Selection for, and characterization of, fluralaner resistance in the house fly, Musca domestica

House flies, Musca domestica (L), are the mechanical vector of >100 human and animal pathogens, including those that are antibiotic-resistant. Given that house flies are associated closely with human and livestock activity, they present medical and veterinary health concerns. Although there are numerous strategies for control of house fly populations, chemical control has been favored in many facilities. Products with pyrethroid active ingredients have been used predominantly for >35 years in space sprays. As a result, strong selection for pyrethroid resistance has led to reduced control of many populations. Reliance on a limited number of insecticides for decades has created fly control problems necessitating the discovery and formulation of new control chemistries. Fluralaner is a relatively new insecticide and acaricide (first reported in 2010), belonging to the isoxazoline class. These insecticides target the glutamate- and gamma-aminobutyric acid-gated (GABA) chloride channels, which is a different mode of action from other insecticides used against house flies. Although is it not currently registered for house fly control in the United States, previous work has shown that fluralaner is highly toxic to house flies and that there was limited cross-resistance found in laboratory strains having high levels of resistance to other insecticides. Herein, we characterized the time and age dependency of fluralaner toxicity, detected cross-resistance in populations from across the United States, and selected a highly resistant (>11,000-fold) house fly strain. We found that the fluralaner LD₅₀ of 18-24 h old flies was 2-fold higher than for 5-6 d old flies. This appears to be due to more rapid penetration of fluralaner into the 5-6 d old flies. Fluralaner resistance was inherited as an intermediate to incompletely dominant trait and was mapped to chromosomes 5 and 3. Resistance could be suppressed to 7-fold with piperonyl butoxide, suggesting that cytochrome P450 (CYP)-mediated detoxification was a major mechanism of resistance. Decreased penetration was also demonstrated as a mechanism of resistance. The utility of fluralaner for house fly control is discussed.