29 



Usinger: Introduction 



Intro, fig. 40. Jeep equipped for power larviciding 

 (Consolidated Mosquito Abatement District). 



Snow-pool mosquitoes may be treated in the spring 

 when the snow is melting and the eggs are hatching 

 or in the fall in anticipation of the spring hatch. For 

 domestic mosquitoes such as Culex pipiens control 

 measures should be continued throughout the year. 

 Unfortunately, with the increase in potency of 

 mosquito larvicides greater hazards have been intro- 

 duced to fish and other aquatic organisms. Ordinarily 

 this problem does not arise because it is considered 

 bad mosquito control to introduce larvicides in areas 

 where fish occur. Nevertheless, the danger exists and 

 evidence is not yet entirely clear as to the limits of 

 tolerance of various species under diverse conditions. 

 However, certain generalizations can be made from 

 the work of Tarzwell (1950) and his associates. Field 



Intro, 

 treating 

 photo). 



fig. 41. Hand sprayer mounted on a motorcycle for 

 catch basins with larvicide (U.S.P.H.S., C.D.C. 



Intro, fig. 42. Application of larvicides by means of an airplane 

 (Kern County Mosquito Abatement District). 



/ 



experiments were conducted at weekly intervals by 

 airplane using a standard rate of 0.1 pound per acre 

 of DDT mosquito larvicide applied as a spray or 

 aerosol. "Studies on the effect of DDT (dichloro- 

 diphenyl-trichloroethane) and certain other new insec- 

 ticides indicate that they are all toxic to fishes if 

 used in large doses. With DDT the type of pond or 

 water in which it is used greatly influences the 

 onset and severity of toxic action on fishes. Vegeta- 

 tion, organic material, type of water, and silt or 

 turbidity are all factors influencing this action. Crabs, 

 crayfish, amphipods, isopods, and Palaemonetes are 

 very sensitive to DDT, being considerably more so 

 than fishes. Among the fishes, some of the Centrar- 

 chidae are the first to be affected, especially the 

 bluegill . . . Although top minnows were among the 

 first fish to be killed, they continued to be present 

 during the period of treatment and were in evidence 

 when most other fish had been eliminated. A few 

 frogs and snakes were killed by routine dosages of 

 0.1 and 0.05 pound DDT per acre. At routine dosages 

 of 0.1 pound per acre, DDD (dichloro-diphen\ 1-dich- 

 loroethane), chlordane, and DDT are toxic to fish 

 and will significantly reduce the population of ponds. 

 At dosages of 0.05 pound per acre, DDT appears to 

 be somewhat more toxic than chlordane or DDD. 

 Studies carried on in 1947 indicated that DDD was 

 considerably less toxic to fish than DDT. These three 

 insecticides appear to have no significant effect on 

 the fish population at dosages of 0.02.5 pound per 

 acre. Toxaphene was found to be very toxic to fishes, 

 giving complete kills at 0.2 and 0.1 pound per acre 

 after two and three applications in deep ponds. Kills 

 were obtained at dosages of less than 1 part in 27 

 million, indicating that this material is as toxic or 

 more toxic to fish than rotenone and may be useful 

 as a substitute for it in fish management work." 

 Doudoroff, Katz, and Tarzwell (1953) added data on 

 other insecticides, stating that, "Aldrin appears to 

 be less toxic to goldfish than toxaphene, but much 

 more toxic than DDT and BHC (benzene hexachloride)." 



