OILING METHODS 57 



When Applied and How Often. — -Oil should be applied 

 whenever and wherever the wrigglers or tumblers are 

 found, even though permanent correction is planned. 

 This will prevent them from being washed out into 

 some other situation where they would be liable to 

 complete their transformation. The frequency with 

 which oil must be applied depends on the rate of de- 

 velopment of the wrigglers and the evaporation of the 

 oil, — both conditions being dependent on the tem- 

 perature. Therefore, more frequent applications are 

 necessary during midsummer, when with the oil men- 

 tioned above, spraying should be repeated at least 

 every twelve days, and under cooler conditions (averag- 

 ing 50 to 6o° F.) every three w r eeks. If it requires 

 only ten days for some mosquitoes to pass through 

 their entire transformation, one might think that ap- 

 plications of oil every twelve days would not be often 

 enough, but it must be remembered that the oil kills 

 all wrigglers and tumblers at the time of contact and 

 the film remains on the water for two or three days, 

 sometimes longer, during which time any adult mos- 

 quito, intending to lay eggs, is killed on coming in 

 contact with the oil. After the oil has evaporated 

 quite largely, the breeding may begin again, but the 

 next application of oil will catch the oncoming brood 

 before the ten days necessary for complete develop- 

 ment have expired. 



Certain conditions are unfavorable to the effective 

 use of oil, such as abundant vegetation, e.g. tule and 



