Fall Armyworm

Fall armyworm is an occasional pest of crops in Iowa, but when it shows up it tends to be very destructive. Outbreak-level populations in 2021 caught many farmers by surprise, but consistent scouting of corn, soybean, and hay crops can ensure it is caught early when treatment has the highest chance of success.

Identification

Adult: Fall armyworm adults are grayish-brown moths with mottled forewings. Males have white triangular spots near the tip and center of each forewing (Photo 1). Moths have a 1 ½ inch wingspan.

Egg: Individual fall armyworm eggs are dome-shaped and about 0.4 mm in diameter. They are laid in clusters that look furry or moldy due to the deposition of scales between and over the egg mass by the female moth (Photo 2).

Larva: Young fall armyworm larvae are greenish white with a black head. The head turns orange during the second instar and narrow, white stripes develop down the length of the back as larvae mature. Their heads may turn darker brown as they mature, and they develop wider stripes along the sides of their bodies. Older larvae can range from black to green. Each body segment has distinct black tubercles, each with a hair. A combination of two characters separate fall armyworm larvae from other caterpillars: a prominent inverted “Y” on the head and four tubercles in the shape of a square near the end of the abdomen (Photo 3). Fully grown larvae reach 1.5 inches in length.

Pupa: The pupa is unlikely to be seen, but it is reddish-brown and inside of a loose cocoon either in the soil or within debris on the soil surface.

Be aware of look-alikes! Fall armyworm moths are difficult for most people to distinguish from other, similar moths. Larvae are often confused with other caterpillars in field crops, like true armyworm and corn earworm (Photo 4).

Biology

Distribution: Fall armyworm is native to the tropical regions of the Americas, but has spread to Africa, Asia, Australia, and some areas of Europe since 2016. In the United States, fall armyworm only successfully overwinters in southern Florida and southern Texas, but they migrate north annually during the summer months.

Life Cycle: Once moths arrive, they lay egg masses on foliage or light-colored structures. Egg masses typically contain 100-200 eggs, and a female can produce 1,500-2,000 eggs during her lifetime. Eggs hatch in 2-3 days, and larvae develop through six instars in about 14 days during the summer. Larvae construct loose cocoons for pupation in the soil, and after about 9 days adults emerge to mate and lay eggs. Adults are usually active for 7-14 days.

During the summer, the life cycle of fall armyworm is usually completed in 30 days. When temperatures are cooler, the life cycle may take as long as 60-90 days. The number of generations depends largely on timing of moth arrival and weather conditions, but there is usually at least one generation each year in Iowa. 

Host Plants: Fall armyworm has a very wide host range but seems to prefer grasses. The most preferred plants are field and sweet corn, sorghum, bermudagrass, and crabgrass, but numerous field crops, fruits, and weeds have been known to serve as hosts. There is some evidence that strains of fall armyworm exist based on host preferences, but little is known about it.

Dispersion and Dispersal: Adults are very strong fliers, which is part of the reason they are able to migrate and disperse so well. Adults will also catch the jet stream for long-distance migration. Migration to northern states is a death sentence for fall armyworm, as there is no evidence that they migrate back south in the fall. Larvae are also highly mobile and known to move between fields in high numbers once plants have been consumed. 

Plant Injury

Larvae are the only life stage of the fall armyworm that causes injury to plants. Like all caterpillars, they have chewing mouthparts and cause defoliation or direct injury to the crop by removing tissues. Small larvae tend to leave some leaf tissues intact, creating a scraped appearance to leaves. When larvae are larger, they make bigger holes and eat from the edge of the leaf inward, leaving ragged edges on leaves (Photo 5). The final instar is when most of the injury occurs. Sometimes, plants can be completely consumed except for the harder parts of plants, like midribs or stalks. When numbers are large, complete defoliation of fields can occur seemingly overnight.

Injury to corn: If corn is still in the whorl stage when fall armyworm arrives, they may feed inside the whorl. This leaves behind a series of holes that are typically very ragged. Leaf feeding is common on vegetative corn plants (Photo 6), but leaf tissue of more mature plants is unsuitable for survival, so larvae tend to settle in the ear zone. Silks are also largely unsuitable for development, so fall armyworm larvae may settle inside the ear to feed on kernels in a similar manner as corn earworm (Photo 7). However, fall armyworm may enter the side of the ear rather than following the silk channels like corn earworm does.

