Cabbage looper
The caterpillar of a grey-brown night-flying moth. It is pale green with thin white lines, and arches its back as it walks. It chews ragged holes between the leaf veins of cabbage, broccoli, kale, lettuce and many other crops. Later it bores into heads and fouls them with frass. It cannot survive northern winters. Moths fly north each year, and in New England numbers usually build only from late July. Kill loopers while they are small: Bt kurstaki works on young larvae, and larger loopers are harder to control. Wild parasites often do much of the work.
Trichoplusia ni larva in its looping walk on a cabbage leaf. Photo: pending.
Identification
Larvae are smooth and light green, with a narrow white stripe along each side and thin pale lines down the back. They grow to about 1½ inches (UC IPM, UKY), or 1½–2 in by UMass's measure. The tell is the walk. There are no legs in the middle of the body, only two pairs of prolegs towards the rear plus the pair at the tip. So the larva arches its middle into a loop, like an inchworm. The cabbage worm, by contrast, is velvety, has a full set of prolegs, and crawls flat.
The moth is mottled grey-brown, about ¾ inch long, with a wingspan of 33–38 mm (about 1½ in). In the centre of each forewing there are silvery-white marks, a U shape and a dot, often joined into a figure 8. It flies at night. Eggs are pale domes about 0.6 mm across, finely ridged, and laid singly, mostly on the undersides of leaves (UF/IFAS notes they can go on either surface, sometimes in small clusters).
Easily confused with
- Imported cabbagewormAlso a pestVelvety green and sluggish, with a faint yellow stripe down the back and four pairs of prolegs in the middle of the body, so it crawls flat rather than looping.
- Diamondback moth caterpillarAlso a pestUnder ½ inch when full grown, pointed at both ends with a forked tail. Touch it and it wriggles violently and may drop off the leaf on a silk thread.
- Cross-striped cabbagewormAlso a pestBluish-grey on top with many black bands across the back and a yellow line down each side, and several feed together on one plant.
Damage symptoms
Young loopers feed on the underside of the leaf, and the first three instars leave the upper surface intact. The fourth and fifth instars chew large holes, usually away from the leaf edge. UF/IFAS says a larva eats three times its weight in plant material a day, so most of the damage comes late in its life. UC IPM notes that young cole plants can lose a lot of leaf without losing yield. The damage that matters comes after heading, when larvae bore into the heads and foul them with their bodies and frass.
- Scraped patches on the leaf underside where young larvae fed through to the upper surface without breaking it
- Ragged holes between the veins, usually away from the leaf edge
- Wet green frass pellets in the leaf bases and heads
- Larvae and frass inside heads and under the wrapper leaves at the base of the head
- Thin white cocoons on the undersides of leaves
Lifecycle & degree days
The looper survives the winter only in the southernmost states. From there it spreads back across most of the US and all of Canada each year, and flights of about 124 miles (200 km) have been estimated. UMass says numbers in New England usually stay low until late July or August, though flights come earlier in some years and not at all in others. The number of generations depends on how long the season is: two to three in Canada, five in North Carolina, and five to seven in California.
- Egg to adult
- 18–25 days at 90–70 °F (32–21 °C)
- Eggs / female
- 300–600
- Larval stage
- About 18–20 days at 73–90 °F (lab, on bean)
- Generations / yr
- 2–3 in Canada, 5 in North Carolina, 5–7 in California
- Overwinters
- Only in the far south; moths migrate north
Host crops
Crucifers come first. UF/IFAS lists damage on broccoli, cabbage, cauliflower, Chinese cabbage, collards, kale, mustard, radish, rutabaga, turnip and watercress. It also records injury to beet, cantaloupe, celery, cucumber, lettuce, pea, pepper, potato, beans, spinach, squash, sweet potato, tomato and watermelon, and to chrysanthemum and snapdragon. UMass adds chard to the non-brassica hosts.
Monitoring & action thresholds
These are UC IPM's commercial cole-crop thresholds, and they tighten as the crop matures. On seedlings and small plants the threshold is about one caterpillar per five plants. Established plants before heading can carry more than nine small to medium caterpillars per plant. From heading onwards it is more than one unparasitised looper per 25 plants, or more than 10% of plants infested. UMass's Northeast figures are looser for heading crops: no need to treat young plants until 35% are infested, then treat after head formation at 15–20% infested, with 10–15% for leafy crops such as kale and collards. No source gives a garden figure. In a garden, protect the heads and the leaves you eat, and let young plants carry some holes.
Turn over the lower leaves to look for single eggs and small larvae. Search the chewed areas, leaf bases and heads for caterpillars. UC IPM samples 25 plants at random and records natural enemies alongside the pest. Its heading threshold counts only unparasitised loopers. In the north, keep checking through late summer, because UMass finds that migrant loopers usually build up from late July.
Biological control
Wild natural enemies do much of the work. In a North Carolina study reported by UF/IFAS, the tachinid fly Voria ruralis was the main killer of later-instar loopers, at about 53% mortality. A virus caused about 12%, fungi about 10%, and the wasp Copidosoma truncatellum 6–7%. In California, egg parasitism by Trichogramma varied but reached about 35%. UC IPM also lists the larval parasites Hyposoter exiguae and Microplitis brassicae. Broad-spectrum insecticides kill these allies. Bt kurstaki is the standard biological spray. Trichogramma wasps can be bought and released against the eggs. Tachinid flies cannot be bought, only conserved.
Cultural controls
Row covers, where practical, stop moths laying eggs on the crop (UF/IFAS). For everything else, timing matters most. UKY warns that larger loopers are more difficult to control, so find them young. UF/IFAS notes that insecticide resistance is a problem but varies widely from place to place.
- Cover brassicas and lettuce with row cover to keep the moths from laying
- Check the undersides of the lower leaves often, from late July in the north, and act while larvae are small
- Look inside heads before harvest; larvae bore in and foul them with frass
- Some cabbage varieties, such as Mammoth Red Rock and Savoy Perfection Drumhead, are relatively resistant, though the resistance fades under heavy feeding (UF/IFAS)
- Avoid broad-spectrum sprays such as pyrethroids. UC IPM rates most of them highly toxic to the predators and parasites that keep loopers down
Chemical, last resort
- Bt kurstakiREI 4 h
- MethoxyfenozideREI 4 h
- SpinosadREI 4 h
- ChlorantraniliproleREI 4 h
Sources
UF/IFAS Featured Creatures EENY-116 (IN273), Cabbage Looper, Trichoplusia ni (Capinera)
UC IPM Pest Management Guidelines: Cole Crops — Cabbage Looper (Natwick, Joseph & Dara, 12/20)
UC IPM, Relative Toxicities of Insecticides Used in Cole Crops to Natural Enemies (12/20)
UC IPM Pest Management Guidelines: Cole Crops — Imported Cabbageworm (12/20)
UMass Vegetable Program, Caterpillars in Brassica Crops (Hazzard, Scheufele & McKeag, 2022)
University of Kentucky ENTFACT-300, Cabbage Pests (Bessin, rev. 1/26)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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