The median lethal concentrations of some persistent insecticides as obtained by a filter-paper-impregnation technique for X. cheopis and P. humanus corporis are given. Their relative toxicity runs in the following order:—
Flea (adults): Gamma BHC > dieldrin > aldrin > chlordane > DDT > pyrethrins > allethrin > toxaphene.
Lice: Dieldrin > gamma BHC > aldrin > chlordane > pyrethrins > allethrin > DDT=toxaphene.
On flea larvae gamma BHC is 36 times as toxic as p,p'DDT.
On flea pupae gamma BHC and DDT powders and wettable suspensions are effective; further tests are required to determine their comparative toxicity.
On mud blocks (London soil) gamma BHC wettable suspension is only 3 times as toxic as DDT compared with 20 times on filter papers impregnated with oil solutions. This is probably due to the more rapid sorption of gamma BHC as well as the difference of surface and formulation.
In the field, at dosages of 2 gr. per m2 of 70 per cent. p,p′ DDT and i43 mg. of gamma BHC from wettable powders, BHC was superior to DDT as it gave a much better control of fleas (P. irritans) over the period of the experiment (24 weeks).
Field trials on body lice by dusting the clothes, while worn, with 40 gr. (per person) of dusts containing, respectively, 10 per cent. (70 per cent. p,p′) DDT dust and 0·4 per cent, gamma BHC, showed that BHC gave more satisfactory results than DDT.
The resistance of lice to DDT shown by Busvine in the local strain of Pediculus and the possibility of its occurrence among fleas may be one of the factors contributing to a better control by BHC.
It was found possible to develop artificially, by selection in the laboratory, a slight degree of resistance in X. cheopis.