Armyworms
Beet armyworm. Photo from Animalia
Introduction
Armyworms are a group of destructive moth larvae (caterpillars) that pose a serious threat to crops across Southeast Asia. Named for their habit of moving in large "armies" across fields, devouring crops as they go, these pests are highly mobile, fast-breeding, and often difficult to control once established.
Several species of armyworms are of major concern in the region, each with unique host preferences and ecological behaviours. Together, they affect a wide range of crops, including rice, maize, cassava, soybeans, vegetables, and sugarcane. They can cause significant economic losses for farmers in the region.
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Along with the Fall Armyworm, several species of armyworms can be found in Southeast Asian countries, including the following:
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Beet Armyworm (Spodoptera exigua)
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Paddy/Oriental Armyworm (Mythimna separata)
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Lawn Armyworm Moth (Spodoptera mauritia)
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Beet Armyworm (Spodoptera exigua)
Spodoptera exigua, also known as the beet armyworm, is a small moth that hides during the day and is active at night. The pest is native to Southeast Asia.
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A highly polyphagous pest that attacks vegetables (e.g., onions, cabbage, chilli), cotton, and ornamental plants.
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Larvae are small, greenish, and often resistant to insecticides.
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More active in dry seasons and nursery settings.
Life Cycle and Identification
Eggs: The beet armyworm (Spodoptera exigua) lays its eggs in clusters, sometimes in several layers, on top of a mass of white, cottony hairs and scales from the moth's body. Each cluster contains 50 to 150 eggs, with an average of 300 to 600 eggs laid by a female. The individual eggs are circular when viewed from above, and usually grey, but can sometimes be greenish or pinkish. They are laid at night, normally on the underside of leaves and low in the crop.
Larva: The caterpillars usually go through five developmental instars. The colour and pattern of the caterpillars can vary widely and depend on factors such as the host plant, the stage of development, and the climate. As the larvae grow, they undergo several colour changes. In the first and second instars, the larvae are pale green or yellow but develop pale stripes during the third instar. In the fourth instar, the larvae become darker on their backs, with a dark stripe running along their sides. A fully-grown beet armyworm caterpillar can reach 25 to 38 mm. It has dark, wrinkled stripes on its back and a yellow band running the length of its body on each side, with a black dot on each segment above the band.​ ​​
Mature larva of beet armyworm, Spodoptera exigua (Hübner). Photograph by John Capinera, University of Florida
​Pupa: The pupation normally occurs in the soil. The pupae are light brown and 15 to 20 mm long. After five or ten days, the adult moth emerges from the pupa.
Adult: The moths are of moderate size. The head and thorax are brown, and the abdomen is grey-brown. The wingspan measures 25 to 30 mm. The hind wings are a more uniform gray or white color and are trimmed with a dark line at the margin. The forewings have a yellowish, and generally with an irregular banding pattern and a light-colored bean/kidney -shaped spot.
Adult beet armyworm, Spodoptera exigua (Hübner). Photograph by Lyle J. Buss, University of Florida
This video by Ta-I Huang and Marco Pena, from the University of Arizona, shows the eclosion of an egg mass of the beet armyworm (Spodoptera exigua). The original video was taken by Dr. Ta-I Huang and edited by Marco Pena and can be found on the Entomological Society of America (ESA) webpage.
Damage and Symptoms
The larvae feed on both foliage and fruit. As they grow older, the caterpillars move to the top of the plant, where they mainly feed on the growing tips. This results in large holes in the leaves, sometimes completely stripping them down to the mid-rib. Additionally, they burrow into the crown or center of the head of lettuce and the buds of cole crops. This extensive damage significantly impacts the crop's growth.
Research
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He, W., Zhao, X., Ge, S., & Wu, K. (2021). Food attractants for field population monitoring of Spodoptera exigua (Hübner). Crop Protection, 145, 105616. See Link
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Fu X, Feng H, Liu Z, Wu K (2017) Trans-regional migration of the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), in North-East Asia. PLoS ONE 12(8): e0183582. https://doi.org/10.1371/journal.pone.0183582.
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Wang X, Feng Q, Zhou X, Zhang H, Wu S, Wu K. Seasonal Migratory Activity of the Beet Armyworm Spodoptera exigua (Hübner) in the Tropical Area of China. Insects. 2024; 15(12):986. https://doi.org/10.3390/insects15120986.
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Satiman, U., Tulung, M., Pelealu, J., Salaki, C. L., Kolondam, B. J., Tallei, T. E., ... & Pinaria, A. (2022). Morphology, diversity and phylogenetic analysis of Spodoptera exigua (Lepidoptera: Noctuidae) in North Sulawesi by employing partial mitochondrial cytochrome oxidase 1 gene sequences. Journal of Advanced Biotechnology and Experimental Therapeutics, 5, 136-147. At Link​
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Oriental Armyworm (Mythimna separata)
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Mythimna separata, commonly called the oriental armyworm) primarily targets cereals—rice, maize, sorghum—but also affects sugarcane, sugar beet, legumes, and various grasses. It is widely distributed in Southeast Asian countries, including Indonesia, Malaysia, Vietnam, the Philippines, Myanmar, and Laos.
Life Cycle and Identification
Egg: The female moth lays between 500 and 900 eggs. The egg stage lasts for 2 to 7 days. The eggs are shiny milky white in color and are laid in rows or masses of 20 to 76, between overlapping leaf-sheaths and attached to the leaf using a sticky secretion. When dry, this secretion appears white and flaky. Each moth may lay 1 to 4 egg masses.
