Genetics of resistance of pyrethroid‐selected houseflies, Musca domestica L.

Four resistance factors were isolated genetically from the NPR strain of houseflies ( Musca domestica L.), 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; and py‐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 alone.