Attack by Oiling 163 



not always possible, in the field, to produce and 

 maintain an oil film on a large body of water at 

 reasonable cost that will insure the death of all the 

 larvae under the film. We have found Culex 

 larvae tmder an apparently good oil film that had 

 been in place several days in succession. The 

 explanation may be found in the fact that the oil 

 film, in the field, is frequently not continuous, and 

 that defective places may be found in it by which 

 the larvae and pupas obtain air. However, a very 

 great reduction in the larvae may be achieved by 

 proper oiling, and Anopheles larvae succumb much 

 more rapidly than the Culex. 



The question of how frequently the oil must be 

 applied, can be answered only after a knowledge 

 of local conditions has been acquired. The rule is 

 to oil at least once within the minimum of time 

 required to mature an egg into an adult. In the 

 Canal Zone for the mosquitoes most frequently 

 encountered, this period is about eight days, and 

 oil was therefore applied weekly. 



In the intermittent method enough oil must be 

 applied to produce a continuous film, heavy enough 

 to withstand the tearing action of small ripples, 

 and the light flotsam and vegetation projecting 

 through it. 



It is difficult, if not impossible, to say just how 



