|Cabbageworm gaping leaf damage!|
|There he is munching away, leaving holes|
Scout your plants often and check under the hood - the under sides of the leaves, this is where you'll find the trouble. Squish the eggs if you're a good scout (have your hand lens with you or your readers close by). If they are already middle schoolers (green larvae stage), then pick off and squish at this size. Some gardeners dust their cole crops with while flour (under sides too) so that the larvae eat the flour while eating the leaf, then bloat too much and die with a very full stomach. Red cabbage seems to be less susceptible than green, but both broccoli and cabbage are their favorite cole crops.
They have several natural predators including ground beetles, paper wasps, spiders and parastic wasps. They drown easily from heavy rains or overhead watering. Use chemical controls as a last resort, try Bt (Bacillus thuringiensis) first, more http://www.ext.colostate.edu/pubs/insect/05556.html