The object of this short communication is to state the result of an enquiry into the solubility of the incrustation of the mussel scale-insect which is destructive to the bark of various cultivated trees, particularly of the apple tree. The control of this pest by means of insecticides to a large extent depends upon getting access to the insect by dissolving the scale with which it covers itself soon after it settles down on the bark for the rest of its life. Whatever may be the development of the insect, unless the scale can be dissolved, at least at its point of contact with the bark, no great result can be expected from the application of insecticides.