Army worms are migratory agricultural pests, representing the larval stage of several moth species. These caterpillars are known for their voracious appetites and tendency to “march” together, causing widespread damage to lawns, gardens, and crops. Because they can decimate large areas of turf or foliage quickly, especially during late summer and early fall, timely detection is necessary to prevent severe plant loss. Checking for their presence involves recognizing their physical form, the specific signs of damage they leave behind, and using methods to force them out of hiding.
Visual Identification of Army Worms
The larva is the most common stage found, growing up to 1.5 inches (38 mm) long when mature. Caterpillars typically range from light green to dark brown or black, often featuring distinct pale white or yellow stripes running lengthwise. A key feature of a mature army worm, particularly the Fall Armyworm species, is a prominent inverted “Y” shape on the front of its head capsule.
The larval body also has four dark spots, known as tubercles, arranged in a square pattern on the last abdominal segment. These caterpillars develop through six growth stages, becoming darker and more patterned as they mature. The adult stage is a grayish-brown moth with a wingspan measuring between 1.25 and 1.6 inches (32 to 40 mm).
Recognizing Early Signs of Infestation
The initial signs of an infestation are often the damage left behind, which can be confused with drought stress or disease. Newly hatched larvae scrape off the green tissue on the surface, leaving the clear membrane intact. This feeding pattern creates translucent spots on the foliage, referred to as “window-paning,” which can give the turf a subtle “frosted” appearance.
As the caterpillars mature, their feeding becomes more aggressive, resulting in ragged or chewed edges on leaves and grass blades. In turfgrass, this feeding leads to circular or irregularly shaped patches of brown, thinning grass that expand rapidly. The presence of moist, green or dark fecal pellets near the base of damaged plants or on the soil surface strongly indicates caterpillar activity. Increased bird activity on a lawn also suggests a problem, as birds actively feed on the larvae.
Practical Methods for Active Detection
When damage is suspected, the most reliable method for confirming army worms is the soap flush technique. This involves mixing a solution of liquid dish soap and water, typically three tablespoons of soap per one gallon of water. This mixture should be poured evenly over a small, measured area of turf, such as a three-foot by three-foot square. Target the edges of damaged patches where the larvae are actively feeding.
The soapy water irritates the larvae, forcing them to emerge from the soil and thatch layer to the surface within about ten minutes. If three to four larvae are observed per square foot, it indicates a population level that may warrant further action. Hand searching is also effective, especially around dusk, at night, or in the early morning when they crawl up to feed.
Another method for proactive monitoring, particularly useful for forecasting population surges, is the use of pheromone traps. These traps contain a synthetic lure that mimics the sexual pheromone released by the female moth, attracting male adults. While not used for immediate confirmation of a localized larval problem, a sudden spike in the number of trapped moths can signal that a large egg-laying event is imminent, with peak larval densities often appearing one to two weeks after the moth peak.