Injury to forages: Fall armyworm injury to alfalfa, cover crops, small grains, and other forages is similar to what is described above. They may also clip off seed heads of small grains in addition to defoliating the plant (Photo 8). Long-term injury to stands or survivability of the crop depend largely on whether the growing point is located above- or below-ground and the extent of feeding on regrowth.

Scouting

Moths can be sampled using pheromone traps to alert to migrating populations. Captures using pheromone traps do not necessarily help predict injury to nearby crops, but can indicate the presence of moths that will likely lay eggs nearby. Scouting early and often for eggs or small larvae is the best way to ensure effective management before larvae are too large to kill (>1/2 inch long).

Corn: During the vegetative stages in corn, visit five areas of the field and inspect 20 plants in a row at each stop. Look at leaves, stalks, and the soil for caterpillars, and inspect plants for signs of injury. If defoliation is severe, consider an insecticide application. Consider an insecticide application when 25% of plants are infested and larvae are less than an inch long and exposed (i.e., not under residue or in the whorl or ear).

During reproductive stages, larvae may be found on any part of the plant, but they are most attracted to the ear. Once larvae are inside the ear, insecticides are generally ineffective.

Soybean: In soybeans, use a sweep net or drop cloth to scout for fall armyworm larvae in several areas of the field. Lump fall armyworms with other defoliators to determine if the threshold has been met. The economic threshold is 30% and 20% defoliation for vegetative and reproductive stages, respectively. There may not be much benefit to an insecticide application after R6 (pod fill), unless fall armyworms are feeding on pods. In later growth stages, if fall armyworm is the primary pest in soybeans, a threshold of eight larvae per foot of row can be used.

Forages: Use a sweep net or visually inspect plants for signs of fall armyworm injury or the caterpillars themselves. If larvae are less than ¾ inch long and there are more than 2-3 larvae per square foot, consider harvesting or using an insecticide, depending on the crop, crop condition, and crop use goals. Continue to scout regrowth.

Scouting tips! It is easiest to find larvae early in the day, late at night, or when conditions are cool and overcast, as larvae will hide under debris to avoid desiccation (Photo 9). Congregations of birds in the field may also give away the location of larvae.

Management

Cultural: Fall armyworm issues may be reduced by managing grassy weeds in and around crop fields. Early planting or early maturing varieties may help crops escape larger fall armyworm populations that develop late in the season if they are able to be harvested early.

Mechanical: For hay crops, early harvest is likely the best management strategy if it is feasible.

Biological: Fall armyworm has many natural enemies, similar to other caterpillars. Birds enjoy eating larvae and large congregations may hint at fall armyworm issues in an area. Many parasitoid wasps and flies affect fall armyworms, and predators that impact other caterpillars also feed on fall armyworms. However, these natural enemies usually do not prevent crop injury under high numbers. Presently, Bacillus thuringiensis (Bt) is the most effective pathogen formulated for control of fall armyworm.

Plant-incorporated: Most Bt corn hybrids have some protection against fall armyworm; however, in some areas resistance to the Cry proteins has been confirmed. The Vip3A trait is still effective but should be used sparingly to reduce the risk of resistance development to the trait. Consult the Handy Bt Trait Table to determine which corn hybrids have activity against fall armyworm.

Chemical: Many insecticides are labeled for control of fall armyworm, but timing is critical for effective larval suppression. Insecticides should be applied when larvae are small (less than ½ inch long, ideally) and when armyworms are actively feeding (early morning or late in the day). Resistance to insecticides, namely pyrethroids, has been noted in several states, so be sure to assess efficacy a few days after application. If fall armyworm is feeding in vegetative corn (before R1), use either a high-volume of liquid insecticide or granular insecticides to ensure they will make contact with larvae feeding deep inside the whorl. For hay crops, if early harvest is the chosen management strategy to limit fall armyworm feeding and scouting reveals that larvae are impacting regrowth, an insecticide may be used to manage the population.

Article originally published in 2025 by Ashley Dean and Erin Hodgson.