Army worms, the destructive larval stage of several moth species, pose a consistent threat to lawns and agricultural fields. These pests consume large amounts of foliage quickly, often moving en masse to find new food sources. Timely and accurate identification of an infestation is necessary for effective management. Distinguishing between specific species is less important than confirming their presence, as the damage they cause and the methods used to find them are similar. Understanding how to test for their activity is the first step toward protecting turf and crops.
Recognizing Early Signs of Infestation
The most common indicator of army worm presence is the damage they leave behind on host plants. Young larvae feed on the tender surface of leaves, creating a characteristic transparent layer called “window-paning” as they eat the green tissue but leave the opposite layer intact. As the caterpillars grow larger, their feeding becomes more aggressive, resulting in ragged, irregular holes and a generally chewed appearance on the foliage. In turfgrass, this feeding leads to sudden, expanding patches of brown or thin grass that can easily be mistaken for drought stress.
Another sign is the presence of frass, which is the granular, sawdust-like excrement of the caterpillars. Frass can be found deposited on the leaves, in the whorl of plants like corn, or on the soil surface beneath heavily infested areas. In agricultural fields, significant damage may also include the “clipping” of young seedlings or the severed stems of plants near the soil line. Infestations often begin or are most noticeable along the edges of a lawn or field, particularly in areas with lush, dense growth.
Visual Scouting and Physical Inspection Techniques
Direct observation is the most straightforward method for confirming an army worm problem, though it requires correct timing. Army worm larvae are primarily nocturnal, meaning they are most active and visible during the late evening and early morning hours. During the day, they retreat to sheltered locations to avoid heat and sun, hiding deep within the plant canopy, under debris, or just beneath the soil surface.
To conduct a thorough visual inspection, focus your search on areas where damage is already visible or where birds, which prey on the larvae, are frequently seen pecking at the ground. To standardize the process, employ a scouting technique, such as inspecting a specific area size, like a square foot (30 cm x 30 cm) of turf or a set number of plants (10 to 20 plants) at five different locations within a field. When larvae are found, their appearance helps confirm the pest: a mature army worm can reach 1.5 to 2 inches long, is generally smooth-skinned, and exhibits longitudinal stripes of various colors. The fall armyworm species is often identified by a distinct pale, inverted “Y” shape on the front of its head capsule, along with four prominent dark spots arranged in a square on its second-to-last body segment.
Specialized Trapping and Sampling Methods
Beyond simple visual checks, specialized methods are used to quantify populations or provide an early warning of a potential outbreak. The “soap flush” or drench method is an effective technique for confirming the presence and density of larvae in turfgrass during the day when they are hiding. To perform this, mix a solution of water with liquid dish soap, typically around two tablespoons of soap per gallon of water.
Pour this solution evenly over a one-square-yard (0.9 square meter) area of turf showing damage or suspected activity. The soapy water irritates the larvae, causing them to wriggle to the surface within 5 to 10 minutes, allowing for an accurate count of the population density. For monitoring in taller crops or grasses, sweep netting can be utilized. This involves walking through the field and swinging a specialized net in a 180-degree arc across the top of the foliage. The captured larvae are then counted to estimate the population per set number of sweeps (often 25 or 100), which informs management decisions.
Pheromone traps are another specialized tool, used for monitoring adult moth populations, which helps predict when a larval outbreak is likely to occur. These traps use synthetic sex pheromones to attract male moths, providing an early alert that females are laying eggs in the area. A surge in moth capture numbers can indicate that peak larval activity will follow approximately one to two weeks later, allowing for better timing of scouting efforts before significant damage begins.