LARVICIDES 291 



This oil can be readily prepared by mixing low grade kerosene, or a 

 similar cheap light fuel oil, with various proportions of either heavy 

 distillate fuel oil or crankcase waste oil. There are now on the market a 

 number of light fuel oils which come close to the aforementioned speci- 

 fications and are cheap enough to be used in mosquito extermination 

 work. 



The actual amount of oil required to produce a uniform killing film 

 depends on the kind of oil used and the nature of the breeding place. A 

 clear water surface will require less oil than a surface covered with veg- 

 etation, debris, and other materials which may offer resistance to the 

 spreading of the oil. Generally, about forty to fifty gallons of oil per 

 acre are required. On places thickly covered with vegetation, debris and 

 sewage, the spreading of the oil may be hastened by mixing in certain 

 chemicals. The addition of about 0.5 to 1.0 per cent cresylic acid or 

 similar tar acids or small amounts of oleic acid, pine oil, turpentine, 

 and similar compounds will materially aid in spreading the oil. 



Long experience showed that such an oil, while efficient against mos- 

 quito larvae, was injurious to some aquatic plants and under certain 

 favorable conditions, such as shallow water, damaged fish. Water fowl 

 damage was occasionally claimed. 



The problem was then attacked of how a low-priced larvicide could 

 be made which would destroy mosquito larvae and at the same time re- 

 duce the amount of oil so as not to damage aquatic plants, fish and wa- 

 ter fowl. Studies showed that the addition of the extract of 1 pound 

 of pyrethrum flowers (analyzing 0.9 per cent pyrethrins or more) to 

 a gallon of oil gives the necessary build-up in toxicity to mosquito lar- 

 vae. The oil should, therefore, contain a minimum of 0.1 per cent actual 

 pyrethrins. Of course, the pyrethrins when in colloidal solution in water 

 are toxic to fish, but when dissolved in oil stay in the oil and cannot be 

 drawn out of it by water. 



Many formulae were tried on both fresh and salt water, and it devel- 

 oped that the soap emulsification, while it worked beautifully in fresh 

 water was decomposed in salt water. To meet this situation, after pow- 

 dered milk was tried, reference was had to Gardinol, and the following 

 formula was adopted as the standard New Jersey Larvicide : 



NEW FORMULA FOR TANK PREPARATION (sTOCK SOLUTION) 



100 gallons of kerosene containing enough 

 pyrethrum extract to equal 100 pounds 

 of flowers (containing at least 0.9 per 

 cent pyrethrins) 



