The Collaborative International Dictionary of English v.0.48:

mealybug \meal"y*bug`\, mealy bug \mealy bug\
   (m[=e]l"[y^]*b[u^]g`), n. (Zool.),
   Any of several homopteran scale insects (as, Coccus
   adonidum, and related species of the families
   Pseudococcidae and Eriococcidae), that cover themselves
   with a white powderlike or cottony wax secretion. They are
   common plant-eating pests in hothouses and are also
   destructive of fruit trees.
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

mealy bug
    n 1: scalelike plant-eating insect coated with a powdery waxy
         secretion; destructive especially of fruit trees [syn:
         mealybug, mealy bug]