Behavioral resistance of house flies, Musca domestica (Diptera: Muscidae) to imidacloprid.

House flies captured at a field site in southern California were examined for physiological and behavioral resistance to the insecticidal toxicant imidacloprid using a no-choice and a choice feeding assay, respectively. Relative to a susceptible laboratory colony of house flies, field-captured house flies demonstrated moderate physiological resistance to imidacloprid using a no-choice feeding assay. In contrast, behavioral resistance of field-captured flies was very high with 72% survival of flies at even the highest imidacloprid concentrations tested using a choice feeding assay. Since the introduction of imidacloprid baits in California during 2003, the overuse of imidacloprid baits in southern California has resulted in the rapid selection, over only 5 years, of a house fly population that is highly resistant to imidacloprid. While house fly resistance is shown to be both physiological and behavioral, observed field failures of imidacloprid fly baits are primarily due to behavioral resistance. Field-deployable kits to assess behavioral resistance of house flies and other important insect pests are needed.