Cabbage moth is the name given to the very common Small White butterfly (Pieris rapae) in North America. True Cabbageworms and Cabbage Moth introduced from Europe (Mamestra brassicae) are small and brown, but both types do the same type of damage. The female lays tiny white eggs (that turn straw yellow before hatching) on the underside of Brassicas. These can easily be seen and knocked off. Left unchecked these eggs hatch into medium-sized green catapillars which eat large holes in the leaves of Brassicas and members of the mustard family.
Using floating row covers prevents the butterfly from landing and laying eggs. Once the caterpillar starts eating, holes in the leaves can be seen if the plants are inspected regularly. The green caterpillar will be stationary along a vein on the underside of the leaf and should be removed. Small plants are the most vulnerable. Undisturbed, the caterpillars grow rapidly and can do serious damage. If you use BTK (Bacillus thuringiensis), remember to spray the underside of the leaves as well. Plants that seem to repel the moth include onion, garlic, tomato, sage, tansy, mint, nasturtium, hemp, hyssop, and rosemary. You may want to plan your cabbage planting to include these varieties nearby. Onions do not repel caterpillars of the European cabbage moth.
