The Collaborative International Dictionary of English v.0.48:

Bark louse \Bark" louse`\ (Zool.)
   An insect of the family Coccid[ae], which infests the bark
   of trees and vines.
   [1913 Webster]

   Note: The wingless females assume the shape of scales. The
         bark louse of the vine is Pulvinaria innumerabilis;
         that of the pear is Lecanium pyri. See Orange
         scale.
         [1913 Webster]

WordNet (r) 3.0 (2006):

bark louse
    n 1: any of several insects living on the bark of plants [syn:
         bark-louse, bark louse]