INSECTICIDES AND REPELLENTS 31 



Against DDT-resistant mosquito larvae 0.05 to 0.1 pound of lindane 

 per acre, dieldrin at the same dosage, and toxaphene at 0.1 to 0.2 

 pound per acre are recommended. However, toxaphene is highly 

 poisonous to fish and should not be used where they are important. 

 EPN and parathion have not yet been approved for use by the Armed 

 Forces, but may be considered in the future for special situations, 

 such as for the control of mosquito populations that show increased 

 resistance to all the chlorinated hydrocarbons. Their use as larvicides 

 has also been suggested to obviate the development of resistance to 

 the hydrocarbons and thus permit continued use of the latter as 

 adulticides. 



Prehatching Treatments 



One method of mosquito control made possible by the long-lasting 

 qualities of DDT is the prehatching (sometimes called preflooding) 

 treatment of both fresh-water and salt-marsh areas where large 

 broods of Aedes mosquitoes develop. The dosage is usually about 

 1 pound of DDT per acre, but varies under different conditions from 

 }i to 2 pounds or even more, and the applications are made when the 

 areas are either flooded or dry. Such treatments have been effective 

 sometimes over an entire season during which repeated floodings 

 occurred. With the mountain and subarctic species of Aedes that 

 have only one annual brood, the eggs of which hatch as soon as pools 

 are formed by melting snow, control has been obtained by applications 

 to the surface of the snow in the late spring or even to the dry or wet 

 ground before snowfall the previous season. 



In one experiment a large area in Alaska was treated by airplane 

 with DDT-oil sprays several times during two summers. There was 

 a marked reduction in breeding during the second and third years, 

 owing apparently to the residual action of DDT. 



For prehatching treatments of wet or dry ground the wettable pow- 

 der is perhaps the most effective, although all the other DDT formula- 

 tions have also proved satisfactory. For application to snow 10- or 

 20-percent DDT dust or dry wettable powder is preferred where dust- 

 ing equipment is available and strong winds do not interfere. 



It is now believed that larvicidal treatments, especially prehatching, 

 may cause resistance to develop more rapidly than spraying directed 

 against the adults only. This possibility should be carefully consid- 

 ered before extensive larvicidal programs are undertaken where re- 

 peated treatments year after year are likely to be required. 



Other Types of Application 



Gelatin capsules containing an emulsifiable concentrate of DDT or 

 other larvicide have been used to a limited extent in mosquito control, 

 especially for treating small, scattered pools. The capsules are thrown 

 by hand into the breeding places, where they swell and burst after 

 contact with the water. DDT cakes prepared with plaster of paris 

 and small insecticide pellets prepared with a clay carrier have been 

 used in a similar manner. Granular bentonite impregnated with a 

 larvicide, which has been used in airplane work (p. 50), may also be 

 broadcast in breeding places by hand or with a hand seeder. 



Other methods of applying mosquito larvicides include the use of 

 drip cans and sawdust soaked with an oil solution of DDT or other 



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