Cabbage Worms

Garden pest · Pieris rapae

Description

“Cabbage worms” commonly refers to the larvae of the cabbage white butterfly, Pieris rapae, also called the imported cabbageworm. These green caterpillars feed on cabbage and other brassicas, including broccoli, kale, collards, cauliflower, and Brussels sprouts. Young larvae make small holes in leaves; older ones chew larger, irregular holes and may leave only the main veins after heavy feeding. They can also hide among the leaves of a forming cabbage head or broccoli head, where their feeding and dark droppings make the crop unappealing. Damage is usually most noticeable from spring through fall wherever the butterflies are active. Established plants often tolerate modest leaf loss, but repeated feeding can slow growth, and seedlings may be badly damaged. The caterpillars do not sting. Although other moth and butterfly larvae are sometimes called cabbage worms, the imported cabbageworm is the species most often meant by the name.

Symptoms

• Holes in leaves
• Insects visible on the plant
• Stunted growth

How to Identify

Look on both sides of brassica leaves for dull, velvety green caterpillars with a faint yellow stripe along the back. The small, pale yellow eggs are laid singly, often on leaf undersides. Adults are white butterflies with dark wing tips and small black spots. Irregular holes and dark green droppings near feeding sites support the diagnosis. Cabbage loopers are also green but move by arching their bodies into a loop; diamondback moth larvae are smaller, more active, and often wriggle or drop on silk when disturbed. Slugs leave slime trails and usually feed at night.

Prevention

Cover brassicas with insect netting or lightweight floating row cover immediately after planting, and seal the edges so butterflies cannot lay eggs underneath. Check transplants and the undersides of leaves before covering them; a cover can trap pests already present. Remove the cover when growing brassicas for seed that need insect pollination. Inspect plants weekly, especially during warm weather, and remove eggs and young larvae before damage builds. Keep plants growing steadily with appropriate water and nutrition so they can better tolerate minor feeding. After harvest, remove heavily infested crop debris to reduce places where caterpillars can pupate. Rotating crops can help with other brassica problems, but it will not reliably exclude butterflies, which readily fly into new beds.

Treatment

Start by hand-picking caterpillars and rubbing off eggs, checking leaf undersides and the folds of developing heads. Drop collected larvae into soapy water or otherwise dispose of them. On small plantings, this and a properly secured insect cover may be enough. If feeding continues, apply a product containing Bacillus thuringiensis var. kurstaki (Bt-k) labeled for edible brassicas. Thoroughly coat the leaves young caterpillars will eat; Bt must be ingested and works best on small larvae. Reapply only as the label directs, particularly after rain. If an infestation remains severe, a labeled spinosad product is another option, but it can harm bees and other beneficial insects. Avoid spraying flowers and apply only when pollinators are not active, following all label directions and harvest intervals. Insecticides will not repair existing holes, so judge success by whether new feeding stops.

Plants Affected

Bok Choy, Broccoflower, Broccoli, Broccolini, Brussels Sprouts, Cabbage, Cauliflower, Chinese Broccoli, Chinese Cabbage, Collard Greens, Daikon, Kale, Kohlrabi, Komatsuna, Mizuna, Mustard Greens, Napa Cabbage, Nasturtium, Rapini, Red Cabbage, Romanesco, Rutabaga, Savoy Cabbage, Tatsoi, Turnip, Watercress

Not sure this is your problem? Try the problem diagnoser.