Beet armyworm
The beet armyworm is a moth caterpillar that feeds on a long list of vegetables, including pepper, tomato, beet, lettuce, onion and cabbage. It rarely survives the winter where frost kills its host plants, so it overwinters mainly in Arizona, Florida and Texas, and further north it arrives in late summer: Kentucky's risk period is August and September. Young larvae feed in groups and skeletonise the leaves; older ones spread out and chew large holes in leaves and into fruit. It is hard to control. Resistance to insecticides is a major problem, and Kentucky Extension says most Bt products used for other caterpillars do not work on it, though the aizawai strain does.
A late-instar Spodoptera exigua on a pepper leaf, with the dark spot above the second true leg. Photo: pending.
Identification
Full-grown larvae are about 1¼ inches (3 cm) long, dull green (sometimes nearly black) with many fine wavy pale lines down the back and a broader white stripe along each side. The underside is often pink or yellow. The best mark is a dark spot on each side of the thorax, just above the second pair of true legs, but it is absent in some populations. Young larvae are pale green or yellow and feed in groups, often under a light web.
The moth is mottled grey and brown with a wingspan of about 1–1⅛ inches (25–30 mm) and a pale, bean-shaped spot on the front wing. Eggs are laid in clusters, covered with cottony white scales from the female's body. Cluster size varies by source: 50 to 150 (UF/IFAS), more than 100 but usually fewer (UC IPM), up to 80 (Kentucky). The southern armyworm, a relative, can look similar; its dark spot, when present, sits on the first abdominal segment rather than on the thorax.
Easily confused with
- Southern armywormAlso a pestIts large dark side spot sits on the first abdominal segment and breaks the side stripe. Beet armyworm’s spot, when present, is on the thorax above the second pair of true legs.
- Fall armywormAlso a pestA white upside-down Y on the front of the dark head, and four dark spots in a square on top of the eighth abdominal segment, near the rear end.
Damage symptoms
The damage changes as the larvae grow. Young larvae feed together and skeletonise the leaves, and the smallest spin fine webbing. Older larvae become solitary and eat large irregular holes. Small transplants can be badly damaged and older plants defoliated quickly. On tomato and pepper they also feed on flower buds and bore into fruit, which becomes more susceptible as it ripens.
- Patches of skeletonised leaf, often under a light web, with groups of tiny green caterpillars
- Large irregular holes in leaves as the larvae grow
- Single or grouped round holes in fruit, especially tomato; larvae may be inside
- Aborted flower buds
- Small transplants stripped
Lifecycle & degree days
In warm weather the life cycle takes about a month, and there are three to five generations a year in California; Florida has reared six generations in five summer months. North of the overwintering states, beet armyworm is an occasional late-summer invader. In Kentucky it cannot overwinter, and the first hard frost kills it.
- Overwinters
- Mainly Arizona, Florida and Texas, where frost does not kill the host plants
- Risk period (Kentucky)
- August and September
- Egg to adult
- About a month in warm weather
- Generations / yr
- 3–5 in California; 6 reared in 5 summer months in Florida
- Eggs per female
- 300–600
Host crops
UF/IFAS lists asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, corn, eggplant, lettuce, onion, pepper, potato, spinach, tomato and turnip among its vegetable hosts, and it is also a pest of cotton and sugarbeet. Beet is a host, but there is no separate beet crop id on this site yet. UF/IFAS reports that as little as one caterpillar per 20 tomato plants can cause economic loss in commercial fields.
Monitoring & action thresholds
UC IPM's fresh-market tomato guidance says treatment may be justified if one or more larvae or egg masses are found in a 5-minute search. For processing tomatoes the threshold is 3.25% damaged fruit. Both are commercial figures. In a garden, the practical trigger is finding egg masses or groups of young larvae, because larvae over about 1/2 inch are very hard to kill with many insecticides.
From midsummer, check leaf undersides for egg masses covered in white, cottony scales, and for patches of skeletonised leaf with groups of tiny caterpillars. Search the youngest leaves and the flowers on pepper and tomato. Crush egg masses and pick off leaves carrying groups of young larvae: at this stage they are all in one place.
Biological control
Beet armyworm has natural enemies, though the sources disagree on how much they achieve. UC IPM counts the ichneumonid wasp Hyposoter exiguae as its most important parasite, and a virus that often reduces populations in autumn and winter. UF/IFAS adds the braconid wasps Chelonus insularis and Cotesia marginiventris, and predators including Orius pirate bugs, big-eyed bugs and the spined soldier bug. Kentucky Extension says it has few effective parasites or predators there. Of the sprays, the one that fits a biological programme is Bt subspecies aizawai, sold as XenTari, which UC IPM says works best on newly hatched larvae. Kentucky says most Bt formulations used against other caterpillars are not effective against beet armyworm. The DiPel DF (Bt kurstaki) label does list Spodoptera exigua, at its higher rates, so the label and the extension advice disagree; this page follows the extension view.
Cultural controls
The larvae become very hard to kill once they are over about half an inch long, so the useful window is short. Early detection matters more than the choice of product.
- Check leaf undersides for cottony egg masses from midsummer, and crush them
- Pick off leaves carrying groups of young caterpillars before they spread out
- Protect small transplants in late summer, when the migrants arrive
- If you spray, use Bt aizawai (XenTari) or spinosad on newly hatched larvae rather than a standard Bt product. UC IPM says not to let spinosad reach flowering plants, except when sprayed between sunset and midnight where the label allows, to protect bees
- Avoid pyrethroids: UC IPM warns they can trigger leafminer and russet mite outbreaks on tomato
Chemical, last resort
- Bt aizawaiREI 4 h
- MethoxyfenozideREI 4 h
- SpinosadREI 4 h
- ChlorantraniliproleREI 4 h
Sources
UF/IFAS EDIS EENY-105, Beet Armyworm, Spodoptera exigua (Capinera)
UC IPM Pest Management Guidelines: Tomato — Beet Armyworm (Natwick et al., 12/13)
UC IPM, Relative Toxicities of Insecticides Used in Tomatoes to Natural Enemies (09/16)
Univ. of Kentucky ENTFACT-308, Beet Armyworm in Kentucky (Bessin, rev. 1/26)
DiPel DF label, EPA Reg. No. 73049-39 (2022)
Univ. of Kentucky ENTFACT-110, Fall Armyworm in Corn
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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