110 THE PESTS OF CULTIVATED PLANTS 



it is impossible for the poison to reach the stomach of the 

 insect. Hence, in order to kill sucking insects, some material 

 that will either smother the insect by clogging up the spiracles 

 or the openings of the breathing system, or kill it by the 

 corrosive action on the body of the insect, must be used. 

 Insecticides of this class are called contact insecticides. 

 Kerosene emulsion, tobacco sprays, and lime sulphur are 

 the most common sprays for the sucking insects. Common 

 examples of sucking insects are the scale insects, the plant 

 lice, and many others. 



Besides the biting and the sucking insects, a third class 

 of insects exists that must be controlled in still another way. 

 This group includes those insects that live in the tissue of the 

 plant, and hence cannot be reached by either the arsenical 

 or the contact insecticides. This class of insects includes 

 all forms of borers, which burrow into the plant and spend 

 their life within the plant tissue. 



There are two methods for the control of the boring insects, 

 namely, by the use of repellants, which are chemicals used to 

 prevent the attack of the insect, and by the digging out by 

 hand the insects after they have entered the plant. The 

 boring insects can be destroyed by first locating the burrows 

 which are usually detected by the exudation of gummy 

 material, covered by the chewings and the castings of the 

 insect. By the use of a sharp knife the bark can be cut away 

 and the larva killed. Another method is to probe in the 

 burrows with a piece of wire, thus killing the larva. Common 

 examples of the boring insects are the peach tree borer 

 and the locust borer. 



Repellants. — Repellants are chemicals with unpleasant 

 odors. These materials are placed either on the ground 

 around the plants or distributed o\'er the plants themselves. 

 By the giving off of unpleasant odors they drive the insects 

 away and in this manner the plants are saved from destruc- 

 tion. Turpentine and carbolic acid are often used as repel- 

 lants. These materials are usually mixed with ashes or any 

 dry powder and either spread on the ground around the plant 

 or dusted over the leaves. Repellants are effecti^■e in con- 

 trolling the striped cucumber beetle and many other insects. 



