Insects Injurious to Vegetables 



CHAPTER I 



VALUE OF A KNOWLEDGE OF ENTOMOLOGY 



GENERAL CONSIDERATIONS 



For an intelligent understanding of the subject of insect 

 control by agricultural practice one must know not alone that 

 certain conditions produce an increase or decrease of certain 

 forms of insects, but how this is accomplished, why the alter- 

 nation of one crop with another is apt to result in insect 

 injury, and why a system of crop rotation that would be of 

 value in the control of one class of insects might be ineffective 

 against another; how fall plowing, though destructive to one 

 species, would not affect a different insect, and so on. In 

 short, a knowledge of economic entomology beyond the fact 

 that arsenicals are the proper remedies for mandibulate or 

 chewing insects, and that kerosene will kill aphides or plant- 

 lice, scale insects, and other soft-bodied insects, is a prereq- 

 uisite to intelligent effort in the control of noxious insects. 

 Before we can hope to avert losses we must know what our 

 insect enemies are, what species are destroying each crop, 

 which ones are responsible for primary injury, which are sec- 

 ondary or merely auxiliary, how injury is accomplished, when 

 injury begins each year, when it ends, as well as other facts. 



Similarly desirable is it to be able to recognize useful in- 

 sects, such as ladybirds, syrphus flies, tachina and ichneumon 

 flies and other parasites, that these may not be unnecessarily 

 destroyed, but, if possible^ encouraged in their useful work. 





