154 The Control of Mosquitoes 



Panama is imported from California. It averages 

 20° Baume. Oil from the same source, testing 

 30° Baume, was much more satisfactory, but only 

 a limited amount could be obtained. The crude 

 oil was mixed with kerosene in varying propor- 

 tions to increase its spreading qualities, but the 

 resulting mixture proved more expensive and less 

 effective for practical field work than treating this 

 particular grade of oil with larvacide which con- 

 tained phenol compounds. It is possible that some 

 other grades of relatively heavy oils may be more 

 advantageously used after mixing them with 

 kerosene, when its cost is not excessive. 



METHODS OF APPLICATION 



The methods of applying oil for larvacidal 

 purposes may be classified as continuous or inter- 

 mittent. 



The most serviceable for continuous effect is the 

 "drip method" in which drops of oil fall upon the 

 surface of the water from a specially designed 

 container with sufficient frequency and in such a 

 manner as to form a continuous thin film of oil 

 over a certain area of water. This layer of oil is 

 usually known as the ' ' oil film. ' ' 



"Drips" are used advantageously where there 

 is a moderate surface current, where the water 



