An arsenic-resistant tick was first noticed in the East London district of South Africa in 1938–39. It was a strain of the one host blue tick, B. decoloratus. BHC was very toxic to the tick, as 50 p.p.m. of the gamma isomer killed adult females in in vitro experiments. Field results in 1946–47 were equally convincing. Weekly dippings in BHC washes containing 50 p.p.m. gamma isomer brought the arsenicresistant tick under control.

In March 1948, BHC was found to be ineffective against blue ticks at “Burnside”, “Mistley” and “Gulu”, farms in the East London district. Later, ticks at “Tharfield” and “Elmhirst” in the Bathurst area survived regular weekly dippings in BHC preparations. In vitro experiments showed that adult female blue ticks taken from these localities could withstand high concentrations of gamma BHC, while females of the same species from other areas were readily killed. Not even 1,000 p.p.m. would give 100 per cent. control of ticks from resistant areas, and different sources of gamma BHC were equally ineffective. Laboratory experiments further indicated that the ticks were still arsenic-resistant. Indeed arsenic resistance appears to be associated with BHC resistance, as no BHC-resistant tick has been found that is not arsenic-resistant. The chlorinated cyclic hydrocarbons toxaphene and chlordane were ineffective against BHC-resistant ticks, but controlled BHC-sensitive ticks in the laboratory. DDT gave moderate in vitro results against both resistant and sensitive ticks.