How To Identify & Control Armyworms

Understanding Armyworms In Your Lawn

Armyworms may sound like a pest from farmlands and forests, but these voracious caterpillars are a common enemy of residential lawns across many regions of the United States. When populations spike, they can destroy a healthy lawn overnight, leaving homeowners confused and frustrated by the sudden browning and thinning of their turf.

We have put together this guide to help our readers learn how to identify armyworms and the damage they leave behind in your lawn so that everyone can defend their turf and prevent pests like these from invading!

What Are Armyworms?

Armyworms are not actually worms, but the larval (caterpillar) stage of certain moths in the Noctuidae family. The most common species that affect lawns in North America are the fall armyworm (Spodoptera frugiperda) and the true armyworm (Mythimna unipuncta). These pests are known for their large population outbreaks and synchronized feeding behavior. They are called “armyworms” because they travel across turf and crops in large groups, like a small army, devouring everything in their path.

While they are more commonly associated with agricultural crops like corn, wheat, and hayfields, fall armyworms have increasingly become a significant threat to home lawns, particularly those with warm-season grasses like Bermuda, Zoysia, and St. Augustinegrass. Cool-season lawns are not immune either, especially during late summer and early fall when infestations are most common.

What Do Armyworms Look Like?

Recognizing armyworms in their various life stages is essential to catching infestations early. When mature, armyworm caterpillars grow to about 1 to 1.5 inches long and have a distinctive appearance. Adult moths, which are the final stage of the life cycle, are relatively small (about 1.5 inches wingspan) with mottled brown or gray forewings and lighter-colored hindwings. These moths are nocturnal and typically lay their eggs on blades of grass, tree bark, or nearby structures.

• Color: Their coloration varies by species and age, but most are green to brownish-gray with darker stripes running lengthwise along the body.
• Stripes: One of the most telltale signs of a fall armyworm is the presence of four black dots arranged in a square pattern on the second-to-last segment of the caterpillar’s body.
• Inverted Y: Look for a pale, inverted “Y” shape on the head capsule, another reliable indicator of the fall armyworm species.
• Behavior: They are usually more active in the early morning and late afternoon. During hot parts of the day, they tend to hide in the thatch layer or soil.

How Do They Damage Your Plants & Grass?

Armyworms can devastate lawns in a very short time, sometimes within a single day. The larvae feed by chewing the grass blades, starting from the tip and working their way down. This feeding habit results in irregular, ragged brown patches that may at first resemble drought stress, heat damage, or even disease.

The damage can progress quickly, especially when large populations are present. Because armyworms feed in groups, one square foot of lawn can be completely stripped of grass blades in hours. The damage is most severe in newly sodded or overseeded lawns, which are especially vulnerable to defoliation. While armyworms rarely kill well-established lawns, their damage can cause significant stress to turf, making it more vulnerable to other pests, diseases, or heat stress.

Signs Of Armyworm Damage:

• Rapid browning of large lawn areas
• Chewed or skeletonized grass blades
• Presence of caterpillars on the soil surface or in the thatch
• Increased bird activity (birds feeding on caterpillars)

Life Cycle Of Armyworms

Understanding the life cycle of armyworms is essential for both prevention and timing control efforts. Armyworms go through a complete metamorphosis, consisting of four distinct stages: egg, larva (caterpillar), pupa, and adult moth. In warm climates or during hot weather, the full life cycle can be completed in as little as 30 days, allowing multiple generations to occur in a single growing season. This rapid reproduction is what makes infestations feel so sudden and overwhelming.

• Egg Stage: Female moths lay clusters of 100–200 eggs on grass blades, tree bark, siding, or even patio furniture. These eggs hatch in just 2–4 days, depending on temperature.
• Larval Stage (Feeding Stage): This is the most destructive stage. The newly hatched larvae begin feeding almost immediately and go through six growth stages (instars) over a period of 2 to 3 weeks. The older they get, the more they consume. Late-instar larvae are responsible for the majority of turf damage.
• Pupal Stage: After feeding, the caterpillars burrow into the soil and pupate. This stage typically lasts about 10 days, during which they transform into adult moths.
• Adult Stage: Moths emerge from the soil, mate, and begin laying eggs to repeat the cycle. One adult female can lay several hundred to over a thousand eggs during her short life.

How To Prevent & Control Armyworms

Preventing armyworm outbreaks begins with monitoring and early detection. Because they move fast and feed voraciously, waiting until visible damage appears can result in significant turf loss. Here are effective strategies to help prevent an invasion of armyworms on your property.

• Monitor Regularly: Walk your lawn often during late summer and early fall, particularly after rainfall or during warm, humid periods. Look for caterpillars, moth activity, and thinning patches.
• Encourage Beneficial Predators: Birds, ground beetles, and parasitic wasps help reduce armyworm populations naturally. Avoid broad-spectrum insecticides that harm these beneficial species.
• Mow Properly: Avoid scalping your lawn. Taller grass helps shade and cool the soil, making it less attractive to egg-laying moths and more supportive of predator insects.
• Irrigate Strategically: While a healthy lawn is less vulnerable, overwatering may attract moths and encourage egg-laying. Water deeply but infrequently to avoid creating overly lush growth that armyworms prefer.

If you are in the Salt Lake City area and need help dealing with armyworm or other pests in your lawn, give us a call here at Holmes Lawn & Pest! We specialize in pest control for the Salt Lake area, and we are happy to help take care of your pest problem today!