Monitoring and selection of resistance to pyrethroids in the Australian sheep blowfly, Lucilia cuprina

Abstract

Field and laboratory populations of the Australian sheep blowfly, Lucilia cuprina (Wiedemann) (Calliphoridae), were surveyed by bioassay for possible resistance to the synthetic pyrethroids, a group of insecticides under development for blowfly control. A normal distribution of LC50 values was found using deltamethrin, the test pyrethroid, with no indication of specific resistance despite widespread use of deltamethrin on sheep to control the sheep body louse, Damalinia ovis (Schrank) (Trichodectidae). There was no cross-resistance to deltamethrin from existing organophosphate (OP) resistance nor from previous use of DDT. Selection with deltamethrin on a combined field strain, CSF85, increased the LC50 gradually over the first twenty generations until it stabilized at approximately 25x that of the unselected CSF85. This laboratory-induced resistance extended to other pyrethroids, cypermethrin (16x), cyhalothrin (25x) and cycloprothrin (10x), and increased the existing resistance of CFS85 to the OP diazinon (11x) and the carbamate, butacarb (83x).