The cabbage webworm is one of a number of insects that feed exclusively on and affect cabbage, broccoli, cauliflower, and other crucifers. It is easily recognized. Cabbage webworm eggs are flattened in shape and are usually laid singly or in small masses on the terminal leaves. The eggs are gray or yellowish green turning pink as they near hatching. There are five larval instars. The mature larva is yellowish gray with five brownish-purple bands running the length of its body. Its head is black. Yellow or light brown hairs sparsely cover the body.
The smallish moth has yellowish-brown front wings marked with white bands and a dark kidney-shaped spot, which afford it camouflage while resting on the ground. The hind wings are grayish-white with a darker margin.
Survival And Spread
Cabbage webworm moths deposit approximately 300 to 350 eggs on host plant buds. Upon hatching, the larvae mine the leaves and also feed on the underside of the leaves producing small holes. Initially, damage appears as small quarter-sized brown, dried areas along the leaf margins.
At about the third instar, larvae begin to web and fold the foliage by webbing the leaf margin back on itself to form a pocket on the underside of leaves in which they feed. The webs often become covered with dirt and excrement. Larger larvae can burrow into buds, stems, and leaves. Webworms can cause extensive damage when they bore through the bud and into the stem of the plant. This can kill the bud and cause lateral budding preventing the plants from developing marketable heads. They sometimes kill the entire plant. When fully grown, larvae pupate among shed leaves or other refuse on the ground.
Fields should be scouted weekly for this pest checking edges and interior sections in all quadrants of the field. The cabbage webworm is most abundant during the warmer months. Controls used for diamondback moth and other cabbage pests are effective against cabbage webworm when timed properly. A base program consisting of Bts as the main insecticide and incorporating some of the newer materials (spinosad, indoxacarb, and tebufenozide) when needed, will provide control.
Sprays should be applied while the larvae are small before they construct their protective silken webs. Once inside the folded leaves, the larva are protected from insecticide sprays. Sprays need to be directed underneath the leaves and down into the bud.
Cultural controls consist of destroying crop residues, and controlling weed hosts. Planting a nectar source for beneficial insects may be helpful (sweet alyssum has been tested in cabbage). For high-value specialty crucifers, floating row covers put in place immediately after transplanting may eliminate damage.
By Gene McAvoy, Regional Vegetable Specialist IV, University of Florida/IFAS