14 N. H. AGRI. EXPERIMENT STATION [Bulletin 243 



during the summer, the breeding pools must be treated every two to four 

 weeks depending on the temperature. For the salt-marsh mosquitoes, 

 the marshes must be inspected after each heavj' rain or extra high tide 

 and the pools treated which show breeding. 



Many kinds and grades of oil may be used, but to be most effective 

 an oil should spread rapidly, form a complete film over the surface of 

 the water, and not evaporate too quickly. Low-grade kerosene is often 

 used; this spreads and kills well, but evaporates so rapidly that its pro- 

 tective effect is soon lost! A very heavy oil evaporates slowly, but is 

 likely to cling together in spots and not form a complete film. Used 

 crankcase oil may be applied to small areas. This does not spread well, 

 however, and is more efficient if diluted with 25 per cent kerosene. The 

 best type of oil has been found to be the petroleum distillate fuel oils. 

 These spread and kill well and the film lasts for eight or ten days. The 

 New Jersey workers have recommended the following tentative speci- 

 fications for the most efficient oil of this last type: 



Specific gra\-ity — 32°-37° Be 



Flash point — 150° F. 



Cold test — 0° F. —pour 



Boiling range — 350°-675° F. 



Color — Straw to yellow 



Viscosity — 50-100 Sayb/100 



Surface tension — 20 dynes per cm. 



There are no oils on the market which meet all these specifications, 

 but there are several whicli approximate them in most respects and which 

 ha\"e given veiy good results. 



The New Jersey workers have also found that the addition of small 

 amounts (0.5%) of coal-tar acids greatly improved the spreading prop- 

 erties of the fuel oil on water covered with vegetation and organic mat- 

 ter. Coal tar or crude cresylic acid containing 95% cresylic acid (Cre- 

 sol) is mixed with the oil at the rate of one part to two hundred parts 

 of oil. The addition of this unsaturated hydrocarbon reduces the inter- 

 facial tension between the water and the oil, thus increasing the spread- 

 ing. 



Most oils used to kill mo.squitoes will injure and discolor vegetation. 

 In ornamental pools, where this is not desirable, gasoline may be used. 

 This evaporates very rapidh' and the treatment must be repeated when- 

 ever breeding is noticed. Stocking such pools with fish is the best meth- 

 od of control. 



The oil may be poured on the surface of the water or thrown on with 

 a tin cup, but much better spreading and a saving of materials is ob- 

 tained by the use of a sprayer giving a fine mist spray. One of the 

 small, portable, compressed-air sprayers is veiy handy for this work. 



