94 INSECT ENEMIES OF THE BOLL WEEVIL. 



Given 10,000 developing stages of the boll weevil in a 1-acre field. 



(A) Collect and burn squares containing 50 per cent of the stages 5, 000 



This destroys all parasites as well as weevils. 



(B) There remain in the field squares containing 50 per cent of the stages 



present 5> ^^^ 



Normal parasitism 5 per cent 250 



Ants and heat kill 40 per cent 2, 000 



Mortality 2, 250 



Weevils to breed 2, 750 



Parasites present in field, 250. 



There is 1 parasite to every 11 weevils. 



This method undoubtedly greatly reduces the total number of 

 weevils in the field, but, as can be readily seen, the proportion of 

 parasites to weevils is the same as if nothing had been done — namely, 

 1 to 11. It was shown that by placing the squares in 16-mesh wire 

 cages the proportion would be 1 to 6.3. Hence it appears that by the 

 caging method the planter leaves in his field a more active agency 

 than he had before to attack the many weevil stages he has undoubt- 

 edly missed, whereas by the burning method he has not in the least 

 improved his field conditions. 



8. THE ECONOMIC SIGNIFICANCE OF THE INVESTIGATION. 



In final summary the following points are emphasized : 



I. The control of the holl weevil hij insect enemies is sufficiently great 

 to give it a high rank in the struggle against the pest. A considerahle 

 portion of the insect control would not he accomplished bij any other 

 factor; hence it is hy no means to he neglected. 



The number of species of insects attacking the developing stages 

 is 49. 



The control in any given place consists of the combined work of 

 several different species. 



Places having the largest number of controlling insects have the 

 highest percentage of control. 



In many places insect control is considerably greater than climatic 

 control or than any other class of factors. 



The average insect control is 20 per cent of all immature stages or 

 two-fifths of the entire natural control. 



The cotton leaf-worm is a valuable enemy of the boll weevil wheij 

 it defoliates the cotton after September 1, a date beyond which new 

 squares can not be expected to mature. It kills many weevils by 

 starvation, kills many others while consuming the squares, and 

 finally forces a premature hibernation which is generally fatal. 



