If you ‘ve got a cabbage-worm problem in your garden, here are some links you should check out:

[1] A description of cabbage butterflies (or cabbage worms):

[2] Controlling cabbage worms

A few comments on what is being said at that page:

Hand-picking is a chore, not necessarily pleasant and somewhat never-ending — until frost kills off the butterflies.

Rotenone is a powerful pesticide, a biological pesticide that degrades quickly — used in organic gardening — but you must be careful about using it.  Don’t eat or breathe it in.  They say that it is safe to use it, provide you wait a day before harvesting.
We have used it and never died yet.

Bacillus thuringiensis, or Bt for short, is safest, as it will not infect people or mammals, but I don’t know how quickly it works on cabbage worms.

When I first planted a garden at the farm (in 1974), I asked people about what they did to protect against cabbage worms when no effective and relatively safe pesticides were available yet.  The old-timers told me that they made garlic tea, which they then diluted with water and sprayed on their cabbage-family plants (all cabbages, cauliflower, broccoli, brussel sprouts, kohlrabi, etc.).  I never used it and don’t know how effective it is.

By the way, a couple of years ago we had an infestation of cabbage worms on our columbine.  I used Rotenone.  It killed all of the cabbage worms within a day.  Wish I had used it earlier, though.  Rotenone is a natural, poisonous alkaloid that is extracted from some plants (e. g.: chrysanthemums).

Here is some information on Rotenone.
[3] http://en.wikipedia.org/wiki/Rotenone

Happy hunting,

Walter