26 INSECTS NOXIOUS TO AGRICULTURE. 
have tried some rule-of-thumb plan which seems to kill most of 
the adult insects, not dreaming that they have left the eggs un- 
harmed and ready to send forth a fresh swarm at hatching-time. 
There is another obstacle which often prevents success in eradi- 
cating “scale.” This is the difficulty of making sure of the 
effects of any remedy. A plan which has answered well in one 
place will fail in another, and this, not only as regards different 
countries, but even in the same district for neighbouring gar- 
dens, or even for neighbouring trees in the same garden. Tree- 
growers must be prepared to find the very same remedy which 
has cleared their neighbours’ trees fail for their own; and in this 
country the author has seen, in one and the same orchard, some 
trees quite cleared, while on others, treated in exactly a similar 
manner, the “scale ”” was scarcely injured. 
It is from want of knowledge of this and the like points that 
persons who have tried various remedies recommended to them 
have complained of failure, and condemned both the remedy and 
their adviser, whilst really neither was in fault. 
Nothing need be said here of carelessness or unskilfulness 
in applying a remedy, beyond the following instance: A person 
whose apple-trees were being very much damaged by Mytilaspis 
pomorum was advised to apply, by way of paimting the trunks 
and branches, a mixture of kerosene and some other ingredient. 
In two or three months he found violent fault with his adviser, 
for he said every tree was dead or dying. On inquiry it was 
found that, from over-zeal or want of knowledge, he had applied 
the mixture as if painting a house, had used it much too strong, 
and, to make assurance doubly sure, had given his trees two 
good coats of the oil. 
An intelligent appreciation of the life-history and habits of 
scale-insects is necessary to enable any one to select and apply, 
with a probability of success, a remedy, aud the details given in 
Chapter I. of this work will be found useful for this purpose. 
It will be apparent from them that, without regarding the 
generic or specific characters of these insects, we may lay down 
a few general principles to start with, thus :— 
I. Whatever damage is done is effected by the sucking of the 
juices of the plant through the rostrum of the insect. It follows 
from this that applications of any fluid to the tree externally, 
with the object of poisoning the insects in their feeding, would 
be useless, as their food is drawn from beneath the surface, 
