Imported Cabbageworm

The imported cabbageworm is the common velvety green caterpillar seen on the leaves of cabbage, cauliflower, broccoli, and other crucifers. The adult is one of the most common butterflies in the Northeast; it is a white butterfly with black spots on the wings. Adults may be seen almost anytime during the summer. From about mid-July on, this insect is likely to be a pest of some importance.

Damage

The caterpillars feed on the leaves when they are young; however, larger larvae move about freely on the plant and eat out irregular holes on the larger leaves usually eat their way into cabbage heads from the bottom, or get into the developing flower (edible part) of broccoli or cauliflower. It is common for high larval populations in July and August to damage plants so severely that they die. The larvae eat large holes in the leaves. As they feed, they frequently move toward the center of the plant in order to feed directly on cabbage heads and on leaves shading cauliflowers from the sun. They are also often found inside broccoli heads when they are cooked.

Management

A combination of cultural and/or chemical control practices are recommended. The use of resistant cabbage varieties, such as Mammoth Red Rock, Chieftan Savoy,Early Globe, Red Acre, Round Dutch and Savoy Perfection Drumhead, provides some protection but not complete control. New plantings should be as far as possible from those of the previous season. At the end of the season, crops should be harvested without delay. Plowing under or destroying plant residues at this time eliminates an important food source for the overwintering generation of cabbageworms.

Scout the crop regularly to check for presence of larvae. Look for the larvae themselves, or find them by finding fresh feeding damage or fresh green frass piles, then searching plant nearby for the presence of the larva. Grow cabbage as an early crop as early grown cabbage is seldom severely injured because it reaches maturity before the imported cabbageworm populations have built up significantly. Late grown cabbage is very susceptible to injury from this insect. From about mid-July on, this insect is like to be a pest of some importance if control measures are not taken.

The imported cabbageworm has many insect natural enemies, including several parasitic wasps and predators of eggs and young larvae. You may find dead caterpillars with a large mass of white or yellow cocoons nearby. The cocoons are from a parasitic wasp. 

Controlling weeds around the garden, especially plants of the mustard family, should help decrease the numbers of this pest. Hand picking larvae can be effective for small or large gardens. Row covers are sometimes used to prevent the butterflies from having access to the plants to lay eggs on. 

On cabbage, cauliflower, broccoli, Brussels sprouts, and other cole crops use insecticides if needed. Bacillus thuringiensis (B.t.), an insecticide derived from a bacterium, is the preferred control for the imported cabbageworm. Others include insecticidal soap, spinosad, carbaryl, malathion, or rotenone; spray or dust as needed. Wait three days before harvesting. Treatment should start when the first cabbageworms are noticed. Monitor the cabbageworm population.

Adapted from: University of Maine Cooperative Extension and Connecticut Agricultural Experiment Station, 1999; Cornell Cooperative Extension, 2006