INSECTS AND DISEASES 47 



tilated into unnatural activity by food supply, to 

 spread from garden to garden and from field to field. 

 Likewise, when we remember the large amount of 

 commerce in these products between widely sep- 

 arated regions of the country, it is easy to under- 

 stand how one pest may find its way with little dif- 

 ficulty from one section to another many hundred 

 miles away. Our present-day insect and disease 



FOUR-ROW POTATO SPRAYER 



problem is one, therefore, that comes as a natural 

 result of the disturbed equilibrium in nature caused 

 by cultivation and the giving over of large areas to 

 the growth of some particular crop. A process of 

 readjustment, however, is constantly going on. A 

 new pest enjoys its widest liberty and causes its 

 greatest destruction while it is comparatively new 

 in any locality. After a while natural enemies 

 develop which thrive and feed upon it, or if too 

 destructive the cultivator ceases to grow the host 

 plant and as a result a material check inevitably 



