Bark louse n. (Zool.) An insect of the family Coccidae, which infests the bark of trees and vines. 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.
... an old practice to wash trees with strong lye when they are affected with the oyster-shell bark louse. The modern method of treating these pests, however, is to spray with some kerosene or oil compound when the young growth is starting, for at that time the young insects are migrating to the new wood and ... — Manual of Gardening (Second Edition) • L. H. Bailey
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