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Knock‐down resistance in Musca domestica, which provides cross‐resistance between DDT and pyrethroids, has been genetically separated from three different resistant populations. After careful purification of each factor, the cross‐over rates between them and the visible mutants brown body and green eye were estimated. This indicated that these factors are probably identical. The influence and implications of the knock‐down resistance factor, kdr, on the total resistance of populations that include it are briefly discussed.

Structure‐activity relationships (SARs) for 10 pyrethroids against susceptible, kdr and super‐kdr strains of houseflies (Musca domestica L.) were investigated by Principal Components Analysis. In the three strains with kdrLatina' all only slightly to moderately (2.6 to 26‐fold) resistant to pyrethroids, no correlation between the structure and Levels of resistance could be discerned. In flies with super‐kdr, SARs were influenced by the nature of the alcoholic portion of the ester. Resistance was strongest to esters of a‐cyano‐3‐phenoxybenzyl alcohol (74 to 430‐fold) and to permethrin (48 to 55‐fold). It was weak (6.2 to 11‐fold) to cyclopentenone derivatives, being barely stronger than for flies with kdr (2‐6 to 6.3‐fold). Two variants of super‐kdr (3D and A2) were distinguished on the basis of their differential response to esters of 5‐benzyl‐3‐furylmethanol. It is presumed that kdrLatina, super‐kdrA2 and super‐kdr3D form an allelic series in which kdrLatina represents ground level insensitivity, and the two super‐kdrs the progressive extension of strong resistance to more types of ester. The strong differences in resistance to different pyrethroid esters by super‐kdr flies provides scope for improving management of resistance to pyrethroid insecticides and for modifying the SAR of pyrethroids to favour weak resistance.