Quick Facts
- What they are: Caterpillars (larvae) of a moth species that feed voraciously on grass blades
- When they're active: June through November, until the first hard frost
- Where they're most prevalent: Southeastern U.S., Gulf Coast states, Texas, and transition zones
- Primary damage signs: Rapidly expanding brown patches with a distinct border; defoliated grass blades
Fall armyworms are periodic turf pests—they may not cause major damage every year, but when they're active, they can cause a lot of damage in a short time. So how do you go about identifying, preventing, and managing fall armyworms?
The problem with armyworms
Occasionally, weather patterns (think hurricanes, severe wind and rain storms, and associated flooding) will create the perfect setting for a severe fall armyworm outbreak. And once these caterpillars get to munching, they can annihilate a lawn within a matter of days.
How to ID fall armyworms
Knowing your enemy—and knowing how to prevent and mitigate damage—starts with proper pest identification. The damaging stage of the armyworm, and the stage you're most likely to notice and ID, is the "larvae" or the caterpillar.
Size: Up to 1.5 inches long
Color: Green or mottled brown with a darker head
Body Markings: Wide black stripes down each side of the back
Distinguishing Marks: Inverted "Y" pattern on the face; four black spots aligned in a square near the back end of each abdomen segment, usually with hairs
Photo Credit: Blake Layton, MSU-ES, Alabama Cooperative Extension System
Adult armyworms are moths!
You may see armyworm moths flitting around between spring and fall. The moths are nocturnal, and are attracted to lights at night. They have:
- Tan or brown bodies
- Mottled brown forewings with a characteristic white spot near the center
- White hind wings with a brown line around the edge
While the name suggests a seasonal pest, there will likely be multiple fall armyworm cycles beginning as early as June in some areas. Larvae can hatch, feed, pupate into moths, and lay more eggs every couple of weeks in many regions!
Photo Credit: Lyle J. Buss, University of Florida, UF | IFAS, Entomology
Common lookalikes
Fall armyworms are often confused with other armyworms and various cutworms. Use the comparisons below to distinguish between similar pests.
Fall Army Worms:
- Caterpillar Appearance: Green/brown with inverted "Y" on face; four black spots in square pattern near back
- Moth Appearance: Light brown bodies with white hind wings and mottled brown forewings.
- Damage Timing: Summer and early Fall (June-November)
- Damage Pattern: Rapidly moving line of damage; complete defoliation
True Armyworms:
- Caterpillar Appearance: Gray to yellowish green tinged with pink; narrow, broken black stripe down center
- Moth Appearance: Reddish brown with a white dot in the center of each wing
- Damage Timing: Spring
- Damage Pattern: Rapidly moving line of damage; complete defoliation.
Black Cutworms
- Caterpillar Appearance: Dark gray to black with a pale stripe; smooth and hairless
- Moth Appearance: Mottled brown
- Damage Timing: Spring through Fall
- Damage Pattern: Circular dead spots that start small and expand out in a circular pattern
Photo Credit: True Armyworms | University of Arkansas Extension.
Photo Credit: Black cutworm on soybean | UMN Extension.
Where do fall armyworms live?
Fall armyworms are most common in the Southeastern U.S., Gulf Coast states (including Texas, Louisiana, Mississippi, Alabama, and Florida), and transition zones. Adult moths will lay hundreds of eggs on the undersides of leaves (especially those overhanging turf), or on light posts, fences, signs, patio or lawn furniture, and even on walls. Larvae will hatch and fall to the turf below, where they will begin to feed. They prefer bermudagrass, but will also feed on fescue, ryegrass, bluegrass, bentgrass, and occasionally zoysiagrass. They can also feed on clover, veggies, and crops, if nearby.
When are fall armyworms active?
Fall armyworms are most active from June through November, until the first hard frost. Adult moths can travel 500+ miles within 24 hours if there are large storms that blow moths along jet streams. This is why large fall armyworm outbreaks are often associated with above average rain and wind storms. Once the larvae hatch, caterpillars feed most actively mid-morning or late evening, but can often be seen feeding mid-day.
