Egg: Diamondback moth eggs are oval and flattened, and are 0.44 mm long and 0.26 mm wide. The eggs are yellow or pale green, and are deposited alone or in small groups of two to eight eggs in depressions on the foliage surface, or occasionally in other parts of the plant. Females can lay 250 to 300 eggs, but the average total egg production is probably 150 eggs. The development time averages 5.6 days.
Larva: The diamondback moth has four instars. The average and range of development time are approximately 4.5 (3-7), 4 (2-7), 4 (2-8) and 5 (2-10) days, respectively. Throughout their development, the larvae remain quite small and active. If they are bothered, they often writhe violently, move backwards and leave the plant in a silk thread. The total length of each instar rarely exceeds 1.7, 3.5, 7.0 and 11.2 mm, respectively, for stages 1 to 4. The average width of the head capsule for these instars is approximately 0.16, 0.25, 0.37 and 0.61 mm. The body of the larva narrows at both ends, and a pair of prolegs protrudes from the posterior end, forming a distinctive "V". The larvae are colorless at the first instar, but thereafter they are green. The body has relatively few hairs, which are of short length, and most are marked by the presence of small white patches. There are five pairs of prolegs. Initially, the habit of feeding first-instar larvae is the extraction of leaves, although they are so small that mines are difficult to detect. The larvae leave their mines at the end of the first instar, move under the leaf and then feed on the underside of the leaf. Its chewing produces irregular spots of damage and the epidermis of the upper leaf often remains intact.
Pupa: Pupation occurs in a loose silk cocoon, usually formed in the lower or outer leaves. In cauliflower and broccoli, pupation may appear on the florets. The yellowish pupa has a length of 7 to 9 mm. The duration of the cocoon averages approximately 8.5 days (range of five to 15 days).
Adult: The adult is a small, slender, gray-brown moth with pronounced antennae. It measures about 6 mm in length and is marked with a broad band of cream or light brown in the back. The band sometimes narrows to form one or more light colored diamonds on the back, which is the basis of the common name of this insect. When viewed from the side, it can be seen that the tips of the wings rotate slightly upwards. Adult males and females live approximately 12 and 16 days, respectively, and females lay eggs for approximately 10 days. Moths are weak aviators, usually fly less than 2 m from the ground, and do not fly long distances. However, they are easily carried by the wind. The adult is the stage of hibernation in temperate zones, but moths do not survive cold winters, as is found in most of Canada. They routinely re-invade these areas every spring.
The damage to plants is caused by feeding larvae. Although the larvae are very small, they can be quite numerous, which results in the complete elimination of leaf tissue with the exception of leaf veins. This is particularly detrimental to seedlings, and can alter head formation in cabbage, broccoli and cauliflower. The presence of larvae in the florets can result in complete rejection of the product, even if the level of elimination of plant tissue is negligible.