SOME ASPECTS OF SPRAYING AGAINST PESTS. 217 
until the flowers begin to unfold. Strictly speaking, it should be 
included in the " active season," but there are certain peculiarities 
which justify its being put in a separate class. It is obvious that one 
kind of tree, say a pear, may pass out of the dormant state into the 
semi-dormant at an earlier period than another kind of tree, such as 
the apple, so that it is possible for a pear to be in the semi-dormant 
state while an apple is still in the dormant. These terms, therefore, 
do not apply to particular times of the annual cycle, but to the state 
of the trees under consideration. This thus brings the condition of 
the trees into proper relationship with the question of spray-fluids, 
so that one would not say, for example, that 2 per cent, caustic soda 
is a suitable winter spray so much as that it is a suitable spray for 
trees in the dormant condition. If one calls it a winter spray it 
should mean that one might apply it with safety in winter. One 
would certainly call January a winter month, and yet certain pears 
may begin to open their flower buds as early as that month in a year 
such as 1916, but spraying with 2 per cent, caustic soda would be a 
dangerous operation. Following this preliminary classification, it is 
now proposed to consider some of the more commonly used fluids. 
Spy ay -fluids applied during the Active Season. 
During this season insects are generally, if not always, in a more 
vulnerable condition than in the dormant, and therefore spray fluids 
used are comparatively weak. 
They may be divided into two classes : 
(1) Stomach poisons. 
(2) Contact insecticides. 
Stomach Poisons.' — The principle of the action of stomach poisons 
is simple. The material is sprayed on the leaves of the tree, and any 
insect feeding on these leaves absorbs a certain amount of poison and 
dies. It is necessary, of course, for the poison to be insoluble in water, 
since soluble poisons are nearly always harmful to the tree. At the 
same time they must be of such a nature that they will be acted upon 
by the insect's digestive juices and brought into a soluble form. The 
earliest of these stomach poisons to be used on a commercial scale 
was Paris green. Its use started some time between i860 and 1870, 
when the Colorado beetle was ravaging the potato lands in America. 
Shortly afterwards the substance known as London purple was intro- 
duced, but never gave such reliable results owing to the frequent 
variation in its composition. For many years, despite its tendency 
to burn, Paris green held the field, and it was not till somewhere 
about 1906 or 1907 that lead arsenate was introduced. This substance 
at once proved its worth, owing to its freedom from burning, its effec- 
tiveness, and to the fact that it could be prepared in the form of a 
paste. So far it is still the most commonly used stomach poison, 
though both in England and in America other bodies have been 
suggested and tried. I do not, however, propose to discuss these here. 
