Susceptibility to DDT and some other insecticides of head lice (Pediculus capitis De Geer) in Czechoslovakia.

For testing the susceptibility of the head louse to insecticides impregnated bunches of polyamide fibres, whose diameter resembled that of human hair (0.1 mm) were used. In this manner a low mortality of control lice collected from infected persons was obtained (2.9% after 16 hours of exposure). Even after 25 years of DDT use no apparent resistance could be demonstrated in 25 tested louse populations and cross resistance to permethrin in 7 tested populations. The diagnostic concentration for resistance to pp'-DDT was established at 1%. Of the tested insecticides the highest toxicity was shown by malathion (LC50 = 0.000 018%), pirimiphos-methyl had on average a twice lower toxicity, trichlorphon 104 times lower, tetramethrin 1220 times lower, permethrin 2.5 times lower and pp'-DDT 630 times lower toxicity.