Cabbage Worms on Broccoli: How to Identify and Control Them
Ragged holes chewed through the leaves and small green caterpillars tucked into the crown are the signature of cabbage worms — the most persistent pest broccoli faces. Several different caterpillars are lumped under the name, but they damage the plant the same way and respond to the same controls. Catch them early, before they bore into the head, and they're entirely manageable.
Imported cabbageworms
What's happening
The velvety, pale-green caterpillars of the white cabbage butterfly — the small white butterflies you see fluttering over the bed. They blend almost perfectly with broccoli leaves and chew large irregular holes, leaving dark green-brown droppings, then crawl deep into the head where they're hard to spot.
How to confirm
Look for the slow-moving fuzzy green caterpillars along leaf veins and midribs, the white butterflies laying eggs, and crumbly dark frass collecting in leaf crevices and the crown.
How to fix it
Hand-pick caterpillars and squash the tiny yellow eggs on leaf undersides — for a few plants this alone keeps them in check. For heavier infestations, spray with Bt (Bacillus thuringiensis), a caterpillar-specific biological control, or insecticidal soap on younger larvae, coating leaf undersides and the head.
Prevent it
Cover the crop with floating row cover from transplant on to block the butterflies from laying eggs, and inspect leaf undersides weekly through the season.
Cabbage loopers
What's happening
Smooth green caterpillars that move in a distinctive looping, inchworm motion, arching their backs as they crawl. They're voracious leaf-eaters that skeletonize foliage and bore into developing heads, often doing more damage faster than cabbageworms.
How to confirm
Watch for the looping crawl that gives them away, larger ragged holes between leaf veins, and the night-flying brown moths that lay their eggs.
How to fix it
Hand-pick where practical, then treat with Bt or neem oil, which disrupts feeding and the larvae's life cycle. Repeat applications every 7–10 days while caterpillars are active, since new eggs keep hatching, and always coat leaf undersides.
Prevent it
Row cover excludes the egg-laying moths, and clearing crop debris at season's end removes overwintering pupae. Encourage parasitic wasps and other beneficials by avoiding broad-spectrum sprays.
Diamondback moth larvae
What's happening
Tiny, slender green caterpillars that wriggle backward rapidly and drop on a silk thread when disturbed. They chew small holes and 'windowpane' patches where they eat one leaf surface but leave the other, and they reproduce in waves through the season.
How to confirm
Look for very small, fast-wriggling larvae, pinhole and windowpane feeding damage rather than big chewed holes, and caterpillars that dangle on threads when you brush the plant.
How to fix it
Bt and insecticidal soap both work on the small larvae; spray thoroughly, including leaf undersides, and reapply on a 7–10 day schedule because generations overlap. Rotating control methods helps since this pest develops resistance quickly.
Prevent it
Use row cover, rotate brassicas to a new bed each year, and keep the patch weeded of related wild mustards that host the pest between crops.
When to worry (and when not to)
A few holes in the outer leaves are cosmetic and the plant shrugs them off — broccoli leaves are tough and most of the plant's energy still reaches the head. Step in promptly once you see caterpillars or frass near the crown, since worms that bore into a forming head ruin the harvest and are nearly impossible to wash out after cutting. A weekly inspection and an early Bt spray almost always keep a planting well ahead of the damage; soaking cut heads in cold salted water before cooking floats out any stragglers you missed.