160 MOSQUITO ERADICATION 



covered with a scum, more oil is required. The amount of oil 

 needed in such a case must be determined largely by experience, 

 since the factors in the case vary so much that no definite state- 

 ment on the matter can be made. 



KIND OF OIL REQUIRED 



The oil used must be thin enough to spread and form a film, 

 since, unless there is a uniform film over the water surface, many 

 of the larvae will not be suffocated. Probably the best oil for 

 the purpose is a light fuel oil; frequently such an oil can be pro- 

 cured thin enough not to require mixing with kerosene; where 

 only a heavy grade of oil is available, it must be mixed with 

 enough kerosene to cut it thoroughly, often from 50 to 75 per 

 cent. Proper mixing requires vigorous stirring until the mixture 

 has merged into a thin and quick-spreading fluid. The main 

 practical considerations are that it be thin enough not to clog up 

 the sprayer and that it spread enough to form a film. 



Kerosene alone is fairly satisfactory, but it has a tendency to 

 evaporate too quickly. It is also generally somewhat more expen- 

 sive than the fuel oil. Furthermore, in using kerosene, there 

 is a tendency to use more oil than is essential, as kerosene has 

 a secondary "spread," or, in other words, covers a larger area 

 after an hour or two than when at first applied. A film that is 

 almost too thin to be measured, but which will give an irridescent 

 reflection in sunlight, will kill Anopheles larvae. 



LARVICIDES 



Larvicides or substitutes for oil may be utilized to advantage 

 under some circumstances. On the other hand, however, it 

 should be remembered that they are often poisonous, less effec- 

 tive and more costly than oil. These considerations naturally 

 limit their profitable use, and LePrince 1 suggests that they be 

 used only under the directions of boards of health or health 

 officers. 



In cases where rains are so heavy and frequent that the oil is 

 likely to be washed away before it has time to kill the larvae, use 

 of a larvicide is indicated, provided conditions are such that there 

 is no danger of poisoning cattle or other stock. Larvicides 



1 "Control of Malaria: Oiling as an Anti-mosquito Measure," U. S. 

 Public Health Service 1919. 



