8 

 LARVICIDES 



ALARViciDE FOR MOSQUITO CONTROL PURPOSES IS material (usually a 

 chemical compound) which when applied to water, where mosquito 

 wrigglers are present, kills them. The ideal larvicide would be a mate- 

 rial or a process that would kill mosquito larvae, be noninjurious to 

 aquatic plants, fish and water fowl, with one application remain effec- 

 tive throughout at least one season, and be cheap enough to be prac- 

 tical over thousands of acres. 



Let us see to what extent our present larvicides approach this ideal. 

 Early in mosquito work it was discovered that kerosene applied to mos- 

 quito breeding water surfaces so as to form a film killed mosquito wrig- 

 glers. Dr. L. O. Howard's publication (13) of this fact had much to do 

 in starting local mosquito control efforts in northeastern New Jersey, 

 notably at South Orange under the leadership of Spencer Miller. Before 

 long, larvicides of character miscible with water followed. As time 

 passed, however, since petroleum oil was cheap, and required for its ef- 

 fect merely the establishment of a surface film, quite regardless of water 

 depth, it became the accepted larvicide. 



Studies were undertaken to develop petroleum blends, that would not 

 only form a film quickly but also remain actively present for the great- 

 est possible length of time. These two qualities were somewhat opposed 

 to each other. The oil finally selected had to represent a compromise. 



Efforts were made to increase the poisonous character of the film by 

 the addition of highly toxic oil fractions. Incidentally studies of toxic- 

 ity of petroleum oil fractions to mosquito larvae proved that the more 

 unsaturated the oil the more toxic it was to the larvae. The wide range 

 in time required to kill larvae ran from several hours with almost com- 

 pletely saturated oils to a few minutes (thirty or less) with highly un- 

 saturated oils. The difference between saturated and unsaturated oils is 

 briefly this. All petroleums are composed of hydrocarbons. If these 

 hydrocarbon molecules have few or no free bonds, the oil is highly satu- 

 rated ; if they have many free bonds, the oil is considered largely un- 



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