Forms of insecticide resistance in houseflies and body lice

Starting with insecticide-resistant strains of house-flies [Musca domestica L.] from Italy (I) and Sardinia (S) [R.A.E., B 41 122] and a non-resistant colony from Rome (N), the author attempted the segregation of strains with different kinds of resistance by selective breeding. When the N and S strains were crossed and the male progeny that survived exposure to γ BHG were back-crossed with susceptible females for three generations, resistance to γ BHC was maintained and some tolerance of chlordane and dieldrin was preserved, while resistance to DDT was lowered practically to normal. Eesistance to chlordane, dieldrin and γ BHC rose 500, 300 and 60 times, respectively, in a line (Ch) derived from the I strain by selection for resistance to chlordane on six occasions over a period of 11 generations, whereas resistance to DDT increased only 2.2 times. Selection by exposing batches of flies of a sub-colony (P) of the I strain to pyrethrins seven times in 12 generations caused only a slight increase in resistance. Flies of this sub-colony were subsequently twice selected for resistance to paralysis by DDT, and resistance to pyrethrins increased somewhat in consequence. An 88 sub-colony was derived from the 8 strain by selecting for resistance to kill by DDT in two successive generations to make the average lethal dose about the same as that for the Ch sub-colony, and an 8 k-d sub-colony was selected for resistance to DDT-paralysis over 11 generations by breeding from flies not readily knocked down by DDT. These various strains were then maintained in isolation without further treatment, and tests were made of their insecticide resistance. In addition to those used in the preceding tests, the insecticides included DANP (1, 1-dianisyl-neopentane),. which is a DDT analogue of interest in connection with the mechanism of resistance to DDT because all the chlorine atoms of the latter are replaced by methyl or methoxy groups [l, l-di(p-methoxyphenyl)-2, 2, 2-trimethyl-ethane]. The median lethal concentrations (MLC) of all the insecticides to each strain and also the rates of knockdown by DDT are given in a table, and the following conclusions are drawn from it. Eesistance to BHC and chlordane are independent of resistance to DDT, but the factors for resistance to BHC, chlordane and çUeldrin are linked, so that selection with one raises resistance to the others. Resistance to paralysis caused by DDT is not closely linked with resistance to its lethal effect [loc. cit.']. Strains I and S have about the same resistance to the lethal action of DDT and so have strains Ch and 88, but the first member of each pair is considerably more resistant to paralysis than the second. Resistance to DANP is correlated with resistance to pyrethrins and to paralysis by DDT. This is an extension of an earlier observation that resistance to DDT analogues in the I strain (which is resistant to pyrethrins and to paralysis by DDT) is, unlike that in the 8 strain, unrelated to the ease of dehydrochlorination. It seems that there is a DDT-resistance factor not concerned with dehydrochlorination [cf. loe. cit.] and associated with resistance to paralysis by DDT and kill by pyrethrins.
Reports having been received of resistance to DDT in body lice [Pediculus humanus humanus L.] in Korea [cf. 41 106], a strain of body lice was reared from eggs obtained from a laboratory colony in Cairo for experiments on alternative insecticides. Spray tests, however, indicated that the MLC of DDT for the Egyptian strain was nine times as great as that for susceptible British lice assessed by the same technique in 1946 [36 217], the MLC of γ BHC being 1.3 times as great. Tests in which lice of the Egyptian strain and a new susceptible strain from London were exposed in batches for 18 hours at 30°C. [86°F.] on filter papers impregnated with insecticides. in a eonstant volume of oil [cf. 41 156] indicated that the Egyptian lice were about normally susceptible to y BHC, chlordane, dieldrin,. aldrin, toxaphene, allethrin and pyrethrins, whereas the MLC of DDT for them was much more than 3 per cent, as compared with 0.6 per cent, for the London strain. As the MLC of DANP for the London strain was 2.5 per cent., whereas none of the Egyptian lice was killed by 10 per cent., the DDT resistance of the latter cannot be accounted for by a dehydrochlorination mechanism. It may have resulted from large-scale dusting of Egyptian villagers with DDT for four years before the Cairo colony was started. The colony had been kept for two years without exposure to insecticide before the eggs from it were sent to London. The Egyptian lice appear to be less resistant to DDT than the Korean ones.