20 MISC. PUBLICATION 336, U. S. DEPT. OF AGRICULTURE 



widening of the flood plain and silting-up of downstream areas, which 

 may create mosquito-producing areas as serious as those remedied. 



Such factors as these must be considered in planning mosquito- 

 control programs, and they emphasize the need of obtaining advice 

 from competent specialists when making the preliminary surveys. 

 The impounclage of water rather than drainage may be employed 

 successfully in many cases, both in salt-water and fresh-water areas, 

 since an open pond with clean margins and containing mosquito- 

 destroying fish is not favorable for mosquito breeding (116) . (PI. 3.) 

 Where the sacrifice of wildlife habitat appears necessary to accom- 

 plish effective mosquito control, a decision must be made as to the 

 greater benefit to be derived. 



Sanitation, as applied to mosquito control, includes such measures 

 as the elimination of artificial and other breeding places of the domes- 

 tic mosquitoes. It also involves the treatment of permanent bodies 

 of water by the removal of aquatic vegetation and other protective 

 harborage for the larvae, to make them unfavorable for mosquito 

 development, 



LARVICIDES 



Various kinds of larvicides are employed where permanent methods 

 of control are not feasible. Although there are many chemicals that 

 will poison the larvae rather easily, the number of materials that 

 are utilized in practical work is comparatively small. 



Petroleum oils have been used extensively and are effective against 

 nearly all economic species. They act as contact poisons and kill 

 the larvae or pupae by entering the breathing tubes. The lighter 

 and more volatile oils, such as gasoline and kerosene, are the more 

 toxic, whereas heavy oils are more lasting. Various mixtures of 

 heavy and light oils have therefore been employed. Light distilled 

 fuel oil (No. 2) is recommended for general use, since it is of fairly 

 uniform quality, easily handled in large or small spraying equipment, 

 and is economical in cost. Since fuel oil varies somewhat in toxicity 

 according to the type of crude petroleum from which it is derived, 

 preliminary tests of its effectiveness against mosquito larvae should 

 be made before it is purchased in large quantities. The addition 

 of about 1 percent of castor oil or crude cresol has been recommended 

 as a means of increasing the spreading power of the oil. 



Paris green is highly toxic as a stomach poison to mosquito 

 larvae (4) and is now used extensively in the control of anopheline 

 mosquitoes. It is effective in very small quantities and, since these 

 larvae feed at the surface of the water, the material can be applied 

 economically as a dust in admixture with an inert diluent. Recent 

 experiments by King and McNeel (80) have shown that this arsenical 

 also is effective against the salt-marsh species and some of the other 

 subsurface-feeding larvae, including Psorophora columbiae and Culex 

 quinquefasciatus, when mixed with water and sprayed on the breed- 

 ing places from a sprinkling can. 



Tests with calcium arsenite suggest that a much cheaper arsenical 

 than paris green may be available as a larvicide. This arsenical 

 should not be confused with calcium arsenate, a common insecticide 

 for plant-feeding insects, most samples of which have had a low 

 toxicity for mosquito larvae. 



