Diamondback moth larvae are small, slender, green caterpillars that usually feed on the undersides of leaves of plants in the cabbage family. They may wiggle vigorously and drop from plants when disturbed. Full-grown larvae are about 1/3 inch long. Adult moths are dark brownish gray and very slender. When the moth is resting with its wings held together, if viewed from above the light gray markings on the wings look like three diamonds along the midline of the the back. The moth's wingspan is about 1/2 inch.
Larvae eat one leaf surface and the inside of the leaf, leaving the other surface intact. This "window-pane" damage is very characteristic.
The diamondback moth overwinters in Illinois as a moth, though the percentage that survive is low in normal to severe winters. Eggs, larvae, and pupae may be introduced on transplants shipped in from southern regions, and northward migration of moths during the season also can extend its range. Flat, yellowish eggs are laid singly or in small groups, often near leaf veins or on stems, but they usually go unnoticed. Larvae initially mine between leaf surfaces, then they feed externally, often consuming all but the upper or lower epidermis. Fully grown, they are about 3/8 inch long. They pupate within a light silken cocoon on a leaf, and a small moth (1/2-inch wing span) emerges a week or so later. Each generation takes 3 to 4 weeks for growth and development, and there can be as many as 6 generations per year in Illinois.
Natural enemies, including parasitic wasps, often keep populations of diamondback moths in check. Avoiding early insecticide applications whenever possible allows the natural enemies to survive and provide control. The use of Bacillus thuringiensis
early in the season kills caterpillar pests and still allows survival of natural enemies.