Cabbage loopers are green caterpillars that chew large, irregular holes in cabbage, broccoli, cauliflower, kale, and other vegetable crops. They receive their common name from the distinctive way they arch the middle of their bodies into a loop while crawling. The species is highly adaptable and feeds on many cultivated plants and broadleaf weeds.
The cabbage looper (Trichoplusia ni) can contaminate vegetables with caterpillars, silk, shed skins, and droppings. Damage becomes particularly important after cabbage heads, broccoli florets, or cauliflower curds begin developing because even limited feeding may make produce unmarketable.
Taxonomy and Classification
- Common name: Cabbage looper
- Scientific name: Trichoplusia ni
- Order: Lepidoptera
- Family: Noctuidae
- Subfamily: Plusiinae
- Type: Leaf-feeding moth caterpillar
- Primary hosts: Cabbage and other cole crops
Cabbage loopers belong to the owlet moth family. Although their name emphasizes cabbage, the caterpillars have a broad host range. They attack vegetables, field crops, ornamentals, and weeds belonging to several plant families.
Cabbage Looper Identification
Fully grown cabbage looper caterpillars are usually 1 to 1.5 inches long. Their bodies are light to medium green and often have a narrow white stripe along each side. Several thinner white lines may extend down the back.
The body is narrow near the head and becomes broader toward the rear. Cabbage loopers have fewer abdominal prolegs than most caterpillars. This arrangement prevents them from crawling with a smooth, continuous motion. Instead, the larva pulls its rear forward, arches its body into a loop, and then extends the front of the body.
Young larvae may be nearly translucent after hatching but turn green after feeding. Older caterpillars blend effectively with cabbage leaves and may remain motionless along a vein when disturbed. Close inspection is often necessary to find them.
Adult Cabbage Looper Moths
Adult cabbage loopers are mottled grayish-brown moths with wingspans of approximately 1.25 to 1.5 inches. Each forewing usually has a small silvery-white marking near its center. The marking may resemble a figure eight, a bent letter Y, or two connected spots.
The adults are most active around dusk and during the night, although they may fly on cloudy days. Moths feed on flower nectar and do not damage vegetable foliage. Their role in the infestation is mating and depositing eggs on host plants.
Eggs and Pupae
Female moths usually deposit eggs individually on the upper or lower surfaces of leaves. Small groups may occasionally occur. The eggs are pale greenish-white or yellowish-white, rounded, and covered with fine ridges. They are flatter than the eggs of certain other caterpillar pests.
Mature larvae form loose, white silk cocoons on the undersides of leaves, within folded foliage, in plant debris, or near the base of a crop. The pupa begins greenish and becomes brown as the adult moth develops.
Distribution and Seasonal Movement
Cabbage loopers occur throughout much of North America and many other parts of the world. They are most persistent in regions with mild winters. In warm southern areas and protected greenhouses, populations may remain active for extended periods.
Cabbage loopers generally cannot survive severe northern winters in large numbers. Adult moths migrate northward during spring and summer, often arriving on favorable air currents and storm systems. As a result, infestations in northern states may appear later in the growing season and vary substantially from year to year.
Host Plants
Cabbage loopers strongly favor cole crops, also called brassicas or cruciferous vegetables. However, they can develop on a wide variety of other plants.
- Cabbage
- Broccoli
- Cauliflower
- Brussels sprouts
- Kale and collards
- Kohlrabi and turnips
- Lettuce and spinach
- Swiss chard and beets
- Peas and beans
- Potatoes and tomatoes
- Cucumbers, squash, and melons
- Celery and certain flowering ornamentals
Broadleaf weeds can maintain looper populations between crop plantings. Moths may move into cultivated vegetables as weeds mature, dry out, or are removed.
Life Cycle
Cabbage loopers undergo complete metamorphosis with egg, larval, pupal, and adult stages. Eggs hatch within several days during warm weather. The young caterpillars begin feeding on the undersides of leaves, where they may remain hidden from gardeners and predators.
Larvae pass through several growth stages called instars. The caterpillar stage commonly lasts two to four weeks, depending on temperature and food quality. Feeding increases dramatically during the final instars.
After completing development, a larva spins a silk cocoon and pupates. The adult may emerge after approximately one to two weeks under favorable conditions. A generation can be completed in about one month during warm weather, allowing several overlapping generations each year.
Feeding Damage
Newly hatched larvae scrape the lower surface of leaves, leaving the upper layer intact. These damaged areas appear as thin, translucent, or brown windows. Larger caterpillars chew completely through the foliage and produce irregular holes between major veins.
