The Collaborative International Dictionary of English v.0.48:

cabbage butterfly \cab"bage butterfly\ (Zool.),
   a white butterfly (Pieris rap[ae] of both Europe and
   America, and the allied Pieris oleracea, a native American
   species) which, in the larval state, devours the leaves of
   the cabbage and the turnip. See also Cabbage worm, below.
   [1913 Webster] cabbage worm

WordNet (r) 3.0 (2006):

cabbage butterfly
    n 1: white butterfly whose larvae (cabbageworms) feed on cabbage