26 INSECTS NOXIOUS TO AGBICULTUKE. 



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 painting 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, and 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. 



