Armyworms are a significant pest that can quickly damage a healthy lawn, moving across turf in large groups and voraciously consuming grass blades. These caterpillars, the larval stage of a moth, cause noticeable devastation in a matter of days, especially during late summer and early fall when they are most active. Because the damage can resemble drought stress or disease, swift identification is crucial for effective treatment. Once an infestation is confirmed, immediate action is necessary to protect the remaining turf and prevent the insects from completing their lifecycle.
Identifying Army Worms and Assessing Damage
Properly identifying the pest is the first step, as brown patches in the lawn can have many causes. Armyworm larvae range in color from green and yellow to brown and gray, typically growing up to 1.5 inches long. A distinguishing feature on the head of the Fall Armyworm is an inverted “Y” shape, along with stripes running down the body. Initial damage appears as small, transparent spots on the grass blades, known as “window-paning,” but large larvae quickly consume the entire leaf, leaving a scalped or scorched appearance.
To confirm the presence of armyworms, especially when hidden in the thatch layer during the day, perform a soap flush test. Mix three tablespoons of liquid dish soap with one gallon of water and pour the solution over a three-foot by three-foot area of the affected turf. The soapy water irritates the larvae, forcing them to wriggle to the surface within minutes for easy counting. An infestation of three to five armyworms per square foot warrants immediate control measures.
Non-Chemical Control and Cultural Practices
For small, localized infestations, non-chemical methods offer an immediate approach to control. Manually removing the caterpillars is possible in very small areas; collected larvae should be dropped into a bucket of soapy water. Strategically mowing the lawn also helps, as cutting the grass short removes the primary food source and exposes the larvae to sun and natural predators.
Encouraging natural enemies provides a sustainable, long-term defense against armyworms. Birds, parasitic wasps, and ground beetles feed on armyworm eggs and larvae, helping to keep populations under control. Adjusting watering practices also makes the lawn less appealing; watering deeply and infrequently, rather than light daily sprinkling, avoids creating the moist, lush environment that adult moths prefer for laying eggs.
Selecting and Applying Insecticide Treatments
When non-chemical methods are insufficient, a targeted insecticide application is necessary to stop rapid turf destruction. Insecticides are categorized into fast-acting and systemic options, and the choice depends on the size of the larvae. Fast-acting contact insecticides, such as products containing pyrethroids, provide immediate knockdown of small to medium-sized armyworms on the surface.
Biological or reduced-risk options, like those containing the bacterium Bacillus thuringiensis (Bt) or the compound spinosad, are highly effective against young larvae. Spinosad works by contact and ingestion, causing armyworms to stop feeding almost immediately, and is less damaging to many beneficial insects than synthetic alternatives. Systemic products, which are absorbed by the grass and poison the larvae when they feed, offer longer residual control, often lasting for weeks.
Application timing is crucial because armyworms feed primarily during the late afternoon and evening. Mowing the lawn and lightly watering it a few hours before treatment helps the insecticide reach the larvae and encourages them to surface. Always apply the product according to the label’s instructions, ensuring thorough coverage of the affected area, and delay irrigation for at least 24 hours to allow the treatment to penetrate and remain active.
Maintaining Lawn Health to Prevent Recurrence
Long-term management relies on cultivating a dense, healthy lawn that is less attractive to egg-laying moths and more resilient to damage. When mowing, keep the grass height at the upper end of the recommended range for the turf type; taller blades promote deeper, stronger root growth. This maximizes the lawn’s natural defenses and recovery ability.
Proper fertilization and aeration are significant factors in maintaining turf vigor. Applying a balanced fertilizer supports steady growth, but avoid excessive nitrogen, which causes a flush of tender new growth highly attractive to armyworm larvae. Aerating the soil periodically reduces compaction, allowing for better water infiltration and root development, helping the lawn recover quickly from minor feeding damage.