All members of the cabbage family routinely get attacked by cabbage butterfly larvae. They are small, smooth green caterpillars that you can often find on the underside of the leaves. They eventually mature into the common white cabbage butterfly. The best way to stop them is to spray every 7 days or so with Bacillus thuringiensis, often called Bt. It's a natural bacteria (hence organic) that will keep the catepillars from being able to digest their food and they die. You can find products with Bt in them at most garden centers. A couple of brands that come to mind are Dipel or Safer. Just read the labels."