i62 The Control of Mosquitoes 



may occasionally be used with advantage. Dis- 

 carded oil-soaked cotton waste is tied into small 

 bundles, immersed in crude oil, and then placed in 

 small seepage streams and outcrops. If there is 

 danger of its being washed away, it can be tied to a 

 stone. These oil-soaked bundles of waste give a 

 thin film of oil to the water passing by or under- 

 neath them during seven to ten days. They may 

 be resoaked in oil and used many times. This 

 method of oiling is used where the volume of water 

 is insufficient to warrant the use of a drip can. 

 Other uses may suggest themselves under varying 

 local conditions. 



As the conditions under which continuous oiling 

 can be successfully carried out are rather limited, 

 it follows that the larger part of oil application to 

 mosquito breeding areas must be done by the 

 periodical application of oil or "intermittent 

 method." 



In the intermittent method the aim is to produce 

 a continuous film, and to retain it in place suffi- 

 ciently long to kill all larvae in the water covered 

 by the oil film. Given a perfect film, it must 

 remain unbroken at least several hours to insure 

 a marked reduction in the larvae under it. In- 

 tentionally we do not say "destruction of all the 

 larvae under the film," because in the tropics it is 



