STREET AND HIGHWAY PLANTING. 103 



Leaf-eating 1 insects are those insects which in some stage of tin Mi- 

 development feed upon the leaves, and in some cases upon the tender 

 growing tips of the shoots. These insects take the plant tissues into 

 their bodies, and hence can be poisoned by those poisons that are known 

 as "stomach" poisons. This means poisons whose toxic effect appears 

 only after introduction within the body of the insect. The basis of these 

 poisons is usually some form of arsenic. Caterpillars and leaf-eating 

 beetles are examples of this class. 



Sucking insects are those insects which do not destroy the plant 

 tissues directly, but rob the plant of its juices, and affect its health 

 only when their number becomes so considerable as to be felt by the 

 host plant. As poisons can be applied only to the surface of plants, 

 and as these insects eat no portion of the plant tissues, it is obvious 

 that stomach poisons would be of no value. Resort is made to "con- 

 tact" poisons. These materials are generally soaps or oils, which clog 

 the breathing pores of the insect and hence smother it. 



Borers are the third class of insects which attack trees, and are in 

 many ways the most difficult to combat. Usually the larvae are the 

 forms that do the damage, though many beetles in the adult stage are 

 equally destructive. When the larvas hatch from the eggs they pass 

 almost immediately through the bark of the tree and bore their galleries 

 through the wood tissues, in some cases just below the bark and in other 

 cases well into the heart wood. As the eggs are not easily destroyed, 

 as they do not feed to any appreciable extent before entering the wood, 

 and as the adults are flying insects, the only time for attack is after 

 the larvae have entered the wood. 



If the borers are very numerous, the limbs or trees should be removed 

 and burned. If the insects are few r and careful attention can be given, 

 hand work will often be effective. A flexible wire will often reach the 

 larva if the wire is pushed flrmly into the opening. If, afterwards, a 

 few drops of carbon bisulfide is put in the opening and the hole closed 

 with putty, the work will be assured. If the number of insects is very 

 small, the bark may be opened along the gallery and the insect killed. 

 As this causes considerable injury to the tree it is less desirable than the 

 other methods. 



Because no method has been devised as yet which will do away with 

 the necessity for great caution and attentiveness on the part of the 

 operator, the control work for borers is both tedious and expensive. 

 It has been demonstrated, however, that a thorough concerted action 

 will reduce the number of larva?, and hence the number of adults; and 

 later spraying will reduce the number of eggs which may be laid, so that 

 after several seasons the borers may be checked and practically exter- 

 minated. 