How to tell if you have fall armyworms in your lawn
Fall armyworms are unique in their feeding pattern. As their name suggests, armyworms move across the turf en masse—much like an army marching across the lawn. The caterpillars wage war on the lawn by feeding on leaf blades, occasionally leaving a "windowpane" behind, but more often completely defoliating the grass within a matter of days. This leaves the crown (growing point) of the grass exposed to the elements, leading to heat and moisture stress, and even death. There is typically a clearly defined line of damage that moves across the lawn rapidly, leaving a dead, defoliated yard in their wake.
Photo credit. Fall Armyworms on the March Through Piedmont Lawns | NC University Cooperative Extension.
How to check for fall armyworms
Follow these steps to confirm a fall armyworm infestation:
- Look for damage signs. Check for large brown patches of defoliated grass with a distinct border that appears to be moving across the lawn.
- Scout at the right time. Go out in the early morning or late afternoon when caterpillars are most active.
- Inspect the damage border. Look closely at the edge between damaged and healthy turf for caterpillars or green frass (caterpillar poo) at the base of the grass.
- Conduct a soapy water flush. Mix 2 tablespoons of dish soap in 1 gallon of water. Pour over 1–2 sq. ft. of lawn along the border of damaged and undamaged turf. Count any caterpillars that come to the surface.
- Assess the severity. Treatment is recommended when you find more than 3-4 armyworms per square foot, or when damage is still occurring.
Natural pest prevention and reduction practices
Unfortunately, fall armyworms can be difficult to control, partly because they act so quickly and can have multiple life cycles throughout the year. There are a few steps you can take to prevent damage, though:
- Maintain a healthy, dense lawn. Follow proper fertilizer and watering practices to grow a healthy lawn that is more resilient to insect injury.
- Check for eggs. Go on an egg hunt, particularly after large storms. Check the undersides of tree leaves, gutters, fence posts, and lawn furniture. If you find any large egg masses that are light gray and fuzzy looking, scrape them off and dispose of them.
Photo Credit: Grace Christopher, NBC | WFMJ.
- Monitor the lawn. Check for signs of damage, including large brown patches of defoliated grass, with a distinct border.
- Scout for caterpillars. If you do notice damage, and suspect you might have fall armyworms, do a little sleuthing. Go out in the early morning or in the late afternoon to see if you can find caterpillars or green frass (caterpillar poo) at the base of the grass, or do a soapy water flush to draw the pests up from the thatch layer.
- Pick your battles. Birds, rodents, wasps, flies, and ground beetles love to feast on a battalion of fall armyworms. This is one of those times that you might want to let one pest take care of another pest for you—although birds and rodents can leave behind holes and mounds, the damage they leave behind is often easier to repair than fall armyworm damage, which can kill the grass.
If your lawn does sustain some damage from fall armyworms, help it bounce back. Irrigate damaged areas to keep the crowns from drying out, and see if that helps the grass repair itself. Reseeding or patching bare or dead spots will also help bring some green back to the lawn.
Frequently Asked Questions
Do fall armyworms kill grass? Yes, fall armyworms can kill grass. They feed on leaf blades and can completely defoliate a lawn within days, exposing the crown to heat and moisture stress. If the crowns dry out before the grass can recover, the grass will die and require reseeding.
How fast do fall armyworms spread? Fall armyworms spread very quickly. A single generation can complete its life cycle in 3-4 weeks, and a high population of caterpillars can defoliate an entire lawn in just a few days. Adult moths can travel over 500 miles in 24 hours during storms, leading to rapid regional outbreaks.
What kills fall armyworms naturally? Natural predators including birds, wasps, parasitic flies, and ground beetles feed on fall armyworms. Allowing these beneficial predators to work can help reduce populations. For direct control, Bacillus thuringiensis (Bt) is a natural bacterial insecticide effective against young caterpillars.
When should I treat for fall armyworms? Treat when you find more than 3-4 armyworms per square foot during a soapy water flush test. Early detection is critical—smaller caterpillars are easier to control, and treating before significant damage occurs gives your lawn the best chance of recovery.
Cited sources
Header image photo credit: Pat Porter, Texas A&M Agrilife Extension.
Armyworms are on the March. Texas A&M Agrilife Extension.
Fall Armyworms in Turf. North Carolina State University.
Fall Armyworm Q&A. The Ohio State University.
Fall Armyworms March Across Ohio. Purdue University.
Managing Black Cutworms in Turfgrass. Texas A&M Extension. Armyworms in Turfgrass.
