Cabbage Worms on Broccoli and Kale: Control Guide

title: "Cabbage Worms on Broccoli and Kale: Control Guide"

—- title: "Cabbage Worms on Broccoli and Kale: Control Guide" slug: cabbage-worms hub: problems category: Problem description: "Cabbage worms and cabbage loopers can destroy brassica crops quickly. Learn to identify each species, find hidden caterpillars, and control them with Bt or row covers." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

"Cabbage worm" is a common name applied to several caterpillar species that feed on brassica crops — broccoli, kale, cabbage, cauliflower, Brussels sprouts, collards, and kohlrabi. The two most prevalent in North American gardens are the imported cabbageworm (Pieris rapae) and the cabbage looper (Trichoplusia ni). A third species, the diamondback moth (Plutella xylostella), is less common but important because it has developed resistance to Bt in some populations.

Understanding which species you have matters because Bt kurstaki is highly effective on all three, but diamondback moth resistance to Bt is real and documented in some regions.

Table of Contents

1. Species Identification
2. Damage Pattern
3. Finding Caterpillars
4. Control Methods
5. Row Covers for Prevention
6. Common Situations Table
7. Frequently Asked

Species Identification

Imported Cabbageworm (Pieris rapae)

Per Penn State Extension's cabbageworm guide, the imported cabbageworm is the larva of the familiar small white butterfly seen in gardens in early spring and summer. Adults are white with one or two black spots on the wing.

Larvae: Velvety green caterpillars, up to 1.25 inches long, with a faint yellow stripe down the center of the back. They are the same green as kale and broccoli leaves — almost perfectly camouflaged. They feed steadily and produce round, dark green fecal pellets.

Eggs: Per Penn State Extension, eggs are tiny, yellow, and bullet-shaped; laid singly on leaf surfaces. They are easier to spot on the undersides of smooth-leaved crops like cabbage than on crinkled kale leaves.

Cabbage Looper (Trichoplusia ni)

Per UC IPM's cabbage looper pest note, the cabbage looper is the larva of a grayish-brown nocturnal moth. Larvae are lighter green than imported cabbageworm, with two white lines along each side, and move by "looping" — arching the middle of the body upward as they crawl, like an inchworm.

Eggs: Pale green, dome-shaped, laid singly on upper leaf surfaces.

Multiple generations: Per UC IPM, 3—7 generations per year are possible in mild climates; in the Northeast, 2—3 generations are typical, with late-season generations in September and October creating fall brassica problems.

Diamondback Moth (Plutella xylostella)

Per NC State Extension's diamondback moth guide, the diamondback moth is a small moth (wingspan about 1/2 inch) with a distinctive diamond pattern on its back when resting. Larvae are pale green and very small (under 1/2 inch), and drop from leaves on a silk thread when disturbed.

Diamondback moth is significant because it has developed Bt resistance in commercial farming areas where Bt is used intensively. For home gardeners, standard Bt still works in most cases, but if Bt applications fail to control what appear to be diamondback moth larvae, spinosad is the recommended alternative.

Damage Pattern

Per Penn State Extension, all three species cause similar damage:

• Window paning: Young larvae feed on the lower leaf surface, leaving the upper surface intact as a thin, translucent "window." As larvae grow, they feed through the leaf entirely.
• Irregular holes in leaf tissue, scattered through the canopy.
• Contamination of broccoli heads: Caterpillars feed inside developing broccoli and cauliflower heads, where they may not be visible until harvest. Per Penn State Extension, caterpillars inside the head are not immediately apparent and become a problem at harvest.

Finding Caterpillars

The challenge with cabbage worms is that imported cabbageworm larvae are nearly perfectly camouflaged against green brassica foliage. Practical search techniques:

1. Check the underside of leaves systematically. Larvae feed on undersides, particularly when young.
2. Look for frass. Per Penn State Extension, dark green, round fecal pellets on leaf surfaces indicate feeding above; follow the frass upward.
3. Inspect the head interior. For broccoli and cabbage, check between florets and the innermost leaves where caterpillars shelter.
4. Inspect early morning. Caterpillars are most active in cool early morning and may be easier to spot.
5. Check routinely. Per UC IPM, once-a-week inspection through the growing season is the standard recommendation. The goal is to catch larvae when they are small, before major damage occurs.

