ECONOMIC PRINCIPLES INVOLVED. 83 



10. Other cotton insects, by their ravages upon the food of the 

 weevil, sometimes reduce the numbers of the boll weevil itseK. 



11. Much valuable work is done by the ants, wliich are present in 

 many fields. 



PART III. THE ECONOMIC APPLICATION. 



The economic application of parasitic control to the boll-weevil 

 problem is dependent upon accurate knowledge of a multitude of 

 conditions. The preceding two parts of this bulletin have been 

 devoted to a statement of the many phases of the parasite situation. 

 It must be understood at the beginning of this part that we consider 

 the utilization of parasites and other insects inimical to the boll 

 weevil as intimately connected with good agriculture. The boll- 

 weevil problem, from a parasite standpoint, is entirely different 

 from any other parasite problem ever studied. In other cases such 

 means as introductions from foreign countries may be utihzed. In 

 the present case the main problem is to devise such agricultural 

 practices as will increase the effectiveness of the parasites already 

 present. 



In order to facihtate the treatment of the economic methods to be 

 suggested, tliis part is also divided into sections, which are as follows : 



1. The economic principles involved. 



2. Interpretation of parasite statistics. 



3. Interpretation of the biological complex. 



4. How to profit by existing conditions. 



5. How to plan for the greatest possible control. 



6. Propagation and artificial intro(J^uctions. 



7. Objectionable practices. 



8. The economic significance of the investigation. 



1. THE ECONOMIC PRINCIPLES INVOLVED. 



The attempt at utilization of insect enemies in economic ento- 

 mology is now receiving so much attention that the authors will 

 set down the principles which appear to have been the foundation of 

 the work they have done. 



1. Insects in a state of nature are more or less completely held in 

 check by natural agencies, in wliich other insects frequently figure 

 as of direct or indirect importance. Many insects are controlled 

 almost entirely by their insect enemies. No insect is without its 

 natural checks. 



2. The relationships between an insect and its enemies can not 

 be expressed by a simple ratio, nor are they in any way invariable. 

 The agencies operating for and against the welfare of a given species 

 are so man}^ and of such inconstant magnitude, due to the activities 



