Armyworms
Order: Lepidoptera
Family: Noctuidae
Description
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, they have chewing mouthparts. In Colorado, problematic species include the beet armyworm (Spodoptera exigua), fall armyworm (Spodoptera frugiperda), and western yellowstriped armyworm (Spodoptera praefica). The exact appearance of caterpillars and moths depends on the species, but the caterpillars are varying shades of tan, green, gray, and black, and they also have a distinct inverted “Y” shape on the head capsule when mature. Moths of all three armyworm species are small to moderately sized brown or gray moths.
Fall armyworm
This species is native to North America. Young fall armyworm caterpillars are up to 6.5 mm (1/4 inch) long, depending on the instar. The earlier instars range in color from light green to dark brown and have a dark head capsule relative to the rest of the body. The caterpillars develop faint white lines along the body as they mature, and the head capsule of young caterpillars does not have the inverted “Y” shape until later in their development. Mature caterpillars can reach lengths of up to 35 mm (1.4 inches) and range in color from pink to dark brown. The caterpillars also develop prominent white lines along the side of the body as they mature and have a dark band between the white lines. Mature caterpillars also have four dark spots that form a square at the rear. Pupae of the fall armyworm are red to brown, and measures 14-18 mm (1/2-7/10 inch) long.
Adults of fall armyworm measure about 1.8 cm (7/10 inch) long and have a wingspan of 32-40 mm (1.25-1.6 inches) long. The forewings of female moths are uniformly gray to brown, while males have gray to brown forewings with triangular white spots at the tip and center of each wing. The hindwings of both sexes are a silver-white color with a narrow dark border along the margins. The eggs of fall armyworm are dome-shaped, have a diameter of 0.4 mm. The eggs are cream, grey, or white and laid in masses covered with hairs.
Western yellowstriped armyworm
The western yellowstriped armyworm is native to North America. Caterpillars are predominantly black with white spiracles on each body segment. There have two prominent pale-yellow stripes and many narrow stripes on each side of the body. After hatching larvae are about 2 mm long but reach lengths up to 5 cm (2 inches) when mature. The pupae are red to brown and measure about 18 mm (7/10 inch) long. Moths have a wingspan up to 45 mm (1.8 inches), with forewings that are dark gray to brown and hindwings that are silver and gray. The eggs are flattened spheres with a diameter of 0.4-0.5 mm and covered in a brown or gray, cotton-like material.
Beet armyworm
The beet armyworm is native to Asia and an introduced species in North America. Caterpillars of beet armyworm are pale green or yellow when young but develop pale stripes along the body as they mature. Mature larvae measure 3.2 cm (1.25 inches) long and have a green back with a pink or yellow underside and white stripes that run along the sides. They also have dark spots or broken lines running along the back. Some individuals are very dark or completely black. They are hairless and have white spiracles with a narrow black border. One key identifying feature is the presence of a small black spot on the thoracic segment behind the head. The caterpillars of beet armyworm lack hairs or spines, which is a feature that can be used to distinguish this species from tomato fruitworm, Helicoverpa zea (Lepidoptera: Noctuidae). Pupae of the beet armyworm are light brown initially but darken as they develop.
The moths of beet armyworm have a wingspan of 25-30 mm (1-1.2 inches) and forewings mottled with grey and brown. Often, there is an irregular banding pattern on the forewings and a light-colored bean-shaped spot. The hindwings are a more uniform white or gray color with a dark line at the margin. The eggs of beet armyworm are green to pink or white spheres that taper at the top. The eggs are covered in a layer of white scales that give the mass a fuzzy or cotton-like appearance. The pupae of beet measure up to 20 mm (8/10 inch) long and light brown initially but gradually darken as they develop.
Quick Facts
- The common name “armyworm” refers to several moth genera, some of which are in the genus Spodoptera.
- Armyworm caterpillars frequently feed in groups. This behavior distinguishes armyworms from other lepidopteran pests of vegetables.
- Caterpillars can feed on many different ornamental plants, vegetables, and weeds. They typically consume 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.
