In New Orleans he cross striped cabbage worm shows up as soon as the weather warms up. Try to control them by picking them off the plant every morning and, yes, squish, them. This should work for quite some time.
If your worm infestation gets out of hand, the use of Bacillius Thuringiensis (BT) is effective. BT is an organic insecticide and will not hurt beneficial insects.
You can purchase BT at your local nursery or plant store.