DDT‐resistant mosquito strains with cross‐resistance to pyrethroids

Abstract

Abstract Various DDT‐resistant strains of mosquito, comprising 8 strains of Aedes aegypti from the Neotropics (tropical and South America) and one each of Anopheles gambiae and An. quadrimaculatus were tested for resistance to pyrethroids. Susceptibility was measured by LT 50 values, after exposure to impregnated papers in the cylinders used in the WHO standardised test for adult mosquitoes. DDT‐resistance levels (by comparison with susceptible strains) ranged from × 2 to × 73, with simultaneous low resistance to permethrin (× 1.4 to × 3.4) except in one strain from Guyana which reached × 30. Further comparisons were made with this Guyana strain and one from Salvador with similar DDT‐resistance (about × 70) but low permethrin‐resistance. Tests with methoxychlor and I,I‐di‐(4‐chlorophenyl)‐2‐nitropropane and with the synergist I,I‐di‐(4‐chlorophenyl)‐2,2,2‐trifluoroethanol suggested that a considerable part of the DDT‐resistance of both strains was due to a dehydrochlorination mechanism. Tests with the synergist piperonyl butoxide (PB) suggested that microsomal oxidase systems were also involved, especially in the Guyana strain. The cross‐resistance in both strains extended to 3 other pyrethroids. Addition of the synergist PB had little effect on the Salvador strain, but substantially reduced permethrin‐resistance in the Guyana strain. It is concluded that an unknown mechanism associated with DDT‐resistance confers a low level cross‐tolerance to pyrethroids; this can be substantially augmented by the development of a microsomal oxidase mechanism, as in the Guyana strain.