Diamondback moths are small, 12 mm (1/2 in.) long, with an 18 mm to 20 mm (3/4 in.) wing-span. They are grey or brown, with light-tan marks on the margin of the forewing. While at rest, the wings are folded, roof-like, over the body. They hold their antennae forward when at rest.
A female may lay 100 to 300 scale-like, greenish-white to creamy-yellow eggs. The eggs are laid singly or in small groups. The mature larva is a small, pale-green caterpillar, about 8 mm (5/16 in.) long. It has a narrow head and hind-quarters (i.e., somewhat spindle-shaped). With five pairs of legs on the rear half of the body. The tail end of the larva has a forked appearance.
Larvae of the diamondback moth feed on the foliage from the seedling stage to harvest. They can greatly reduce the quality and yield of the crop. Older larvae feed on flowers, young pods, and the surface tissue of stems and mature pods, usually from mid-July to early August.
The larvae pupate in a loose, silken cocoon on the plant, with the new adult moths emerging a week or so later.