not effective this year. This suggests the buildup of some resistant populations, but I didn't verify that personally."

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Armyworm Life Cycle

The life cycle of the fall armyworm includes egg, larvae (caterpillars), pupae and adult. Eggs are laid in clusters of around 50-200 eggs per cluster, and you can see them on light-colored structures like porches, barns and fenceposts, and on trees and shrubs around the yard. In summer, they can undergo a single generation — from egg to moth — within a month, and eggs hatch within 48 hours — which, of course, means the earlier in the season they hatch, the larger the population in late summer and fall.

Although the gray and tan caterpillars start out small, they munch more and more grass as they grow, until they top out at around 1.5 inches (3.8 centimeters) long. An armyworm can be identified by a white, upside-down Y on its face.

armyworm moth
Armyworm eggs are laid in clusters of around 50-200 eggs per cluster, and you can see them on light-colored structures like porches, barns and fenceposts.
UGA College of Agricultural and Environmental Sciences

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The Fall Armyworm Threat

Ask anybody who's really into their lawn, and they'll tell you fall armyworms are bad news.

"They extensively feed on leaf blades and stems of bermudagrass," says Joseph. "Of course, they prefer some grass species over others if given a choice, with the larger caterpillars feeding more aggressively than the young caterpillars. It is all about numbers. If the infestation is severe, we are talking about hundreds of caterpillars. They can make the lawn from green to brown within two days. Usually, the bermudagrass recovers within 3 to 4 weeks if sufficient irrigation and nutrients are provided."

Unlike some moth and butterfly larvae that only feed on one species of host, the fall armyworm feeds on over 100 different plants, most of which are grasses like corn, wheat, rice and sorghum, but also crops like alfalfa, beets, tomatoes, potatoes and soybeans. During a large outbreak, they can do an immense amount of damage in just a few days — in 2021, the U.S. Environmental Protection Agency approved the special use of a normally restricted herbicide to control fall armyworms in rice fields because the damage was so great.

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What Homeowners Can Do

Since fall armyworms can decimate a lawn in just a few days, if you enjoy your lush green sod, it's a good idea to keep your eye out for them.

"First, have a plan to monitor the pest," says Joseph. "Scouting for egg masses on the structures around the lawn is a good strategy for homeowners. They also lay eggs on the trees and shrubs. The lawn may appear ragged and leaves completely stripped because of caterpillar feeding. Pouring soapy water on the suspected turfgrass spot will bring the worms to the surface. Older larvae are easier to find than younger ones. Insecticide applications might be necessary if the population is large. Contact local university agents for insecticide recommendations."

In most cases, the long-term health of grass is usually not affected, and the grass will recover eventually with proper care. However, after a severe case of armyworms that were not treated early enough, you may need to resod or reseed to repair the damage.

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