Common Cabbage pests

The classic trio of caterpillars are the imported cabbageworm (velvety green larvae of the white butterfly), the cabbage looper (a green inchworm that arches as it crawls), and the diamondback moth larva (small, wriggly, and tapered); all three chew ragged holes in leaves and bore into heads, leaving green frass behind. Cabbage aphids form gray-green waxy clusters on leaf undersides and in the crown, stunting plants and fouling the head. Flea beetles pepper young leaves with tiny shot-hole punctures, cabbage root maggots tunnel into roots and cause sudden wilting of transplants, and the shield-shaped harlequin bug sucks sap and leaves pale stippled blotches. Cutworms sever seedlings at the soil line, while slugs, rabbits, and deer will graze tender foliage.

Common diseases

Black rot, a bacterial disease, shows up as yellow V-shaped lesions along the leaf margins with blackened veins, and it can rot heads from the inside. Clubroot is a soilborne pathogen that swells roots into distorted galls and causes plants to wilt in the heat and stay stunted. Downy mildew produces yellow blotches on the upper leaf surface with fuzzy gray growth underneath, and Alternaria leaf spot forms dark target-like rings on leaves. Fusarium yellows wilts and yellows one side of the plant, and white mold (Sclerotinia) creates watery rot with cottony white fungal growth, especially in cool, wet, crowded plantings.

Prevention

Rotate brassicas so they do not return to the same bed for three or four years, since clubroot and black rot survive in soil and debris. Give plants full spacing for good airflow, water at the base rather than overhead to keep foliage dry, and clean up crop residue and cull piles at the end of the season. Start with certified disease-free seed and transplants, choose resistant or tolerant varieties where available, and keep soil pH near neutral (around 7.0 or above) to suppress clubroot. Time plantings to sidestep peak pest pressure and control cruciferous weeds like wild mustard that harbor the same pests and diseases.

Organic control

Floating row covers installed at transplanting are the single best defense, excluding cabbageworms, loopers, flea beetles, root maggots, and harlequin bugs before they can lay eggs. Scout and hand-pick caterpillars and egg clusters, and treat active caterpillar feeding with Bt (Bacillus thuringiensis) or spinosad, applying in the evening and reaching leaf undersides. Knock down aphids with a strong water spray or insecticidal soap, and encourage lady beetles, lacewings, and parasitic wasps that prey on aphids and caterpillars. Neem can help with soft-bodied pests, and if downy mildew or Alternaria appears in wet weather a copper-based product labeled for brassicas may be used; always read and follow the product label for any material.

Scouting & monitoring

Begin checking twice a week as soon as transplants go in, looking on leaf undersides for the yellow eggs and small caterpillars of cabbageworms, loopers, and diamondback moths. Watch for shot-hole flea beetle damage on young plants, sudden wilting that signals root maggots, and the first waxy aphid colonies in the crown as heads form. Inspect leaf margins for the yellow V-shaped lesions of black rot and pull suspect plants promptly. Yellow sticky cards and a few pheromone traps can help you gauge when moth flights peak so you time controls before populations build.

Tips

Get row covers on at transplanting and keep them on until pests taper off, since exclusion beats spraying every time. Rotate away from where brassicas grew, keep the pH up, and remove infected plants quickly, because clubroot and black rot are managed by prevention, not rescue treatments.

General home-garden IPM guidance, focused on prevention and least-toxic control. Always read and follow the product label for any pesticide, including organic products. Page updated Aug 4, 2026.