The presence of white butterflies fluttering around the garden signals the start of a possible infestation of imported cabbageworm. The adult cabbage butterfly, Pieris rapae, is white with black tips and one or two black spots on the forewings. The larva (the cabbageworm) is a bright green caterpillar clothed in fine, short hairs that give it a velvety appearance. The imported cabbageworm will attack all growth stages in cole crops.

Symptoms and Diagnosis

When the adult butterflies are observed fluttering around the garden, the larvae and the damage they do will soon be obvious. All stages of growth in cole crops can be attacked by the larvae, but they are often observed on the underside of developing leaves. The outer leaves may become riddled with large ragged holes from the larvae's feeding. If leaves are parted, masses of greenish-brown excrement can be found at the base of the leaves. The larval feces often contaminate edible portions of the plant.

Life Cycle

The imported cabbageworm overwinters as a pupa attached to plant debris. The adult butterfly emerges the following spring and flies about the garden area during daylight hours. The elongated, yellow, ribbed eggs (shaped somewhat like a football) are laid on the underside of the foliage. The eggs hatch in 7 days. The emerging caterpillars are velvety green. They feed on all cole crops for about 15 days. After this, they pupate, and new adults emerge in about 10 days. There are 2 or 3 generations of imported cabbageworm each year.

Integrated Pest Management Strategies

1. Handpick the cabbage worms off of the underside of the leaves while plants are young. This procedure is effective when plants are young or if only a few larvae are present.

2. Use row covers. Position row covers or netting over plants to prevent egg-laying by the butterflies. Start controls before the white cabbage butterflies are seen fluttering around the yard.

3. Fall sanitation. Clean up and remove infested plant material after harvest to eliminate overwintering sites of the pupae.

4. Use Btk spray or dust. Dust plants with Bacillus thuringiensis kurstaki (Btk). Begin treating the plants when white butterflies are first noticed and the cabbage worms are still small. It may take several days before the insects die after feeding on Btk, but feeding will usually stop shortly after the plants are treated.

5. Chemical controls. Spray with insecticidal soap, carbaryl (Sevin), malathion, cyfluthrin, or permethrin when plants and larvae are young and damage is increasing. The larvae are more easily killed when young and this will minimize the amount of insecticide needed for control. Carbaryl (Sevin), the active ingredient in Sevin, is very toxic to honey bees; do not use it near flowering plants.

Organic Strategies

Strategies 1, 2, and 3 are strictly organic approaches. For an organic approach to Strategies 4 and 5, consult the Organic Materials Review Institute (OMRI™) for appropriate Bacillus thuringiensis kurstaki (Btk) and insecticidal soap products.