1. Those that chew and swallow some portion of the leaf; as, for example, the elm leaf beetle, and the tussock, gipsy, and brown-tail moths.
2. Those that suck the plant juices from the leaf or bark; such as the San José scale, oyster-shell, and scurfy scales, the cottony maple scale, the maple phenacoccus on the sugar maples, and the various aphides on beech, Norway maple, etc.
3. Those that bore inside of the wood or inner bark. The principal members of this class are the leopard moth, the hickory-bark borer, the sugar-maple borer, the elm borer, and the bronze-birch borer.
The chewing insects are destroyed by spraying the leaves with arsenate of lead or Paris green. The insects feed upon the poisoned foliage and thus are themselves poisoned.
The sucking insects are killed by a contact poison: that is, by spraying or washing the affected parts of the tree with a solution which acts externally on the bodies of the insects, smothering or stifling them. The standard solutions for this purpose are kerosene emulsion, soap and water, tobacco extract, or lime-sulfur wash.
Fig. 97.—A Gas-power Spraying Apparatus.
The boring insects are eliminated by cutting out the insect with a knife, by injecting carbon bisulphide into the burrow and clogging the orifice immediately after injection with putty or soap, or in some cases where the tree is hopelessly infested, by cutting down and burning the entire tree.