The Behaviour and Mortality of Anopheles maculatus and Culex fatigans in experimental Huts treated with DDT and BHC

Abstract

Experimental huts fitted with window traps, of the same basic design as those described by Thomson (1948), modified for Malayan conditions, were used in studying the behaviour and mortality of A. maculatus in relation to DDT and BHC. Other species of Anophelines formed a large proportion of the human bait trap catch but rarely entered the huts.In untreated huts, A. maculatus fed at all hours of the night with a peak between 9 p.m. and midnight. The large majority left before 8 a.m. in search of outdoor resting places, at least 50 per cent. before dawn. It appeared that attraction to light was the strongest, but not the only factor influencing exit from the huts.Large numbers of Culicines (mostly Culex fatigans) were recovered. In untreated huts about 80 per cent. were blood fed ; 28 per cent. were in the window traps.DDT (33 per cent.) and BHC (“ Gammexane P530 and P520 ”) water dispersible powders were tested.An application of 100 mg. DDT and 20 mg. γ BHC (P530) per sq. ft. was unsuccessful, due in part to the small numbers of mosquitos recovered. No conclusions are drawn from the results.DDT at 200 mg. and BHC (P520) at 40 mg. γ isomer per sq. ft. were both effective for at least 12 to 16 weeks against A. maculatus. With DDT, 63 per cent. initial, and 83 per cent. 24-hour corrected mortalities were recorded over 16 weeks. With BHC, though the initial mortality dropped from 100 per cent. in weeks 1–4 to 19 per cent. in weeks 12–16, the 24-hour mortality, almost 100 per cent. for weeks 1–10, was still 88 per cent. for weeks 12–16.DDT apparently had little effect on the feeding of A. maculatus ; with BHC there was a reduction in the percentage fed.DDT was relatively non-lethal to C. fatigans, but had a marked irritant effect driving them into the light trap and the biting rate was reduce to 45 per cent. BHC killed all C. fatigans which entered for 2 weeks but most of its toxic effects were lost by the eighth week. The biting rate was reduced to 27 per cent. for weeks 1 to 8 and the majority of mosquitos were recovered in the light trap.Ants, cockroaches, spiders, etc., were found to affect the number of mosquitos recovered and special precautions had to be taken against losses.The results suggested that BHC when freshly applied acts as a repellent to mosquitos.Mansonia uniformis was shown to be susceptible to both DDT and BHC residual deposits.