Fall armyworms are destructive caterpillars that feed on more than 80 plant species, with a strong preference for grasses and corn. Native to the Americas, they have now invaded over 80 countries across Africa, Asia, the Pacific, and parts of Europe, causing an estimated $9.4 billion in economic losses in Africa alone, where they can reduce maize yields by up to 73 percent. Whether you’re a farmer watching for crop damage or a homeowner noticing your lawn being devoured overnight, understanding this pest’s behavior is the key to stopping it.
What Fall Armyworms Look Like
Fall armyworm larvae are smooth-skinned caterpillars that range from light green to dark brown or nearly black. The most reliable identification feature is an inverted Y-shaped marking on the head capsule. Along the body, you’ll see four dark spots arranged in a square pattern on the upper surface of each segment near the tail end. Young larvae are tiny and pale, easy to overlook. Fully grown larvae reach about 1 to 1.5 inches long.
Adult fall armyworm moths have a wingspan of about 1.5 inches. Males have a distinctive white patch near the tip of each forewing, while females are duller, with mottled gray-brown wings. The moths are strong fliers and migrate northward each spring and summer, sometimes covering hundreds of miles on storm fronts. Two genetic strains exist: a corn strain that feeds primarily on corn, sorghum, and cotton, and a rice strain that prefers rice and Bermudagrass.
Life Cycle and Timing
Fall armyworms develop through four stages: egg, larva, pupa, and adult moth. Female moths lay clusters of 100 to 200 eggs, often on the undersides of leaves, covered with a fuzzy layer of scales. In warm conditions, eggs hatch in as little as one to three days. The larval stage, when all the feeding damage occurs, lasts 10 to 18 days across six growth stages called instars. After the caterpillar is done feeding, it burrows into the soil to pupate, a resting phase that lasts 7 to 16 days before the adult moth emerges.
Temperature drives the speed of the entire cycle. The optimal range for survival is 21 to 25°C (roughly 70 to 77°F), while population growth peaks between 25 and 28°C (77 to 82°F). In tropical and subtropical climates, multiple generations can cycle through a single growing season. In temperate regions like the northern United States, fall armyworms cannot survive winter and rely on annual migrations from southern areas, typically arriving in late summer or early fall.
Crops and Plants They Attack
Fall armyworms have been recorded feeding on over 80 plant species, but grasses are their clear favorite. Corn is the most economically significant host. Other commonly damaged field crops include sorghum, rice, cotton, soybeans, peanuts, alfalfa, wheat, oats, millet, sugarcane, and Bermudagrass. Among vegetables, sweet corn takes the heaviest hits, though peppers, tomatoes, and crucifer crops (like cabbage and broccoli) are also targets.
In home landscapes, Bermudagrass and other warm-season turf grasses are frequently attacked. Lawns can appear healthy one day and be devastated the next because the caterpillars feed aggressively, especially at night.
How They Damage Plants
Young larvae start by scraping tissue from one side of a leaf, leaving a thin, translucent “window” of remaining tissue. This early damage is easy to miss. By the second or third instar, larvae begin chewing holes through leaves and eating inward from the edges. Older, larger caterpillars cause extensive defoliation, leaving plants with a ragged, torn appearance.
In corn, the damage pattern is distinctive. Larvae work their way into the whorl (the rolled-up central leaves), producing masses of wet, sawdust-like frass. Later in the season, they can bore through husks and feed directly on kernels. In peppers and tomatoes, larvae tunnel into fruit, causing premature drop and rot. When populations are high, caterpillars can defoliate entire plants and move in mass across fields, consuming nearly all vegetation in their path, which is exactly how they earned the “armyworm” name.
How to Scout for Them
Early detection makes the difference between manageable control and a lost crop. Scouting for eggs or small larvae (under half an inch) is critical because larger caterpillars are far harder to kill and have already done most of their damage. The best times to find larvae are early morning, late evening, or on cool, overcast days. During the heat of the afternoon, they hide under debris and leaf litter to avoid drying out. Flocks of birds congregating in a field can also signal a larval infestation.
