Cabbage Worms on Broccoli and Kale: Control Guide

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

Imported cabbageworm (Pieris rapae) larva and frass on a brassica seedling tray — Photo: Downtowngal on Wikimedia (CC BY-SA 3.0)

By Thomas Joseph Published 2026-06-10 Updated 2026-06-13 Reviewed by Thomas Joseph 8 min read

"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.

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Species Identification
Damage Pattern
Finding Caterpillars
Control Methods
Row Covers for Prevention
Common Situations Table
Frequently Asked

Reader questions

Questions from other readers about this guide. Ask your own — if it's a good question, the answer goes here for the next reader.

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. --- > 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.

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Cabbage Worms on Broccoli and Kale: Control Guide

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Cabbage Worms on Broccoli and Kale: Control Guide

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