Predicting resistance and managing susceptibility to cyromazine in the Australian sheep blowfly Lucilia cuprina

Abstract

Four cyromazine-resistant variants of Lucilia cuprina were selected after ethyl methanesulfonate mutagenesis and screening above the concentration of cyromazine lethal to susceptibles. Resistance is controlled by a single gene in each variant. Two resistance loci have been identified, one (Cyr 4) closely linked to the marker 'reduced eyes' on chromosome IV, the other (Cyr 5) closely linked to the 'stubby bristles' marker on chromosome V. Concentration-mortality line analysis shows resistance ratios are low (1.5-3x). One variant [Cyr 4(2)] is viable as a homozygote, the others are lethal [Cyr 4(1)] or, at best subvital [Cyr 5(1) and Cyr 5(2)]. Competition experiments between resistant heterozygotes and susceptibles show that resistance to cyromazine is selected for over a limited range of concentrations. The capacity of laboratory studies to predict likely resistance mechanisms before they evolve in the field is discussed. The use of genetic, toxicological and relative fitness data arising from these studies to devise the most effective strategies of insecticide usage while minimising the evolution of resistance is emphasised.