- Management involves cultural and biological controls. Chemical control of armyworms is challenging due to the development of resistance.
Beet armyworm (Spodopter exigua). Note the dark markings running along the back. Image credit: John Capinera, University of Florida, Bugwood.org
Fall armyworm (Spodoptera frugiperda) on corn. Note the inverted “Y” shape on the head capsule, which is a shared feature of older armyworm caterpillars. This marking is not visible on young caterpillars. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Fall armyworm moth. Note the white markings at the tips of the forewings. Image credit: John Capinera, University of Florida, Bugwood.org
Adult of beet armyworm. Note the lightly colored bean-shaped spot on the forewing. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Armyworm egg. Note the white cotton-like appearance. Image credit: Eddie McGriff, University of Georgia, Bugwood.org
Braconid wasp attacking an armyworm caterpillar. Note that the caterpillars are feeding in a group, which is a distinct behavior of armyworms. Image credit: Debbie Waters, University of Georgia, Bugwood.org
Predatory stink bug feeding on an armyworm caterpillar. Image credit: Ronald Smith, Auburn University, Bugwood.org
Life history and habits
Fall armyworm
This species does not typically overwinter in Colorado. The fall armyworm can feed on over 350 plant species, including but not restricted to cauliflower (Brassica oleracea botrytis), collard (Brassica oleracea viridis), turnip (Brassica rapa), bell pepper (Capsicum annuum), chickpea (Cicer arietinum), potato (Solanum tuberosum), tomato (Solanum lycopersicum), pea (Pisum sativum), bean (Phaseolus vulgaris), and spinach (Spinacia oleracea). Moths are most active at night, making them difficult to spot. They are strong fliers that can disperse long distances each year. Females usually lay eggs on the undersides of leaves, although egg masses can also be laid on the tops of leaves, stalk of young plants, and even fence posts or telephone poles when the pest density is high. Each egg mass contains between 100-200 eggs, and each female produces an average of 1,500 eggs throughout her lifetime. Older caterpillars can be cannibalistic, which tends to limit their density to one or two per plant. Caterpillars typically develop through six instars and younger caterpillars can disperse to new plants via wind. Mature caterpillars pupate in soil or within leaf debris on the soil surface.
Western yellowstriped armyworm
The western yellowstriped armyworm can feed on many vegetables including beets (Beta vulgaris), carrots (Daucus carota), chives (Allium schoenoprasum), rhubarb (Rheum rhubarbarum), squash (Cucurbita spp.), asparagus, potatoes, beans, peas, and tomatoes. Adults are mostly active at night when they mate and lay eggs but may fly during the day when disturbed. Egg masses are laid on the upper surface of leaves. Caterpillars of this species usually develop through six instars. In some regions of the United States, populations can build to outbreak levels in crops of the Fabaceae family. Mature larvae pupate in the soil about 5 cm (2 inches) deep.
Beet armyworm
Like the fall armyworm, this species does not overwinter in Colorado and reinvades the state annually. Preferred hosts of the beet armyworm include but are not restricted to asparagus (Asparagus officinalis), eggplant (Solanum melongena) onion (Allium cepa), pea (Pisum sativum), sweet potato (Ipomoea batatas), lettuce (Lactuca sativa), pepper, potato, and tomato. The moths are nocturnal and tend to lay eggs at night on the undersides of leaves or near blossoms. Each egg mass contains 15-150 eggs, and a single female produces between 300-600 eggs throughout her life. After hatching, young larvae feed in groups near the tips of new growth and often adhere leaves together with webbing. Like the fall armyworm, caterpillars of the bet armyworm will cannibalize each other when colonizing nitrogen-deficient plants, and research has shown that the moths of cannibalistic caterpillars tend to produce more eggs than moths of non-cannibalistic caterpillars. Larvae develop through five instars, although more stages have been reported in some areas. Mature larvae tunnel 2.5-7.5 cm (1-3 inches) in the soil to pupate.
