Synthetic pyrethroid resistance in Haematobia irritans subsp. exigua (Diptera: Muscidae): knockdown-recovery observed, a new putative resistance-linked mutation detected, and a complete voltage-gated sodium channel gene transcript sequenced

Haematobia irritans subsp. exigua (Diptera: Muscidae), locally known as buffalo flies, are the costliest pest of cattle in Australia. Control of H. i. exigua is primarily through insecticides despite resistance to many control chemicals now confirmed. Elevated resistance to synthetic pyrethroids in Queensland H. i. exigua populations has been observed, which led to this study to investigate if new voltage-gated sodium channel (VGSC) mutations might be involved. The complete VGSC coding region of H. i. exigua has been sequenced. The gene consists of 25 putative exons plus 7 alternate-splice sites. Five genetic assays were developed that amplify H. i. exigua DNA and target 26 resistance-linked mutations characterized in other insects. -Deltamethrin and α-cypermethrin tested flies from a field population of H. i. exigua were screened and a new putative resistance-linked mutation, T929I along with previously characterized kdr (L1014F) were found in 82% of the resistant flies. The T929I mutation has been shown to provide a synergistic enhancement of kdr in other insects and may be providing a synergistic enhancement of resistance to α-cypermethrin in H. i. exigua. A new class of resistance, knockdown-recovery, was observed in H. i. exigua populations. Knockdown-recovery in other insects has been linked to the elevated expression of detoxifying enzymes. This alternative pathway may explain how 18% of flies without VGSC resistance mutations survive high concentrations of insecticide.