Cabbage Looper

Cabbage looper larvae can be distinguished from most other caterpillars in cole crops by their distinctive looping movement. They move by arching the middle portion of their body and pulling their rear end forward to meet the front legs.

The caterpillars are green, usually with a narrow white stripe along each side and several narrow, pale lines down the back. They are smooth-skinned, with only a few long bristles down the back, and may grow to be up to 1.5 inches long.

Cabbage loopers have two pairs of prolegs (leglike appendages, or fleshy stubs) on the fifth and sixth abdominal segments (middle of the abdomen). This distinguishes them from imported cabbageworms, which have four pairs of mid-abdominal prolegs, on segments 3 to 6. True loopers have one pair of mid-abdominal prolegs, on segment 6.

Last-instar cabbage loopers pupate in a silken cocoon, usually attached to leaves. Adults (moths) are mottled brownish or gray with an 8-shaped, silvery marking on each front wing. Eggs are dome-shaped with numerous fine ridges and laid singly, mostly on the undersurface of leaves.

Cabbage looper has many generations per year and develops throughout the year in California cole crops. The highest numbers commonly occur during fall.

Although seedlings are occasionally damaged, most injury occurs after heading. Loopers chew ragged holes in leaves and bore through and contaminate heads and leaves with their bodies and frass. Young plants between seedling stage and heading can tolerate substantial leaf damage without yield loss.

Many natural enemies generally keep cabbage loopers below economically damaging levels at least until heading, unless they are disrupted by broad-spectrum insecticides applied for other pests. Monitor the abundance of loopers and natural enemies to determine whether biological control of cabbage looper is sufficient, and to determine whether insecticide application is needed after heading.

Important natural enemies include the egg-parasitic wasp Trichogramma pretiosum, the larval-parasitic wasps Copidosoma truncatellum, Hyposoter exiguae, and Microplitis brassicae, and a parasitic tachinid fly, Voria ruralis. A naturally occurring entomopathogenic virus is sometimes effective, turning infected caterpillars into elongate, dark, liquidy sacks that hang from leaves.

Consider maintaining hedgerows of insectary plants to attract natural enemies of cabbage loopers.

Use biological control and sprays of Bacillus thuringiensis and the Entrust SC formulation of spinosad in an organically certified crop.

Combine the monitoring for cabbage looper and imported cabbageworm, beginning after seedling emergence or after transplanting.

1. Check 25 plants selected randomly throughout the field.
2. Look for eggs and small larvae on the underside of lower leaves.
3. If leaves have holes, search nearby for caterpillars, opening chewed heads if present.

Monitor for natural enemies when monitoring caterpillars. If looper numbers are close to the treatment threshold but there are many parasitized or diseased individuals, delay treatment for a few days. Pulling apart some apparently healthy caterpillars may reveal that they contain maggotlike parasite larvae not yet old enough to cause the caterpillar to appear parasitized. Monitor again in a few days to see whether natural enemies are reducing looper numbers.

The location and extent of chewing may indicate the presence of cabbage looper caterpillars and how numerous they may be. However, to make pesticide application decisions, use the combined number of counted healthy cabbage loopers and imported cabbageworms to decide whether insecticide application is necessary.

Apply insecticide:

• To seedlings or small plants, if the numbers of medium-to-large caterpillars are high enough (approximately 1 caterpillar per 5 plants) to stunt crop growth.
• To well established plants and before heading or at Brussels sprouts formation, if more than 9 small-to-medium caterpillars are found per plant.
• At heading, if more than 1 nonparasitized or noninfected looper or other caterpillar is found in 25 plants.
• From heading to harvest, apply insecticide if more than 10% of the sampled plants (2–3 plants out of 25) are infested with at least 1 caterpillar.

If insecticide application is warranted, use the caterpillar-specific Bacillus thuringiensis to avoid damaging natural enemy populations. Bacillus thuringiensis and moderately selective insecticides (such as chlorantraniliprole and spinetoram) are very effective against cabbage looper and imported cabbageworm, especially when applied to early-instar (young) caterpillars. If significant numbers of other caterpillar species are present (see the beet armyworm and diamondback moth sections), apply an insecticide for these pests. Pesticide applications for other caterpillars, such as beet armyworm and diamondback moth, will also control cabbage looper.