PREDICTING RESISTANCE AND MANAGING SUSCEPTIBILITY TO CYROMAZINE IN THE AUSTRALIAN SHEEP BLOWFLY LUCILIA-CUPRINA

Four cyromazine-resistant variants of Lucilia cuprina were selected af ter ethyl methanesulfonate mutagenesis and screening above the concent ration of cyromazine lethal to susceptibles. Resistance is controlled by a single gene in each variant. Two resistance loci have been identi fied, one (Cyr 4) closely linked to the marker 'reduced eyes' on chrom osome IV, the other (Cyr 5) closely linked to the 'stubby bristles' ma rker on chromosome V. Concentration-mortality line analysis shows resi stance 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 heteroz ygotes and susceptibles show that resistance to cyromazine is selected for over a limited range of concentrations. The capacity of laborator y studies to predict likely resistance mechanisms before they evolve i n the field is discussed. The use of genetic, toxicological and relati ve fitness data arising from these studies to devise the most effectiv e strategies of insecticide usage while minimising the evolution of re sistance is emphasised.