Their heads are black and bear a V-shaped mark. When they first hatch, larvae feed on either side of the partly folded leaves of the plant buds. After a few days, they begin to feed beneath a protective web made from silk-like threads that they form. Sometimes the larvae are found on the outer leaves or in the angle between the main plant stalk and the leaf. They can be detected by debris and webs at the point of feeding.
Cabbage webworms tunnel into and kill the buds of young plants.
Destruction of the original bud causes the production of secondary buds that cannot mature by harvest-time. Less severe injury may disfigure the head produced from the original bud. Feeding on the outer leaves of older plants usually does little harm. Treatments applied for other pests usually keep the webworm under control. The front wings are mottled yellowish-brown to brown and are marked with zigzag lines of dark brown.