134 Control of Parasites 



Among these methods, the last requires the least knowl- 

 edge, and, if properly applied, promises the surest success. 

 At the same time, it is practical only in the case of more or 

 less isolated trees and becomes expensive in that of large 

 specimens. Nevertheless, with the development of prac- 

 tical apparatus and of serviceable poisons, this method is 

 now so well tested, and demonstrated as practical in com- 

 bating all leaf-destroyers on street and lawn trees as well 

 as in orchards, that any community which has pride in its 

 appearance and is without the apparatus for applying these 

 insecticides, must be considered behind the times. 



The orchardist, especially, who is not provided with this 

 means of keeping his trees healthy is no better than the 

 farmer without a cultivator, and may even be considered a 

 pubKc nuisance. 



The majority of the injurious larvae which feed on leaves 

 are biting insects and eat their food, hence can be directly 

 killed by poisoning this; the sucking insects, which, like 

 plant-lice, plant-bugs, scale-insects and mites, suck the 

 juices, and whose digestive organs cannot be reached by 

 the poison, can, nevertheless, be combated by its use, if 

 they are directly hit by the poison, so that their soft skins 

 are penetrated, or if their breathing apparatus is clogged 

 by it, or if they are otherwise disabled by the spray. 



There are quite a large number of poisons or insecticides 

 in use, each having some ad\'antages, and some being prefer- 

 able for specific use; but, on the whole, for general practice, 

 three remedies stand out as preeminently effective and 

 acceptable, namely, the arsenical poisons, the kerosene 

 emulsion, and hydrocyanic acid gas, which latter, however, 

 is probably rarely practical outside the orchard. 



Since the first-mentioned poisons are injurious to plants 

 as well as animals, caution in their use is necessary; they 



