TREE INSECTS AND THEIR CONTROL 187 
The sap-sucking type, however, cannot be reached by 
poisons, so they must be killed by contact insecticides; 
therefore, they have to be destroyed by materials applied 
after they have begun operations, and every insect must 
be touched by the appropriate insecticide for it to be 
effectual. 
For the leaf-chewers and sap-suckers wholesale de¬ 
struction is possible, because their destructive work is in 
the open, on the outside of bark or foliage. 
For borers and others, which conduct their operations 
within the wood or underground, the measures of elimi¬ 
nation are aimed at the individual insect, by means of 
digging it out or by killing by the use of a wire probe 
or injection of carbon bisulphide. 
A point to be remembered in applying remedies by 
spraying is that the application must be thorough. 
Thoroughness means completely covering the leaves, in 
order to make certain that all the insects will be reached 
by the poison placed on their food, or killing the body of 
each insect by smothering with a contact insecticide. 
Effectual remedial measures require knowledge of the 
habits of the type of insect to be controlled. This knowl¬ 
edge should at least include the ability to distinguish 
between the leaf-chewers, the sap-suckers and the borers, 
in order to determine the steps that must be taken. In 
compiling the suggestions for treatment, which appear in 
subsequent pages of this chapter, an effort has been made 
to give data which will enable the layman to identify and 
classify the insects that may appear on his trees. 
Spraying materials may be bought at seed stores and 
other places. For small operations it is best, with most of 
them, to use the commercial preparations rather than to 
undertake to prepare them at home. In buying them, the 
highest quality should always be insisted upon. The 
