16 
For scale insects and some others, as the pear Psylla, which hiber- 
nate on the plants, two or more strengths are advised with most of the 
liquid insecticides recommended, the weaker for summer applications 
and the more concentrated as winter washes. The summer washes for 
scale insects are most effective against the young, and treatment 
should begin with the first appearance of the larve of the spring or 
any of the later- broods, and should be followed at intervals of seven 
days with two or three additional applications. The first brood, for 
the majority of species in temperate regions, will appear during the 
first three weeks in May. Examination from time to time with a hand 
lens will enable one to determine when the young of any brood appear. 
The winter washes may be used whenever summer treatment can not 
be successfully carried out, and are particularly advantageous in the 
case of deciduous plants with dense foliage which renders a thorough 
wetting difficult in summer, or with scale insects which are so irregular 
in the time of disclosing their young that many summer treatments 
would be necessary to secure anywhere near complete extermination. 
In the winter also, with deciduous trees, very much less liquid is 
required, and the spraying may be much more expeditiously and thor- 
oughly done. In the case of badly infested trees, a vigorous pruning 
is advisable as a preliminary to treatment. 
As winter washes for temperate regions the kerosene mixtures and 
whale-oil soap solutions, particularly the latter, have so far given the 
best results. These stronger mixtures may be applied at any time 
during the dormant period of vegetation, and, with deciduous trees, 
preferably immediately after the falling of the foliage. In the growing 
season any of these stronger washes would cause the loss of foliage and 
fruit, and the more concentrated probably the death of the plant. - 
THE GAS TREATMENT. 
Hydrocyanic acid gas. 
The use of hydrocyanic acid gas originated in California, and was 
perfected by a long period of experimentation by an agent of this 
division, Mr. D. W. Coquillett. It has not been followed to any extent 
elsewhere, however; but in southern California it is held to be the 
best treatment for citrus trees and is now better understood and more 
satisfactory than ever before. It is especially applicable to citrus 
trees, the abundance of foliage and nature of the growth of which 
enables comparatively heavy tents to be thrown over them rapidly 
without danger of breaking the limbs. With deciduous trees it has 
not been practicable to use this gas to any extent, except in the case of 
nursery stock, which may be brought ‘ogether compactly and treated 
in mass under tents. This gas is alsuv she principal agency employed 
in disinfecting material coming into California from abroad. Recently 
it has been introduced into the East, more particularly in connection 
with nursery stock, to combat the San Jose scale. 

