Attack by Oiling 171 



ally on the common grasses, their growth is in- 

 hibited and in time the grasses die. This property 

 of the oil is used to advantage on the sides of 

 ditches and in shallow depressions where water 

 stands during rainy periods, when draining and 

 filling is not economically feasible. 



Oil in any form is not an entirely satisfactory 

 larvacide for the following reasons : 



Its effectiveness depends on a perfect film. 

 Films are unreliable. Perfect films can be ob- 

 tained only under exceptional circumstances, and 

 even a perfect film frequently develops defects. 



The larvacidal action of oil is slow. In some 

 instances several hours must elapse after the oil is 

 applied before the larvas of Culex axe killed. 

 Dtu-ing this time many opportunities may arise 

 for the development of defects in the film. The 

 light oils are expensive, and the heavier oils do not 

 always spread well, unless treated in some manner 

 to decrease their specific gravity and viscidity. 



A relatively large amount of oil is needed to 

 cover a given water surface, thus increasing the 

 cost of handling, especially where the oil has to be 

 transported a long distance by hand. 



There is always more or less danger from fire, 

 while the oil is in storage. 



The application of oil in the quantity necessary 



