24 CIRCULAR 9 7 7, U. S. DEPARTMENT OF AGRICULTURE 



CONTROL OF MOSQUITOES 



As a result of research at the Orlando laboratory during 1943, DDT 

 was recommended to the Armed Forces for the control of mosquitoes. 

 This new insecticide proved outstandingly superior to other materials 

 as a larvicide, in space sprays for large-scale use against the adults, 

 and as a residual treatment for buildings and vegetation to destroy 

 mosquitoes as they alighted or rested on the sprayed surfaces. Later 

 some of the other chlorinated hydrocarbons, such as benzene hexachlo- 

 ride, dieldrin, and toxaphene, were found to be highly toxic to 

 mosquitoes, but they came into only limited use until some mosquito 

 populations became resistant to DDT. 



Except against these resistant mosquitoes, DDT has continued to 

 be the most widely used insecticide in the control of both anopheline 

 and culicine mosquitoes. Since it is a contact as well as a stomach 

 poison and is soluble in a number of organic solvents, including pe- 

 troleum oils, it can be used in various ways that are not practical with 

 larvicides such as paris green. The fact that it is effective in very 

 small amounts simplifies transportation and supply problems, which 

 are especially important in combat areas. It is now used by the 

 Armed Forces in four forms — (1) in petroleum oils or other hydro- 

 carbons, (2) as concentrates for use in preparing aqueous emulsions, 



(3) in dusts with an added wetting agent for aqueous suspensions, and 



(4) in dusts without a wetting agent. 



Resistance to DDT was found among populations of salt-marsh 

 mosquitoes in several counties in Florida where this insecticide had 

 been used for several years. A similar development was reported for 

 several species found in irrigated pastures in California, for a domestic 

 species of Culex in Italy, and for several species of Anopheles in differ- 

 ent countries. This problem is expected to be of increasing impor- 

 tance, judging by the spread of DDT resistance in the house fly and 

 the body louse, and means for combating it must be considered. 



Although this discussion of mosquito control is limited to the use of 

 insecticides, measures for permanent elimination of breeding areas 

 should be undertaken whenever possible. The threat of decreasing 

 effectiveness of chemical control emphasizes this principle. 



CONTROL OF MOSQUITO LARVAE 



DDT Sprays 



Technical DDT in an oil spray or in an aqueous emulsion of an oil 

 solution is the most widely used mosquito larvicide. In an oil spray 

 it is usually dissolved in kerosene or fuel oil in concentrations ranging 

 from 5 to 1 percent or less. When available, No. 2 fuel oil is employed 

 by the Armed Forces for outdoor use. Concentrated solutions in 

 other solvents are sometimes made and diluted with kerosene or fuel 

 oil as needed (see p. 13). 



Larvicides are generally applied with sprayers. Owing to the re- 

 markable larvicidal action of DDT, very little spray is needed, and 

 this must be realized to take full advantage of the potential saving 

 in materials and labor. Oil without DDT is applied at 10 to 25 gallons 

 per acre for control of culicine larvae. Comparable results can be ob- 

 tained with 5 quarts per acre of a 1-percent DDT solution or 1 quart 



