Relation to Deleterious Chemical Agents 453 



tack the grape-vine and there are at least half as many 

 fungous diseases of the same plant. Numerous instances 

 might be cited in which the number of fungous and insect 

 pests of a particular crop is as great as those indicated. 1 

 On the other hand, there are cultivated crops which re- 

 quire very little consideration with respect to parasites of 

 any kind. 



It is only in relatively recent times that spraying opera- 

 tions have developed, especially spraying to prevent fun- 

 gous diseases. This type of control has been in large part 

 due to a careful investigation of the relations between 

 plants and toxic solutions on the one hand, and to the de- 

 velopment of effective spraying devices on the other. In 

 nearly all cases protection against fungous diseases and 

 insect pests is effected by covering the surfaces of fruit, 

 leaves, and stems with a poisonous substance, which 

 should, while relatively noninjurious to the host, prevent 

 the effective germination and penetration of the spores, 

 or kill the insects concerned. 



In controlling insects, it is to be remembered that there 

 are two great classes with respect to the method of attack ; 

 these are : 



(1) Chewing insects which bite off and eat the vegeta- 

 tive parts of the plant ; for example, cabbage worms, tent 

 caterpillars, and potato beetles, which would be killed by 

 poisons sprayed upon the plant. 



(2) Sucking insects, or those which get their food by in- 



1 Some estimates of the amount of damage annually sustained by the 

 crops of the United States have been made, and taking as a basis the 

 prices at which the crops actually sell, it seems to be demonstrated that 

 the vast sum of one billion dollars may be aggregated. 



