Trials of Residual Insecticides against Anophelines in African-type Huts

Since November 1951, trials of residual insecticides have been carried out at Taveta, Kenya, against Anopheles gambiae Giles and A. funestus Giles in native-type huts fitted with exit traps.DDT wettable powder at over 2·5 g. DDT per sq. metre in a hut with plaster of absorbent mud gave, for eight months, kills of over 50 per cent. of the female Anophelines that entered.BHC wettable powder in huts with absorbent walls at 0·24 g. γ isomer per sq. metre gave kills of over 50 per cent. for nine months or more. On non-absorbent walls the kill fell rapidly during the third month and was negligible by the fifth. Insecticide persisted in walls of active material after the roof of inactive materials had become relatively innocuous.Wettable powders combining DDT and BHC were not effective unless the deposit of DDT was at least 2·1 g./m.2. For an equivalent cost, BHC alone was more effective on active walls and dieldrin on inactive ones.BHC in urea-formaldehyde resin at 2·5 g. γ BHC per sq. metre was persistent but the persistence was not commensurate with the cost of the material and difficulty of application. The insecticide persists longest on mud surfaces, presumably because it is absorbed from the skin of resin and later released as vapour. Absorption would render permanently innocuous a nonvolatile insecticide.Aldrin wettable powder was found considerably less persistent than BHC on absorbent walls.