The Collaborative International Dictionary of English v.0.48:

Bollworm \Boll"worm`\, n. (Zool.)
   The larva of a moth (Heliothis armigera) which devours the
   bolls or unripe pods of the cotton plant, often doing great
   damage to the crops.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bollworm
    n 1: any of various moth caterpillars that destroy cotton bolls