Cabbage Worms: Protecting Brassicas

GardenDraft Team · June 18, 2026 · 6 min read

Part of: Plant Problems & Pest Guides

If you grow cabbage, broccoli, kale, or any of their relatives, you've probably seen the small white butterfly fluttering over the bed on a sunny day. That's the start of the cabbage worm problem: she's laying eggs, and within weeks velvety green caterpillars will be chewing ragged holes through your brassicas and burrowing into developing heads. The good news is this pest is predictable, which makes it preventable.

Know the cabbage worm

"Cabbage worm" usually means the larva of the cabbage white butterfly: a velvety green caterpillar that blends almost perfectly with the leaf, plus a close cousin, the cabbage looper, that moves in an inchworm arch. Look for ragged holes in the leaves, dark green droppings tucked in leaf folds, and caterpillars hiding along the midrib. They hit the whole brassica family — cabbage, broccoli, kale, Brussels sprouts, and kin.

Row cover is the cleanest defense

The most effective control is to never let the butterfly reach the plants. Drape floating row cover over the bed right after transplanting and seal the edges — brassicas don't need insect pollination, so you can leave the cover on all season. No eggs laid means no worms to fight later. It's the same row cover you'd use for season extension, so a single piece of fabric does two jobs.

If they're already there

For plants already under attack, two tools handle it. Handpicking works well once you train your eye — check leaf undersides and midribs every few days and drop the caterpillars in soapy water. For heavier pressure, Bt (Bacillus thuringiensis) sprayed on the leaves is a soil bacterium that kills only caterpillars and leaves bees, ladybugs, and you unaffected. It works best on young, small caterpillars and breaks down in sunlight, so spray while the worms are little and reapply after rain or every week or so.

Plan them out of the problem

Cabbage whites fly, so crop rotation won't stop them on its own — but moving brassicas each year still pays off against the soil-dwelling pests and diseases that do build up in place. Strongly scented companion plants like thyme and dill can also mask the crop and draw in parasitic wasps. For the broader lineup of who else might be chewing, see the garden pest ID guide.