Adults:
The imported cabbageworm adult is a white butterfly tinged with yellow on the undersides of the wings. The male has one black spot on the top of the forewing, whereas the female has two black spots on the forewing. The butterfly has a wing span of approximately 1-3/4 inches (44 mm).

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Eggs:
Newly laid eggs are an off-white, later turning light to dark yellow in color. The bullet-shaped egg stands on end and has longitudinal ridges. Eggs are normally laid on the underside of the leaves.

Larvae:
The larvae, which hatch in about 3 to 7 days, are small velvety green caterpillars. They feed for 2 to 3 weeks and pass through five instars. A mature larva is dark velvety green with a light yellow stripe down its back and a broken stripe along each side of the body. When mature, they are approximately 1 1/4 inches (32 mm) long. Larvae have five pairs of abdominal prolegs and three pairs of anterior legs. Larvae are normally sluggish when prodded, and small larvae seldom leave the leaf where they were hatched. Later instars may move to other parts of the plant or even to nearby plants.

Pupae:
When nearing pupation, larvae will frequently crawl some distance away and attach to the underside leaves where they change into a chrysalis. The chrysalis is a naked structure, 3/4 inch (18 mm) long, greenish in color, but eventually turning light brown. It is tapered at the end and attached to the leaf by silken threads. The distal end is thicker and has a pointed protrusion.