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 
public nuisance. 
The majority of the injurious larvee 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 advantages, and some being prefer- 
able for specific use; but, on the whole, for general practice, 
three remedies stand out as preéminently 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 
