Answer: It all depends on what's causing the holes! Imported Cabbage Worms, Cabbage Loopers, Slugs and Flea Beetles are common pests of cabbage family crops. The first two are green or striped caterpillars that blend well with the color of the foliage and can be hard to see. They make large, irregularly shaped holes. Their "frass" (droppings) are dark green - almost black -- and are easy to notice. The adults are moths that lay tiny eggs on leaves. The worms and loopers can be controlled by handpicking and by applying Bt (Bacillus Thuriengiensis) according to the label instructions. Bt is a bacterium that acts as a stomach poison, so the critters will continue to feed until the poison takes effect.
Slugs work at night or in the cool parts of the day. The irregular holes they make are "signed" with trails of shiny slime.Slugs can be hand picked at night, trapped, or baited with commercial bait being careful to follow the label instructions (especially if you have children or pets in the garden).
Flea Beetles chew very tiny "shot holes" in leaves. These little shiny black beetles are so named because they're about the size of a flea and jump like fleas when disturbed. They can be controlled by a variety of methods including growing crops under floating row covers, mulching at transplant time (a four inch deep layer of hay or straw changes the microclimate environment and provides a physical barrier to the beetles as well as creates a cool moist microclimate they dislike). Since the beetle overwinters on crop debris, fall cleanup is also important in keeping the population low. If these methods don't reduce the problem, you may have to resort to Sevin (carbaryl) sprayed at transplant time and repeated as needed according to the label instructions.
Good luck with your cabbage!
Q&A Library Searching Tips