THE VALUE OF EXPERT KNOWLEDGE 251 



Spraying Machines 



Purchasers will find detailed and recent information in the 

 publications of the United States Department of Agriculture. 

 See, for example, the article by Dr L. O. Howard in the Year- 

 book for 1896, which gives descriptions and figures of the chief 

 machines now in use. 



THE VALUE OF EXPERT KNOWLEDGE 



Remedies for injurious insects should not be applied blindly, 

 without considering how the enemy can be best attacked. 

 Does the egg, the larva, the pupa or the fly furnish the best 

 opportunity? The way in which the insect feeds must also 

 be taken into account. A leaf-eating larva may be destroyed 

 by poisoning its food, but this is not the way to treat sucking 

 insects, which feed upon nutritive fluids drawn from the inner 

 tissues. "Contact poisons," such as petroleum emulsions or 

 caustic washes, will be more promising in such cases as these. 

 All farm and garden insects breathe by spiracles, and where 

 other means fail, they can generally be forced to inhale 

 poisonous vapours. Liquids, even if non-poisonous, may 

 destroy by obstructing the spiracles, and soot, as well as 

 other fine powders, is sometimes effective in the same way. 

 The structure and i^ode of life of the plant attacked must 

 also be studied. It is in most cases useless to spray leaves 

 which throw off" all liquids, and poisons, incautiously ad- 

 ministered, may do more harm to the crop than the insect 

 pest. 



The value of special knowledge and experience in deahng 

 with destructive insects is so great that in the United States 

 insect-killing is often practised as a profession, and experts 

 undertake the treatment of trees and crops at a fixed charge. 

 It is much to be desired that the practice may spread to 

 other countries. The cost of knowledge and skill is often 

 insignificant in comparison with the advantages secured. 

 The cotton crop -of the United States was formerly, in bad 

 years, injured to the value of thirty million dollars by the 

 leaf-worm, but in consequence of treatment with arsenicals 

 such losses are now unknown. The fruit-growers of Call- 



