Armyworm caterpillars tend to feed in groups, a behavior that distinguishes them from other vegetable pests such as the imported cabbageworm and cabbage looper. Like all caterpillars, armyworms have chewing mouthparts. The caterpillars of all three armyworm species are varying shades of tan, green, gray, and black. Some caterpillars, like those of the western yellowstriped armyworm, are predominantly black with pale yellow stripes on each side of the body. Young fall armyworm caterpillars are green and yellow but turn brown and grey as they mature. They also have a distinct yellow “Y” shape on the head capsule. However, this is not always a reliable characteristic for identification.
Caterpillars of beet armyworm are pale green or yellow when young but darken as they mature and develop stripes along the body. They also have dark spots or broken lines running along the back. In southern states, beet armyworm is commonly misidentified as the southern armyworm (Spodoptera eridania), but upon closer inspection is readily distinguished by the large dark spot that interrupts the stripe on the first abdominal segment, a feature that is only present on the southern armyworm. Caterpillars of both species lack hairs or spines, which can be used to distinguish them from caterpillars of the corn earworm (Helicoverpa zea).
Adults of all three armyworm species are small to moderately sized brown or gray moths. Eggs vary in appearance, with fall armyworm and beet armyworm producing white, light gree, or pale-yellow eggs covered with white hairs and scales. Western yellowstriped armyworm covers its eggs in a brown cotton-like material.
- The common name “armyworm” refers to several pestiferous moth genera, one of which is the genus Spodoptera. In this region of the United States, problematic species include the beet armyworm (Spodoptera exigua), fall armyworm (Spodoptera frugiperda), and western yellowstriped armyworm (Spodoptera praefica).
- Armyworms frequently feed in groups. This behavior distinguishes armyworms from other lepidopteran pests of vegetables.
- Caterpillars can feed on leaves, fruit, and the crowns of seedlings. The long list of susceptible vegetables includes asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, chickpea, corn, cowpea, eggplant, lettuce, onion, pea, pepper, potato, radish, spinach, sweet potato, tomato, and turnip.
Caterpillar of fall armyworm. Three species of armyworm (Spodopteran spp.) can attack vegetables in Colorado. They are the beet armyworm, fall armyworm, and western yellowstriped armyworm. These pests can attack a wide range of vegetables including many solanaceous and Brassica spp. Image credit: Clemson University – USDA Cooperative Extension Slide Series , Bugwood.org
Caterpillar of fall armyworm. Note the inverted “Y” shape on the head capsule. Image credit: Steve L. Brown, University of Georgia, Bugwood.org
Caterpillar of beet armyworm on a pepper leaf. Note the feeding injury and presence of frass (green pellets). Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Armyworm egg. Note the furry appearance. Image credit: Eddie McGriff, University of Georgia, Bugwood.org
Adult of beet armyworm. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Feeding injury on pepper leaves due to beet armyworm. Image credit: David Riley, University of Georgia, Bugwood.org
Braconid wasp attacking an armyworm caterpillar. Image credit: Debbie Waters, University of Georgia, Bugwood.org
Life history and habits
Moths lay fluffy egg masses on seedling crowns and the leaves of older plants. Upon hatching, the young larvae feed together near the cluster of eggs and gradually disperse as they mature. As larvae mature, they can also feed on green fruit and may enter fruits soon after hatching in later generations. Pupation occurs on or below the soil surface in a cell.
Unchecked feeding of armyworm caterpillars can lead to defoliation, shredded leaves, skeletonization of leaves on larger plants, and the presence of irregular holes. Caterpillars can bore into the heads of certain leafy vegetables and chew on flower buds, stems, and the upper portion of roots. Severe infestations of fall armyworm can kill plants or reduce their growth and significantly reduce yield. Caterpillars can also feed on the crowns of seedlings and maturing fruits, most notably tomatoes and cucurbits. Feeding of larvae produces gouges in fruit that can be shallow or deep. Newly hatched larvae chew small pinholes in leaves, sometimes resulting in a “transparent window” on the leaf.
Pheromone traps can be used to detect adults. Visually inspect leaves and surrounding weeds regularly for eggs, young larvae, and signs of feeding injury. Maturing larvae may be found in the soil or migrating toward the center of the plant. Trapping moths and scouting plants for eggs and larvae can help inform decisions related to management.
Handpicking larvae can provide some control. Maintain a weed-free landscape surrounding the growing site and be sure to till crop residues in the fall to reduce survival of overwintering pupae.
Many natural enemies exist for armyworms including predators, parasites, and microbial pathogens, but because armyworms dwell in soil this prevents. Pathogens appear to be a viable biological control agent, for example the nuclear polyhedrosis virus can provide some protection under greenhouse conditions. Certain nematodes can kill soil dwelling larvae during the prepupal stage.
There are organic and synthetic insecticides available for chemical management of armyworms in vegetables. Before purchasing any insecticides, it is important to read the label and verify that the product is intended for armyworm control on the plant of interest. For more information on chemical management of armyworms in vegetables, consult the Utah State University factsheet.
Capinera, J. 2020. Beet Armyworm. University of Florida – Featured Creatures. Available https://entnemdept.ufl.edu/creatures/veg/leaf/beet_armyworm.htm
University of California. (n.d.). Armyworms – Spodoptera spp. University of California – IPM. Available http://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/armyworm.html
Utah State University. (n.d.). Armyworms on Vegetables. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/veg-armyworms
Gore, A. (2022). Armyworms. Clemson Cooperative Extension. Available https://hgic.clemson.edu/factsheet/armyworms/