PART III. 



INSECTICIDES, PREVENTIVES, AND 

 MACHINERY. 



CHAPTER I. 



PREDACEOUS INSECTS, PARASITES, AND FUNGOUS DISEASES. 



It has appeared throughout this book that, while we have a 

 great many insects that are feeders upon vegetation, there are 

 also a great number that are predaceous upon the herbivorous 

 forms, and yet many others that are parasites, living upon or 

 within others of the same class. It is a tempting proposition 

 that we may, perhaps, be able to array the forces of nature 

 against each other ; that we may keep predaceous forms, and set 

 them free at the proper season to feed upon those injurious to 

 farm crops. I say it is a tempting proposition, and it looks 

 feasible, but that it should be successful involves a great deal 

 more than is apparent at first glance. In the course of the ages 

 through which this world has existed there has been gradually 

 established, by the influence of surroundings, a certain ratio be- 

 tween all existing life, vegetable and animal. There is a struggle 

 among the plants themselves as to which shall keep the ground, 

 and upon the plants feed animals, including insects, which form 

 a factor in determining the relative abundance of the diflerent 

 species. With the development of the plants their enemies also 

 have developed, and always in proportion to the amount of injury 

 that the plant can stand. That this must be so is evidenced by 

 the fact that the plants exist at all. One of nature's ways of pre- 

 venting the plant-feeding insects — for we will confine our re- 

 marks to insects from this point on — from obtaining control of 

 and exterminating the plants is to provide enemies of various 

 418 



