PLACE OF OILING IN ANTI-MOSQUITO WORK 



159 



larva-concealing vegetation (to facilitate control by means of 

 fish) is being carried out, Also swamps may be oiled pending 

 construction of drainage ditches. 



APPLYING THE OIL 



Ordinarily, oil need be applied only around the edges of large 

 bodies of water and of swift-flowing streams, since larvae rarely 

 are found in deep water and are readily wafted away by a swift 

 current, 



Fig. 



108. — Oiling a grassy stream until vegetation can be removed for fish 



control. 



Should deep water be covered with algae, debris or aquatic 

 plants, such as water lily, closely matted grass, etc., breeding may 

 take place — since the larvae will be more or less concealed from 

 the fish — and oil should be applied heavily over the whole surface, 

 unless other measures are put into effect, 



In slowly moving streams, quiet pools, back-waters, etc., are 

 often found which may prove to be breeding places. These 

 should be well oiled, unless other steps are taken to prevent 

 breeding. 



Also in cases, where the flow of streams is obstructed by sticks, 

 weeds, debris and other matter, the oil cannot be depended upon 

 to penetrate the mass, unless the whole surface is covered liberally. 



Ordinarily, it is not necessary that the film of oil be thick; 

 indeed, much oil is wasted in this way. Where the water surface 

 is clean, a very thin film, such as will show a faint display of 

 colors in the sunshine is sufficient. Where the water is foul or 