For corn, check five areas of the field, inspecting 20 consecutive plants at each stop. Examine leaves, stalks, and the soil surface for caterpillars and feeding signs. In soybeans and forages, a sweep net or drop cloth works well for sampling. Pheromone traps can be hung to detect migrating moths. Catching moths in traps doesn’t predict exactly how much damage will follow, but it confirms that adults are in the area and likely laying eggs nearby.
When to Take Action
Not every fall armyworm sighting justifies intervention. Action thresholds vary by crop:
- Corn: Treatment is warranted when 25% of plants are infested and larvae are still under an inch long and visible on the plant surface (not buried in the whorl or ear).
- Soybeans: The threshold is 30% defoliation during vegetative growth stages, dropping to 20% once the plant enters reproductive stages. Alternatively, eight larvae per foot of row signals a problem in later growth.
- Forages and lawns: If you find more than two to three larvae per square foot and they’re under ¾ inch long, it’s time to act. For hay fields, cutting the crop early can be an effective response.
Timing is everything. Once larvae exceed half an inch, they become significantly harder to control with any method. The window for effective intervention is narrow.
Cultural and Physical Controls
Several farming practices reduce fall armyworm pressure without chemicals. Early planting helps crops escape peak moth migration and egg-laying periods. Deep plowing (at least four inches) after harvest exposes pupae in the soil to sun, heat, and predators. Allowing two to three days after plowing maximizes this effect. Destroying crop residues after harvest eliminates sheltering sites for eggs, larvae, and pupae. Crop rotation with non-grass crops breaks the cycle for the corn strain in particular.
Push-pull technology, developed primarily for African smallholder farms, is one of the more creative strategies. Legumes like Desmodium are planted between rows of corn to “push” moths away with chemical signals the plants emit. A border of Napier grass surrounds the field to “pull” the moths toward it instead. The Napier grass produces a sticky substance that traps the larvae. This system also improves soil nitrogen and provides animal fodder, making it a practical choice beyond pest control alone.
Maintaining plant diversity around fields, including flowering plants that provide pollen and shelter, encourages natural enemies to stick around. Adequate soil moisture and proper fertilization produce vigorous plants that tolerate more feeding damage before yields suffer.
Natural Enemies
Fall armyworms have a range of natural predators and parasitoids, though none provide complete control on their own. Parasitic wasps are the most important group. Several species of braconid wasps lay their eggs inside fall armyworm larvae, and the wasp larvae consume the caterpillar from the inside. Tachinid flies operate similarly, depositing eggs on or near caterpillars. In a survey across Ghana’s farming regions, researchers found seven parasitoid species attacking fall armyworms, though the overall parasitism rate was low, around 3.6%.
Ground-dwelling predators matter too. Big-headed ants were the most abundant predator found in Ghanaian cornfields, present on nearly 4% of farms surveyed. Assassin bugs also prey on fall armyworm larvae. Birds, ground beetles, and spiders contribute to suppression in fields where habitat diversity supports them. While biological control alone rarely stops an outbreak, these natural enemies can slow population growth between chemical interventions and reduce the severity of infestations over time.
Insecticide Resistance Is a Growing Problem
Fall armyworms have developed resistance to multiple classes of insecticides in regions where the same products are used repeatedly. A population studied in Puerto Rico showed extreme resistance levels: 500-fold resistance to one diamide insecticide, 223-fold to a carbamate, and 160-fold to another diamide compared to susceptible populations. Even pyrethroids, among the most commonly used products, showed 25- to 48-fold resistance in that population. Only a few compounds, including certain spinosyn and avermectin-based products, still provided reasonable control there, with resistance ratios under tenfold.
In Mexico, resistance patterns varied by region but were generally lower. Some populations showed 20-fold resistance to organophosphates and 19-fold resistance to pyrethroids. These numbers reinforce the importance of rotating between different chemical classes rather than relying on a single product. If you spray the same insecticide repeatedly, you’re essentially selecting for survivors that pass their resistance on to the next generation.
For effective chemical control, target larvae when they’re small (under half an inch), apply in the evening when caterpillars are most active and exposed, and rotate between products with different modes of action. This rotation strategy slows the development of resistance and extends the useful life of available insecticides.