Problem type: Insect
Name of problem: Cabbage white butterfly/imported cabbageworm
Plant name(s): Cabbage, Brussels sprouts, broccoli, cauliflower, radish, rutabaga, summer turnip, lettuce, and nasturtium
Symptoms / Characteristics:
Imported cabbageworm larvae feed on the leaves, creating large, jagged, irregular shaped holes. Similar damage may also occur on the outer leaves of the developing head as the mature larvae penetrate the interior. Larval excrement covering the edible leaves and flower heads can render the crops unsightly and unmarketable.
Imported cabbageworm larvae typically appear in mid-June. These pale green caterpillars have a yellow-orange stripe extending the length of the back and faint lateral bands. The skin has a velvety appearance. They are extremely slow moving, even when agitated, and have a characteristic sideward head swing. After only a couple of weeks of feeding, the caterpillars reach a mature length of approximately 3 cm. Once maturity is reached, the caterpillar pupates by using silk webbing to attach to a leaf underside or to a vertical surface such as a fence or wall. The chrysalis (pupa) can range from green to brown, depending on maturity and site of attachment. On the prairies there may be two to three generations per year.
The adult imported cabbageworm, known as the cabbage white butterfly, is not directly harmful to the vegetable crops. It may be seen at any time during the growing season, as there may be several generations of the insect per year. The adult butterfly is white with dark spots or markings on the wings. Males have one spot on each forewing and females have two. The wings also have black tips and may span up to 5 cm. Whitish, teardrop-shaped eggs are deposited individually on the host leaves, as opposed to in masses. They are typically located on the leaf underside close to the mid-vein.
Control / Preventions:
Tilling crop residue deeply into the soil may effectively eliminate overwintering pupae but will not prevent new adults from flying into the area and colonizing existing crops. Covering young plants with netting or mesh will help prevent adult butterflies from depositing eggs on the leaf surface of host plants. In late May, begin inspecting plants for eggs and newly hatched larvae. Manual removal of eggs and caterpillars can be effective but tedious, and may not be feasible in larger plantings. An early application of a biological insecticide such as Btk (Bacillus thuringiensis var. kurstaki) can effectively control the cabbageworm. Chemical controls are available but are not recommended for use on edible crops. The imported cabbageworm has many natural predators such as braconid wasps, yellow jacket wasps, ground beetles, stink bugs and spiders.