Control Methods

Bacillus thuringiensis (BT spray) var. kurstaki (Bt)

The first-line organic treatment for all three species. Per Penn State Extension, Bt kurstaki is a naturally occurring bacterium that produces proteins toxic to caterpillars when ingested. It is non-toxic to humans, mammals, birds, beneficial insects, and pollinators.

Application:

• Apply when larvae are young — first to second instar is most effective.
• Thoroughly coat the leaf surfaces, including undersides.
• Apply in the evening; Bt degrades in UV sunlight and caterpillars are most active in the morning after a nighttime application.
• Per Penn State Extension, reapply after rain and every 5—7 days while larvae are present.
• Bt must be ingested to work — it does not kill on contact.

For diamondback moth resistance: Per NC State Extension, if Bt appears ineffective and diamondback moth is suspected, switch to spinosad (Entrust or Monterey Garden Insect Spray). Spinosad is organic-acceptable and effective against diamondback moth.

Handpicking

Per Penn State Extension, handpicking is practical for small plantings. Drop caterpillars into soapy water. Crushing eggs on inspection reduces the next generation.

Spinosad

Per UC IPM, spinosad is more toxic to beneficial insects than Bt, but significantly less toxic than pyrethroids. Apply in the evening to minimize exposure to bees and other pollinators. Effective against all three species including Bt-resistant diamondback moth populations.

Conventional Insecticides

Per Penn State Extension, pyrethroids (permethrin, bifenthrin) and carbaryl (Sevin) are effective on all species. Reserve for severe infestations where other options have failed — they are broad-spectrum and will kill natural enemies as well.

Natural Enemies

Parasitic wasps (Cotesia glomerata and related species) are important natural enemies of imported cabbageworm. Per Penn State Extension, parasitized larvae are sluggish and eventually die as the wasp larvae emerge and spin white cocoons. Do not kill caterpillars bearing cocoons. Avoiding broad-spectrum insecticides supports the parasitoid population.

Row Covers for Prevention

Per Penn State Extension, floating row cover placed over brassica crops from seeding or transplanting through harvest prevents adult butterflies and moths from laying eggs on the plants. This is the most effective prevention for the entire cabbage worm complex and does not need to be removed for pollination (brassicas used as vegetables are harvested before they flower).

Common Situations Table

Frequently Asked

How do I get rid of cabbage worms in broccoli heads?

Per Penn State Extension, to remove caterpillars from harvested broccoli, soak heads in cold, salted water (1—2 tablespoons salt per quart of water) for 30 minutes. Caterpillars will float or become visible. For prevention, apply Bt weekly once plants head up, focusing spray between the florets to reach caterpillars before they are buried in the head.

Does Bt kill all caterpillars?

No. Per UC IPM, Bt kurstaki is specific to caterpillars (Lepidoptera larvae) and some other insect larvae. It does not harm adult insects, beetles, flies, bees, or other non-caterpillar species. However, it will kill caterpillars of desirable species (swallowtail butterfly larvae, etc.) that contact treated foliage, so avoid applying it on plants where desirable Lepidoptera larvae are present.

How many times should I spray Bt?

Per Penn State Extension, Bt degrades in UV light and loses activity within 3—7 days under normal conditions. For consistent control during an active infestation, apply every 5—7 days and reapply after rain. Three to four applications over the course of the season is typical for a garden with consistent cabbage worm pressure.

Are white cabbage butterflies harmful?

The adults — the small white butterflies with black spots — are the parents of imported cabbageworm larvae, so yes, their presence in the garden means egg-laying is occurring. Per Penn State Extension, adult butterflies begin laying eggs as soon as brassica plants emerge, and the first generation of larvae appears in spring. The butterflies are common and widely distributed; they will be present in any garden that grows brassicas.

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Recommended gear: Best Floating Row Covers for Pest Exclusion (2026) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Sources

1. Penn State Extension &mdash; <a href="https://extension.psu.edu/imported-cabbageworm">Imported Cabbageworm</a>.
2. UC IPM &mdash; <a href="https://ipm.ucanr.edu/PMG/PESTNOTES/pn7411.html">Caterpillar Pests of Cole Crops</a>.
3. NC State Extension &mdash; <a href="https://content.ces.ncsu.edu/diamondback-moth">Diamondback Moth</a>.