It is the larvae of the diamondback moth that can cause significant economic damage. Larvae have two distinct feeding styles. The only evidence is a long tortuous white trail which is the result of underlying dead tissue. As the larvae grow they become too large to feed as a leaf miner. It is at this point that larvae begin to burrow through the leaf tissue resulting in many small holes in the leaf.
Economic damage is most severe after heading begins.
Larvae will burrow into the developing head destroying its marketability. The recommended treatment threshold for diamondback moth depends upon the plant growth stage. From seedling to cupping the recommended threshold is when 20 percent of the plants are infested. After heading begins, the recommended threshold decreases to 5 percent infestation. The corner stone of good pest management is monitoring.