Army worms are the destructive larval stage (caterpillars) of several moth species. They are named for their tendency to move across land in large, coordinated groups, consuming foliage rapidly. These pests pose a significant threat to agricultural crops and lawns, causing extensive damage in a short timeframe. Controlling an infestation requires a strategy combining timely intervention with targeted methods to eliminate the current population and prevent future outbreaks.
Identifying Army Worms and Their Damage
Confirming the presence of army worms is essential, as their damage can resemble other lawn issues. The larvae are typically green, brown, or black, reaching up to 1.5 inches in length, and often feature distinctive stripes along their bodies. A key identifying feature on the head of the fall army worm species is a whitish, inverted “Y” shape. They feed heavily during the night or on cloudy days, hiding in the soil, thatch, or under debris during bright sunlight.
The initial damage from newly hatched, smaller larvae appears as a “window pane” effect on grass blades, where the caterpillar chews the green layer, leaving a transparent film. As the army worms mature into their final, most destructive stages (instars), they become voracious feeders. They are capable of stripping foliage and leaving only stems behind, creating brown, bare patches that appear rapidly. Early detection before this final feeding stage is important for successful management.
Timing Treatment for Maximum Effectiveness
Successful army worm control depends on applying treatment when the larvae are most vulnerable, specifically during their younger stages (instars). Older larvae, especially those in the final instar, are considerably more difficult to eliminate with insecticides and consume the vast majority of plant material. The optimal window for intervention is when the larvae are less than half an inch long, before they inflict extensive damage.
Scouting for small larvae can be performed using a simple soap-flush method. Mix a small amount of dish detergent with water and pour it over a square yard of turf; army worms present will typically emerge to the surface within minutes. Treating adult moths or pupae buried in the soil is ineffective for immediate control, as moths do not feed on foliage and pupae are protected from surface treatments.
Immediate Eradication: Chemical and Synthetic Options
For severe or widespread infestations requiring rapid pest reduction, synthetic chemical options provide the fastest knockdown. Commonly used active ingredients include pyrethroids, such as bifenthrin, permethrin, and lambda-cyhalothrin, which act as nerve toxins and kill quickly upon contact. Carbaryl is another effective synthetic option used for army worm control.
Application should be timed for the late afternoon or early evening when army worms are actively feeding and moving to the surface. Apply the treatment with enough water to ensure it penetrates the grass canopy and reaches the soil surface where larvae hide during the day. While pyrethroids offer quick results, they may have a short residual effect, sometimes necessitating re-treatment during a heavy outbreak.
Biological and Organic Control Methods
Non-synthetic control methods are preferred for minor infestations or by those seeking environmentally conscious alternatives. Bacillus thuringiensis (Bt) is a biological option toxic only to caterpillars. Bt must be ingested by the feeding army worm, acting as a stomach poison that disrupts its digestive system. This biological insecticide is most effective on smaller, younger larvae and requires reapplication because it breaks down quickly when exposed to sunlight.
Beneficial nematodes, which are microscopic, insect-killing roundworms, offer an organic control strategy, particularly against larvae that burrow into the soil or thatch. Species like Steinernema carpocapsae actively seek out and infect the army worm larvae, providing sustainable population suppression. For targeted spot treatments, insecticidal soaps and horticultural oils can be applied directly to the larvae, leading to suffocation or disruption of the insect’s cell membranes. These organic options generally work slower than synthetic chemicals and their efficacy is highly dependent on proper application and environmental conditions.
Long-Term Prevention Through Cultural Practices
Proactive cultural practices modify the environment to discourage future army worm activity. Maintaining a healthy lawn and soil structure makes the turf less appealing to egg-laying moths and more resilient to feeding damage. This includes managing the height of the lawn, as army worms prefer the more succulent, longer grass blades found in shady areas.
Proper irrigation practices also play a role, as excessive moisture can create an environment attractive to the adult moths for laying eggs. Reducing the accumulation of dense thatch provides a protective hiding place for larvae and pupae. Encouraging natural predators, such as birds, parasitic wasps, and ground beetles, helps regulate the army worm population naturally.