52 INSECTS INJURIOUS IN 1902. 
How often should one apply the oil? It would seem that this 
is a matter easily determined by observation, for as long as a film 
of oil can be seen upon the surface, no further treatment will be 
necessary, and the volatility of the oil must be dependent in a 
great measure upon meteorological conditions. It has been sug- 
gested that one application every four weeks during the summer 
is sufficient. I believe that a more frequent treatment, perhaps 

Fig. 46.—Pupa of Mosquito killed by fungus, enlarged. 
once in three weeks is safer in this vicinity. As to the date of 
first application, that too is easily determined by observation. As 
soon as the wigglers are first seen or even earlier when the adult 
mosquitoes are noted about the water preparing to lay their eggs, 
then the oil should be applied. It is claimed that this method can 
be used with safety in the case of large tanks, the water in which 
is intended for drinking purposes, providing the water is drawn 
from the bottom of the tank, or a better way perhaps is to put a 
screen over such tanks. It is very evident that there must be co- 
