High populations can damage seedlings severely enough to kill them or slow growth enough to inhibit uniform maturing of the crop. Older larvae may burrow into the head from the top. Maximize the use of these natural enemies by limiting treatments, especially between thinning and heading.
Check fields twice a week once seedlings emerge.
Treat seedlings or small plants if populations of medium-sized to large loopers are large enough to stunt growth. Treat well established plants only if you find an average of more than one-half larvae per plant. For cabbage looper control, the best time to apply insecticide is in the afternoon. The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies. Do not apply within this many days of harvest.