Between December 1947 and May 1949, six colonies of body lice from various sources were cultured in the laboratory. Three of the strains originated from natural infestations in England and the others came from laboratory cultures in Hamburg, Basle and Tunis. After being bred for at least one generation in the laboratory, these lice were subjected to a standard test for resistance to DDT. Only one strain showed any evidence of abnormal resistance; this was the strain from Tunis where it had been laboratory-cultured for over 12 years.
Colonies of Aëdes aegypti from Karachi, Poona and Delhi were being cultured at the Virus Research Institute, Lagos, together with a local Nigerian strain. Extensive tests showed no difference in average susceptibility of the four strains to DDT, though there was some evidence of differences in the spread of resistance through the various populations.
A colony of A. aegypti was started with larvae taken from Ilaro, a Nigerian town which had been regularly sprayed with BHC for 2½ years. Comparative tests showed no difference in resistance to γ BHC between this colony and a laboratory culture at the Malaria Service Laboratory, Lagos.
Colonies of house-flies were initiated and maintained at Lagos. Tests with insecticides gave the following results:
(a) M.d. vicina from Ilaro were much more resistant to γ BHC than similar flies from Yaba, near Lagos, or M.d. domestica from England.
(b) The M.d. vicina from both Lagos and Ilaro were considerably more resistant to DDT than M.d. domestica from England. This is surprising, as no DDT-spraying has been done at Ilaro and, so far as can be ascertained, virtually none in Lagos.
(c) The M.d. vicina flies from Lagos were considerably more susceptible to γ BHC and dieldrin than M.d. domestica from England. M. sorbens was much more susceptible to γ BHC and to DDT. These differences could be explained by the smaller average sizes of the two susceptible species.