Army worms are the destructive larval stage of several moth species. They are known for moving in large, synchronized groups that consume vegetation quickly, causing extensive damage to turf, gardens, and crops seemingly overnight. Understanding the precise timing of their emergence is the first step in protecting vulnerable plants from widespread defoliation. Their life cycle is highly dependent on environmental conditions, making monitoring local weather patterns valuable for predicting when to begin scouting for initial signs of an infestation.
Identifying the Key Species of Army Worm
Three primary species are responsible for most damage, and their host preferences and peak activity times vary.
True Armyworm
The True Armyworm primarily feeds on grasses and cereal crops like oats, rye, and fescue. Its larvae are typically greenish-brown to black, marked by a narrow, pale white stripe running along the top of the body. They are nocturnal feeders.
Fall Armyworm
The Fall Armyworm is known for feeding on over 100 different plant species, including corn, turfgrass, and sorghum. A distinctive marker is a prominent, inverted white “Y” shape on the front of its head capsule. These larvae also possess four dark spots arranged in a square pattern on the second-to-last body segment.
Southern Armyworm
The Southern Armyworm prefers broadleaf plants like vegetables and ornamentals over grasses. Its larvae are usually light green or blackish-green and can be identified by a series of dark triangles that run along the side of the body.
Triggers for Seasonal Emergence and Peak Activity
The appearance of army worms, particularly the True and Fall species, often results from mass migration from southern overwintering locations like Florida and Texas. Adult moths are strong fliers, carried northward on warm, moist air currents in the spring. These migratory moths seek out lush, dense vegetation to deposit their egg masses, initiating the cycle in cooler climates.
The emergence of the destructive larval stage is directly tied to accumulated heat, a concept measured by growing degree-days (GDD). For the Fall Armyworm, the lower thermal threshold for development is around 10.9°C to 13.8°C (51.6°F to 56.8°F). Once temperatures consistently exceed this baseline, the rate of egg hatching and larval growth accelerates significantly.
The entire life cycle from egg to adult moth can be completed in as little as 24 to 30 days during the warmest summer months. This rapid development leads to multiple generations, or broods, occurring within a single season, often four to six in warmer regions. The initial generations that appear in late spring or early summer are often smaller and cause less noticeable damage.
The most destructive outbreaks typically occur with the second and third generations, which peak in late summer and early fall, from August through October. By this time, the population has multiplied substantially, and the larvae are larger, consuming the vast majority of their lifetime food intake in the final four days of their development. Larval activity continues until the first hard frost halts the life cycle.
Practical Steps for Monitoring and Detection
Field scouting is the most reliable method for confirming the presence of army worms once the seasonal emergence window has opened. Since many species feed at night, the best time to inspect plants and turf is in the early morning or late afternoon when they are most active. Look for signs of damage, such as the “windowpane” effect on leaves, where only a transparent membrane remains after the leaf tissue is consumed.
In turfgrass, the soap flush test is a simple technique that forces hidden larvae to the surface. This involves mixing three tablespoons of liquid dish soap into one gallon of water and pouring the solution over a three-foot by three-foot area of damaged turf. The soapy water irritates the caterpillars, causing them to emerge from the soil or thatch within minutes.
For broader monitoring, especially in agricultural settings, pheromone traps can detect the arrival of adult moths. These traps use synthetic sex scents to attract and capture male moths, providing an early warning sign before egg masses are laid. Regular inspection allows for proactive intervention before the destructive larval stage begins mass feeding.