How do I deal with cabbage worms on my kale and other brassicas?
Bt (Bacillus thuringiensis) spray applied to the leaf undersides is the most targeted control for cabbage worms — hand-picking and row cover are also effective without any spray at all.
Imported cabbageworm (Artogeia rapae) is the pale green caterpillar of a small white butterfly. The butterfly lays eggs on brassica foliage; the caterpillars hatch and feed, starting on the leaf undersides and working their way through the entire leaf. A single large caterpillar can consume a significant amount of leaf in a day, and multiple caterpillars on a young plant can cause serious damage quickly. Look for the characteristic small, green pellets of frass on leaves and soil below.
Bt (Bacillus thuringiensis var. kurstaki) is the most targeted biological control available. It's a naturally occurring soil bacterium that, when ingested by caterpillars, produces proteins that destroy their gut lining. It does not affect humans, birds, mammals, or most beneficial insects. Apply it to the undersides of leaves — where caterpillars feed — in the evening. It degrades in sunlight within 1–3 days, so reapplication after rain is needed. It works best on young, small caterpillars; large ones are harder to kill.
Hand-picking works well if you check plants every 2–3 days. Turn over every leaf and look closely — the caterpillars are well camouflaged against green foliage. Drop them in soapy water. If you find eggs (small, ribbed, yellowish ovals on leaf undersides), remove those too. This approach is realistic for small plantings and removes the caterpillars before they've done significant damage.
Row cover installed over young plants before the adult butterflies begin flying in spring prevents egg-laying entirely. It's the most reliable long-term approach for gardens with consistent pressure. Remove it when temperatures warm significantly, or during the heat of summer — brassicas under tight row cover can overheat.
- AnthracnoseSunken, dark circular lesions on ripening fruit, sometimes with salmon-colored spores in the center.
- Black RotV-shaped yellow lesions at brassica leaf margins with blackened veins inside — a bacterial disease that moves through the vascular system.
- Brown Marmorated Stink BugSunken, corky dimples on fruit and pods caused by a mottled brown shield bug feeding through the skin.
- Cabbage MaggotBrassica transplants wilting and dying as white maggots tunnel through roots at or below the soil line.
- Imported CabbagewormRagged holes in brassica leaves with pale green caterpillars and green frass nearby.
- What is eating holes in my kale and cabbage leaves?Small round shot-holes suggest flea beetles; ragged large holes with frass (green droppings) point to cabbage worms or imported cabbageworm — check both sides of leaves to identify which you have.
- I have aphids on multiple plants — do I need to spray everything?Aphids tend to colonize plants under stress and naturally crash when beneficial insects find them — water sprays and patience are often more effective than pesticides.
- Something is cutting off my seedlings at the base overnight — what is it?Cutworms — fat gray or brown caterpillars that live in the soil and feed at night — cut young stems at or just below the soil surface, and a simple collar around each stem can stop them.