PART IV 

 THE CONTROL OF INJURIOUS INSECTS 



The effective control of injurious insects demands a knowledge 

 of the habits of the insects themselves so that they may be attacked 

 at the most vulnerable point. As insects differ greatly in their habits 

 and life-history, methods of control must also differ greatly, not only 

 with regard to individual insects but also with regard to the crops that 

 are attacked. For example, it is obvious that insects that live mostly in 

 the ground and injure roots must be controlled in a different way from 

 insects that feed on foliage. Moreover, insects that attack the cereal 

 crops of the farm cannot profitably be treated in the same way as insects 

 that attack the fruit and leaves of garden or orchard crops. 



It has been said that the aim of the economic entomologist should 

 be the control of injurious insects at a profit. It may not be possible 

 to achieve this aim in all cases, for insects must be controlled sometimes 

 for esthetic reasons, not entirely for the losses they cause. Moreover, 

 recent investigations go to show that many so-called minor insects 

 if left untreated simply because it does not pay to treat them, may turn 

 out to be harmful in some indirect way. 



The term "at a profit" used above is rather indefinite. It is 

 sometimes impossible to say whether the control of injurious insects 

 is profitable or not for there are many factors to be considered. A con- 

 trol, seemingly unprofitable from the money standpoint for a particular 

 year, may be profitable from the standpoint of the health of the trees 

 and of future production of fruit. Moreover, orchardists recognize the 

 cumulative effect of continuous spraying. 



Factors of Insect Control 



The factors operating in the control of noxious insects may be 

 grouped as follows: 



I. Climatic. — The range and number of insects are strongly 

 influenced ].y temperature, rainfall, winds, etc. (see Part I). 



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