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How to identify it
- The caterpillar is usually black or almost black, with two prominent stripes and many fine, bright yellow stripes along each side. It is about 1.5 to 2 inches long when grown.[1][2]
- Eggs are laid in clusters on the upper side of leaves and covered with gray, cottony material.[1]
- The adult is a brown moth that flies mostly at night, though moths may flush up as you walk through an infested field.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Beet armyworm | UC IPM notes that the western yellowstriped armyworm is the larger of the two caterpillars in tomatoes, which makes it harder to control.[2] |
Where it lives
In California it can be abundant in Central Valley alfalfa from June to early September and also breeds in the low desert, and the Pacific Northwest insect management handbook covers it as a pest of lentils.[1][3]
Biology and life cycle
Eggs hatch in a few days, and the caterpillars reach full size in two to three weeks. They pupate on or just under the soil surface.[1]
In California there are at least five generations a year in the low desert and four in the Central Valley.[1]
Populations are cyclic and reach large numbers only every few years.[1]
In the Pacific Northwest handbook's account, weeds such as lambsquarters and wild mustard attract egg-laying females and feed the caterpillars, and the insect spends the winter as a pupa.[3]
Damage and signs
- In alfalfa, the caterpillars skeletonize leaves, eating the tissue and leaving the veins largely intact.[1]
- In tomatoes it feeds on both leaves and fruit. It rarely bores deep into fruit but eats at the surface, leaving irregular holes. Damage is sporadic and worst from July to mid-September in California.[2]
- Caterpillars often move into tomatoes from alfalfa fields when the alfalfa is cut, or from beans and other crops when they are harvested or dry out. As weeds dry up, large numbers can also march onto crops.[2][3]
Health risk
These crop guides treat it only as a plant pest and do not describe any harm to people.[1][2][3]
Control
Prevention
In alfalfa, early harvest and cutting the field borders are among the main ways to prevent armyworm damage. Be aware that mowing an infested alfalfa field can send armyworms moving into nearby crops.[1][3]
Sanitation
Control weeds such as lambsquarters and wild mustard in and around plantings, since they attract egg-laying females and feed the caterpillars.[3]
Traps and physical control
Fall tillage can help destroy pupae overwintering in the soil.[3]
Biological control
Natural enemies often keep armyworms under control. Predators include bigeyed bugs, spiders, minute pirate bugs, damsel bugs, assassin bugs and lacewings, and at least ten parasites attack it; the small wasp Hyposoter exiguae is thought to be the most important. Viral diseases, including a nucleopolyhedrovirus, can also knock populations down, though none of these can be counted on every year.[1][2][3]
Pesticides
Bacillus thuringiensis (Bt) works much better on small caterpillars, so watch for infestations early. Bt and, in tomatoes, the Entrust formulation of spinosad are acceptable for organic crops. Home-garden options listed in the Pacific Northwest handbook include azadirachtin (neem oil), pyrethrins and carbaryl; follow the label for your crop.[3][1][2]
Checking fields and gardens
In alfalfa, UC IPM recommends sweeping fields two to three times a week from early summer through September, counting healthy and parasitized caterpillars. A pesticide is suggested only if cutting is not practical soon and counts average 15 or more nonparasitized armyworms per sweep.[1]
A parasitized caterpillar can be spotted by pulling off the head of an older caterpillar and squeezing the body contents toward the head end: the larva of the Hyposoter wasp is light, translucent green.[1]
In tomatoes, start checking for caterpillars when fruit reaches about an inch across. Pheromone traps show when the main moth flights happen but do not predict damage.[2]
Common questions
- Why did armyworms suddenly show up in my tomatoes?
- Western yellowstriped armyworms often move in from nearby alfalfa when it is cut, or from beans and other crops as they are harvested or dry out.[2]
- Do I need to spray every armyworm I see?
- Not usually. Natural enemies frequently keep this armyworm under control, and it reaches large numbers only every few years. Bt sprays work best when caterpillars are small.[1][3]
Sources
- University of California Statewide IPM Program, “Western Yellowstriped Armyworm (Alfalfa Pest Management Guidelines).” ipm.ucanr.edu/agriculture/alfalfa/western-yellowstriped-armyworm/ (accessed 2026-11-09)
- University of California Statewide IPM Program, “Western Yellowstriped Armyworm (Tomato Pest Management Guidelines).” ipm.ucanr.edu/agriculture/tomato/western-yellowstriped-armyworm/ (accessed 2026-11-09)
- Pacific Northwest Pest Management Handbooks (Oregon State, Washington State and University of Idaho Extension), “Lentil-Western yellowstriped armyworm (Pacific Northwest Insect Management Handbook).” pnwhandbooks.org/insect/vegetable/vegetable-pests/hosts-pests/lentil-western-yellowstriped-armyworm (accessed 2026-11-09)
- iNaturalist, “Spodoptera praefica taxon page (order and family).” www.inaturalist.org/taxa/231426 (accessed 2026-11-09)