AbstractFour resistance factors were isolated genetically from theNPRstrain of houseflies (Musca domesticaL.), which resists natural pyrethrins, and were characterised toxicologically. The four factors were : pen, which reduces the rate of penetration of insecticides through the cuticle;kdr‐NPR, a general pyrethroid resistance mechanism unaffected by the synergist sesamex;py‐ses, a mechanism of resistance to natural pyrethrins that can be suppressed by sesamex; andpy‐ex, a factor that gives strong resistance to synergised natural pyrethrins and to the new synthetic esters, e.g. resmethrin, but little or none to natural pyrethrins
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