118 



THE MEXICAN COTTON-BOLL WEEVIL. 



The records for Calvert and Brenham show a very remarkable 



f)ower of endurance in some weevils, the average survival for the two 

 ots of 20 and 8 weevils being over 30 and 60 days, respectively. 



NATURAL CONTROL. 



Considerable attention has been given to the study of the natural 

 forces which control the boll weevil. These studies have revealed a 

 large amount of important data, some of which have been used in 

 several bulletins. In the present publication it is possible to give 

 only a summary of the most important results. 



In general, the natural agencies which control the boll weevil may 

 be classified as climatic (consisting principally of heat which kills 

 directly and also indirectly by rendering the food supply unsuitable, 

 and dryness, the effects of which are intermingled with those of heat), 

 plant resistance, parasites and other insect enemies, diseases, and 

 birds. Each of these agencies will be discussed separately, but a 

 general summarization may be of value. Table LV is a summary of 

 the observations made in the years 1906 to 1909 on weevil stages from 

 many localities. It deals with the mortality of immature stages from 

 all causes exclusive of plant proliferation. 



Table LV. 



-Annual mortality of immature boll weevils in all classes of cotton forms. 

 1 906-1 909. 



Inasmuch as the material used in making the examinations was 

 derived from many sources and in different proportions each year, a 

 system of weighting the different kinds of material was devised. 

 Table LVI presents a summarization of this weighting in terms of 

 percentages of mortality: 



Table LVI. 



•Weighted average mortality of the boll weevil, 1906-1909, due to various 

 causes. 



1 The average determined in 1906 (see Bull. 59, Bureau of Entomology) is used to apply to other years. 