Injury
Unchecked feeding of armyworm caterpillars can lead to the presence of irregular holes on foliage, defoliation, shredded leaves, and skeletonization of leaves on larger plants. Young caterpillars of fall armyworm consume leaf tissue on one side of the leaf while leaving the opposite side intact. As they increase in size, they begin creating holes in leaves and feed from inward from the leaf edge. Severe infestations of fall armyworm can kill plants or reduce their growth and significantly reduce yield. Armyworm caterpillars can also bore into flower buds and feed on the crowns of seedlings and maturing fruits, most notably tomatoes and cucurbits. Tomato is highly susceptible to feeding injury of the beet armyworm, especially when fruits are nearing maturity, and infestations that begin early in the growing period can be problematic. Feeding of larvae produces gouges in fruit that can be shallow or deep and increases the likelihood of microbial infections. Newly hatched larvae chew small pinholes in leaves, sometimes resulting in a “transparent window” on the leaf. In head forming crops, caterpillars can burrow into developing buds and the center of the head.
Monitoring
Trapping moths and scouting plants for eggs and larvae can help inform decisions related to management. Pheromone traps are available for all three species and can be useful in detecting peak flights. However, pheromone traps will not accurately predict injury, and further scouting will be necessary. As a precaution, crops can be examined biweekly for the presence of eggs, larvae, and feeding injuries. In some crops, the use of sweep nets two to three times per week can be an effective approach for monitoring the abundance of young caterpillars. Plant debris on the soil surface can be inspected for older caterpillars and pupae. When disturbed, caterpillars have a tendence to drop from plants. When armyworm caterpillars are found, it is recommended that they be checked for the presence of parasitoid eggs.
In fresh market tomatoes, sampling should begin when fruits start appearing. If one or more larvae or egg masses are found during a five-minute timed search, treatments may be necessary. In tomatoes designated for processing, fruits can be sampled once they are 2.5 cm (1 inch) or more in diameter. At least 100 fruits should be randomly selected in different locations throughout the field. Selecting red fruit only when most fruits are green should be avoided. When feeding injuries are observed, it is important to identify whether the pest is a species of armyworm or tomato fruitworm. A fruit can be considered injured when it has one or more holes deeper than 2.5 mm (1/10 inch), is contaminated with feces, or if larvae are present in the fruit. Observations can be recorded on this monitoring form.
Cultural control
In gardens with low densities of armyworms, egg masses and caterpillars can be handpicked and destroyed by squishing the caterpillars or placing them in a vial with ethanol. Fields can be tilled immediately after harvest to kill larvae and pupae residing in the soil. The weeds along field borders should be removed and destroyed since these alternative hosts provide a refuge area for armyworm caterpillars.
Biological control
Armyworms have several natural enemies that can suppress their densities, including parasitic flies and wasps, ground beetles (Coleoptera: Carabidae), predatory stink bugs (Hemiptera: Pentatomidae), and certain Orius spp. (Hemiptera: Anthocoridae), birds, fungi, nematodes, viruses, and bacteria. It is important to note that limiting the use of insecticides will conserve natural enemies of armyworms and reduce the amount of injury inflicted to plants. In addition, maintaining crop diversity can help attract natural enemies.
In seasons with favorable weather, these natural enemies can suppress populations of armyworms. Cold, wet springs can limit their effectiveness and lead to increasing armyworm populations. The bacterium Bacillus thuringiensis kurstaki (Btk) is available in commercial formulations and can be effective against young caterpillars that are less than 1.3 cm (1/2 inch) long to be effective. For Btk to work, it must be consumed by the caterpillars. In greenhouses, applications of the fungus Beauveria bassiana, a nucler polyhedrosis virus, and entomopathogenic nematodes (Rhabditida: Steinernematidae and Heterorhabditidae) can be effective. When caterpillars die due to viral infection, they hang from plants and ooze body contents. The infected caterpillars can be collected, ground, mixed with water, and sieved to create a solution that can be sprayed onto plants.