Heavy populations may skeletonize leaves, stunt plants, or prevent young cabbage plants from forming normal heads. Larvae can move into cabbage heads, broccoli florets, and cauliflower curds, where they become difficult to see and remove.
Cabbage loopers produce substantial quantities of moist greenish-brown or brown droppings called frass. Frass accumulates on leaves and inside vegetable heads, reducing quality even when direct feeding is limited. Damaged tissue may also become more vulnerable to decay.
Signs of Cabbage Loopers
- Green caterpillars moving with a looping motion
- Small translucent patches on leaf surfaces
- Large, ragged holes between leaf veins
- Chewing damage inside cabbage or broccoli heads
- Greenish-brown droppings on foliage
- Silk or caterpillar skins on marketable plant parts
- Pale eggs on the undersides of leaves
- Loose white cocoons attached to foliage
- Gray-brown moths with silver figure-eight markings
Cabbage Loopers vs. Similar Caterpillars
Imported Cabbageworms
Imported cabbageworms are velvety green caterpillars that move without forming the dramatic loop associated with cabbage loopers. Their adults are white butterflies with dark wing markings and are active during the day.
Diamondback Moth Larvae
Diamondback moth larvae are much smaller, narrow at both ends, and likely to wriggle rapidly when disturbed. They may drop from a leaf while hanging from a silk thread. Cabbage loopers are larger and generally crawl away by arching their bodies.
Monitoring
Inspect plants at least weekly throughout the growing season. Check both leaf surfaces, paying particular attention to lower leaves, veins, growing points, and the developing edible portion of the plant. Look for eggs, small larvae, fresh holes, frass, and cocoons.
Direct caterpillar counts provide better information than feeding damage alone. Old holes may remain visible after loopers have died, pupated, or been removed by predators. Record the number of infested plants and the crop’s growth stage when deciding whether control is necessary.
Commercial treatment thresholds vary by crop and development. Young transplants and vegetables approaching harvest require greater protection, while established plants may tolerate substantial outer-leaf feeding before heads or florets develop.
Natural Enemies
Cabbage loopers are attacked by parasitic wasps, tachinid flies, spiders, predatory bugs, birds, and insect diseases. Important parasitoid groups include Trichogramma, Hyposoter, and Copidosoma species.
Naturally occurring viruses can cause diseased caterpillars to become pale, soft, or darkened before dying. These pathogens sometimes spread rapidly through dense looper populations. Broad-spectrum insecticides can disrupt natural control by killing predators and parasitoids.
Management and Control
Inspect Transplants
Examine vegetable transplants before purchasing or placing them in the garden. Check leaf undersides for eggs and small caterpillars. Removing an infested leaf before planting can prevent loopers from becoming established.
Use Row Covers
Floating row covers or insect netting can prevent female moths from laying eggs. Install covers before moths arrive and seal the edges against the soil. Row covers should not be placed over plants that already contain loopers.
Handpick Caterpillars
Hand removal is practical in small gardens. Search the undersides of leaves and the areas where leaves join the main stem. Drop collected caterpillars into soapy water or dispose of them in a sealed container.
Sanitation and Weed Management
Remove crop residue after harvest to eliminate pupae and remaining larvae. Manage broadleaf weeds that can serve as hosts. Inspect crops after nearby weeds are mowed because adult moths may move into cultivated plants when alternative hosts disappear.
Biological Insecticides
Products containing Bacillus thuringiensis var. kurstaki, commonly called Bt, can suppress young cabbage loopers when the caterpillars consume treated foliage. Bt works best against small larvae and requires thorough coverage of feeding surfaces.
Spinosad and other caterpillar-control products may be options when their labels permit use on the crop. Any pesticide must be applied according to its label, including harvest intervals, application rates, protective equipment, and pollinator precautions.
Resistance Management
Cabbage loopers have developed resistance to some insecticides, particularly where the same products are used repeatedly. Commercial growers should rotate products with different modes of action and base applications on monitoring rather than a fixed calendar.
Conclusion
Cabbage loopers are green, broad-host-range caterpillars recognized by their arching movement and pale lengthwise stripes. They chew ragged holes, enter vegetable heads, and contaminate produce with frass. Weekly inspection, row covers, hand removal, sanitation, natural enemies, and treatments directed at young larvae provide effective control. Protecting the developing and marketable portions of cabbage, broccoli, and cauliflower is more important than eliminating every hole from the outer leaves.