DESTROYING AND REPELLING MOSQUITOES 91 



How often to apply the oil. It has been shown in 

 our study of the life history of Culex pipiens that the entire 

 life cycle may be passed in ten or twelve days. In the 

 case of Anopheles, the life cycle lasts from eighteen to 

 twenty days. These facts alone indicate the necessity 



Fio. 32. Spraying a ditch for mosquitoes with a knapsack sprayer. 



of frequent spraying. I have found that once in two 

 weeks is often enough for sewage ditches. No doubt 

 the oil is effective along the shallow edges of these ditches 

 for some time after it is applied. That is to say, the oil 

 along the edges does not run off immediately because of 

 no current. On quiet pools oil is effective for several days 

 after it is applied because it does not readily evaporate. 



