Characterization of imidacloprid resistance in the house fly, Musca domestica.

House flies are a major pest of agriculture and public health. They have evolved resistance to most major classes of insecticides. Neonicotinoids are currently the most widely used class of insecticides. Imidacloprid, a neonicotinoid, is formulated as bait and used extensively for house fly control. Field collected house flies were selected with imidacloprid to create a resistant strain (KS8S3).  Females demonstrated high levels of resistance (2300-fold) compared to males (130-fold). Imidacloprid resistance was not suppressible by treatment with piperonyl butoxide, suggesting P450 monooxygenases were not involved in the resistance. Cross resistance to other neonicotinoids was observed, although the levels of resistance were lower than found for imidacloprid. There was no cross-resistance to spinosad. Linkage analysis found that resistance was due to factors on autosomes 3 and 4. Imidacloprid resistance in KS8S3 decreased over time without exposure to imidacloprid; indicative of a fitness cost. Given that resistance was not suppressible by PBO and there are at least two resistance mechanisms, it is likely that at least one resistance mechanism is a target site modification.   Efforts to identify the mutations responsible for resistance are underway.