1G8 MOSQUITO ERADICATION 



are submerged, the oil rising to the surface. It is the opinion 

 of the writer, however, that usually equally as good work can be 

 done with the devices already described. 



FREQUENCY OF OILING 



The frequency with which oiling should take place depends 

 upon several different factors. Ordinarily, one oiling a week or 

 every 10 days is sufficient, and this was the basis upon which 

 most of the oiling was done in the extra-cantonment work of the 

 U. S. Public Health Service during the World War. 



The resistance of larvae and pupae to oil varies with the 

 different kinds of oil used and with the different genera of 

 mosquitoes. The Anopheles succumb more easily than the 

 Culex. Frequently, a good film of oil will kill virtually all the 

 larvae in a pool in an hour or so. On the other hand, the writer 

 has observed Culex still active in a cess-pool 2 days after it had 

 been well oiled.. The oil, in this case, may have been prevented 

 from forming a perfect film by the floating matter in the pool 

 or by the gas bubbling to the surface. 



Whatever the period necessary to kill the larvae and pupae 

 may be, once they are dead, several days must elapse before a 

 new breed can reach the pupal stage. The exact time will, of 

 course, depend upon the temperature and other factors. If 

 the weather is very hot and, through early disintegration of the 

 film, new eggs are deposited promptly, a second brood of larvae 

 may appear within 3 or 4 days and, within a week, may reach 

 the pupal stage. Again, if the weather be cold and cloudy the 

 development of the new brood will be correspondingly delayed. 



With the above considerations in mind, it has been suggested 

 that oiling should be done by inspection — that is, that there 

 should be no set times for oiling, which should be done only when 

 it has been determined by careful inspection that oiling is 

 necessary. However, in view of the obvious risks of this method 

 arising from possible changes in weather conditions, possible 

 incompetence or carelessness of inspectors, etc., the writer's 

 experience has been that best results are obtained by regular 

 weekly oiling of each place that requires it — that is, has not 

 dried up or has not been treated effectually by some other method. 

 If the project is a small one, all the oiling may be done on 1 day 

 of each week; if a large one, it may be necessary to have a gang 

 to oil all the time, visiting each place once a week. 



