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

The Collaborative International Dictionary of English v.0.48:

cabbage worm \cab"bage worm`\, cabbageworm \cab"bage*worm`\n.
   (Zool.),
   the larva of several species of moths and butterflies, which
   attacks cabbages. The most common is the toxic green larva of
   a white butterfly, the cabbage butterfly, (Pieris
   rap[ae]). The cabbage cutworms, which eat off the stalks of
   young plants during the night, are the larv[ae] of several
   species of moths, of the genus Agrotis. See Cutworm.
   larva of a cabbage butterfly.
   [WordNet 1.5]