Monitoring Susceptibility of House Flies (Musca domestica L.) in the United States to Imidacloprid

Neonicotinoids are currently the fastest-growing class of insecticides. In 2004, imidacloprid (a neonicotinoid) was registered for house fly control in the United States. This study was undertaken to examine the relative susceptibility of laboratory strains of house flies to imidacloprid using a feeding assay, to determine the relative susceptibility of field collected house flies before imidacloprid use, and to determine the susceptibility 1 and 2 years after imidacloprid use. We found substantial variation in imidacloprid susceptibility in laboratory strains and in populations collected from the field before imidacloprid use. We observed an increase in susceptibility at one site from 2004 to 2005 and a decrease at one site from 2005 to 2006, although all populations in 2005 and 2006 were within the range of responses observed in 2004. The implications of these results to the future use of imidacloprid for house fly control are discussed.