Chemical control
Insecticidal control is only warranted when cultural and biological controls are not sufficient to prevent damage. In some parts of the United States, insecticide resistance is a major challenge in managing armyworms. In addition, young caterpillars are more susceptible to insecticides but, in the case of beet armyworm, often reside within webbed foliage which can protect them from contact insecticides. Older larvae often occur on open leaves and are more exposed to insecticides but are more tolerant of insecticides. When selecting insecticides, it is recommended to opt for insecticides that are target specific, degrade rapidly, and pose a low health risk. To prevent resistance from developing in the armyworm population, insecticides with different modes of action should be rotated. In addition to traditional insecticides, neem products can be used for controlling the larvae of beet armyworm.
References
Acheampong, S. 2019. Western Yellowstriped Armyworm. British Columbia: Ministry of Agriculture. Available https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/agriculture-and-seafood/animal-and-crops/plant-health/western-yellowstriped-armyworm.pdf
Baker, J. 2021. Beet Armyworm. North Carolina State – Extension. Available https://content.ces.ncsu.edu/beet-armyworm
CABI. 2019. Fall Armyworm Field Handbook: Identification and Management. Food and Agriculture Organization of the United States. Available https://www.cabi.org/wp-content/uploads/FAW-pocket-guide.pdf
Capinera, J. 2020. Beet Armyworm. University of Florida – Featured Creatures. Available https://entnemdept.ufl.edu/creatures/veg/leaf/beet_armyworm.htm
Capinera, J. 2020. Fall Armyworm. University of Florida – Featured Creatures. Available https://entnemdept.ufl.edu/creatures/veg/leaf/beet_armyworm.htm
Gore, A. 2022. Armyworms. Clemson Cooperative Extension. Available https://hgic.clemson.edu/factsheet/armyworms/
Government of Canada. (n.d.). Western Yellowstriped Armyworm. Invasive Species Online: Kanagan. Available https://www.oiso.ca/species/western-yellowstriped-armyworm/#:~:text=Western%20Yellowstriped%20Armyworms%20have%20four%20life%20stages-%20adults%2C,are%20silvery-grey.%20Adults%20are%20about%201.5%20cm%20long
Government of Western Australia. 2020. Fall armyworm larval identification guide. Department of Primary Industries and Regional Development. Available https://www.agric.wa.gov.au/sites/gateway/files/Fall%20armyworm%20larval%20identification%20guide%20DPIRD.pdf
Texas A&M. (n.d.). Beet Armyworm. Texas A&M Extension. Available https://texasinsects.tamu.edu/lepidoptera/beet-armyworm/#:~:text=Caterpillars%20are%20overall%20green%20or%20marked%20in%20shades,the%20second%20body%20%28thoracic%29%20segment%20behind%20the%20head
Tilmon, K., and A. Michel. 2023. Fall Armyworm in Ohio Field Crops. Ohio State University – Extension. Available https://ohioline.osu.edu/factsheet/ent-0093
UCIPM. 2017. Armyworms – Spodoptera spp. University of California – Agriculture and Natural Resources. Available http://ipm.ucanr.edu/PMG/GARDEN/VEGES/PESTS/armyworm.html
UCIPM. 2017. Western Yellowstriped Armyworm. University of California – Agriculture and Natural Resources. Available https://ipm.ucanr.edu/agriculture/alfalfa/western-yellowstriped-armyworm/
USU. (n.d.). Armyworms. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/notes_ag/hemp-armyworms#:~:text=Management%201%20Keep%20crop%20area%20weed-free.%202%20Till,flower%20buds.%205%20Approved%20insecticides%20are%20available.%20
USU. (n.d.). Armyworms on Vegetables. Utah State University. Available https://extension.usu.edu/pests/ipm/notes_ag/veg-armyworms
USU. (n.d.). Fall Armyworm. Utah State University – Extension. Available https://extension.usu.edu/vegetableguide/sweet-corn/fall-armyworm#:~:text=Management%20Cultural%20Use%20traps%2C%20including%20blacklight%20and%2For%20pheromone%2C,dry.%20Plant%20early%20and%20plant%20early%20maturing%20varieties.