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]

WordNet (r) 3.0 (2006):

cabbageworm
    n 1: toxic green larva of a cabbage butterfly [syn:
         cabbageworm, Pieris rapae]