Larvae: The head is grayish brown with dark reticulation. The general body color varies among different individuals, with some being a dirty pale brown, while others appear very dark and, in some cases, almost velvety black. A dark brown median dorsal line is interrupted by narrow white dashes and interfused with pale lines on the margin. Laterally, a dark brown stripe is followed by a white stripe just above the spiracle line. Below the line of spiracles is a yellowish stripe with a pinkish tint, contrasting with the dark brown line above and the dusky brown area beneath. The body is sparsely covered with fine hairs placed on minute black tubercles. Larval development is completed in 14-22 days.
Hairless, brownish caterpillar of Paddy armyworm with a narrow, white dorsal line (NBAIR)
Pupae: The pupae are initially cream-colored and then change to dark brown before pupating in an earthen cell in the soil or inside the leaf sheath of the plant. The pupal period lasts for 8-9 days.
Adult: Moths have a wingspan of 35–50 mm; pale greyish‑yellow wings with distinct spots. Male moths are smaller and lighter in colour than female moths.
Adult of Paddy armyworm moth (Source: epitree via Flickr)
Symptoms and damage
During the vegetative stage of rice, M. separata causes significant damage by consuming leaves and developing grains and anthers. The larvae also sever panicles during grain-filling, resulting in yield loss. Research indicates that a population of 15 larvae per hill can lead to yield reductions of 38% and 93% at the booting and panicle stages, respectively. Moreover, for each additional larva per hill, the number of grains per panicle decreases by 4 and 9 during each growth stage. In severe infestations, damage could exceed 60% (Dale, 1994). However, the prevalence of M. separata has decreased in the past 30-40 years due to the expansion of irrigated rice cultivation.
Research
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Cang X, Zhao S, Yang X, Yuan H, Liu J, Liu D, Yang X, Wu K. Migration Monitoring and Route Analysis of the Oriental Armyworm Mythimna separata (Walker) in Northeast China. Agronomy. 2023; 13(1):172. https://doi.org/10.3390/agronomy13010172 - see here
Check out this short clip of the Oriental Armyworm from the Youtube Channel of the Laboratory of Insect Physiology, Kyoto University.
Paddy or Lawn Armyworm (Spodoptera mauritia)
The paddy armyworm can be highly destructive, affecting rice, grass and other cereal crops across Southeast Asia. The larvae are voracious feeders that move in large groups, or "armies", rapidly consuming foliage and stems, often leading to significant crop damage or complete defoliation. It is particularly active during the rainy season and thrives in warm, humid climates. It exhibits high mobility and reproductive capacity, with outbreaks often sudden and severe. Adult moths are nocturnal, while the larvae feed mostly at night, hiding in soil or debris during the day.
Life cycle and Identification
​Eggs:
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Appearance: Eggs are small (~0.5 mm), spherical to dome-shaped, and laid in clusters.
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Colour: Creamy white when freshly laid, turning darker as they mature.
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Location: Laid on the underside of leaves in overlapping rows; often covered with protective scales or hairs from the female’s abdomen.
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Incubation time: 3–5 days depending on temperature.
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Larva:
The larval stage is the most damaging phase.
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Instars: 5–6 instars; larvae grow up to 35–40 mm.
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Coloration:
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Early instars are greenish and semi-translucent.
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Later instars are dark brown to greyish with longitudinal stripes.
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Key Features:
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Prominent white, yellowish, or dark stripes along the body.
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A distinctive inverted “Y” marking on the head (though less prominent than in Spodoptera frugiperda).
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Smooth body with fine hairs and well-developed legs.
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Behavior:
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Gregarious in early instars, spreading out in later stages.
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Nocturnal feeders; hide in plant debris or soil during the day.
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Cause rapid and extensive defoliation.
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​S. mauritia larva; first pale green, later brown with dark dorsal and lateral stripes, up to 38mm long. MERLE SHEPARD
​Pupae:
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Location: Pupation occurs in the soil at depths of 2–6 cm.
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Appearance: Smooth, reddish-brown pupa about 18–20 mm long.
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Duration: 6–10 days under favorable conditions.
Adult Lawn armyworm, Spodoptera mauritia Photograph by Victor W Fazio III on I NautalistAU
Adult:
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Wingspan: 30–40 mm.
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Coloration:
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Forewings: Brownish-grey with wavy markings and dark spots; patterns vary slightly but typically mottled.
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Hindwings: Whitish with a dark margin.
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Behavior:
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Nocturnal and strongly attracted to light.
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Females are highly fecund, laying hundreds of eggs over their lifespan.
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Symptoms and Damage
The caterpillars damage paddy crops by cutting off leaf tips, leaf margins, leaves, and even the plants at the base, and can decimate a crop.
Research
Photo of Paddy armyworm in India. G.K. Upamanya, N.Kalita and R. Sarma (2013)
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Sarma AK, Damgaard C, Neog P. Retrospection of Outbreaks of Spodoptera mauritia Boisduval in NER India: The Solution Lies in Ecological Engineering, Not in Insecticides. Sustainability. 2021; 13(22):12824. https://doi.org/10.3390/su132212824
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Sarma, A. K., Damgaard, C. F., Saikia, M., & Neog, P. (2024). Rapid assessment and management of outbreak of Rice Swarming Caterpillar by drone: a BIG need of the hour. Frontiers in Sustainable Food Systems, 8, 1361012. See Link