Dynamics of insecticide resistance alleles in house fly populations from New York and Florida.

The frequency of insecticide-resistance alleles for two genes (Vssc1 and CYP6D1) was studied in field collected populations of house flies from two different climates. While the frequency of these resistance alleles in flies at dairies from four states has recently been reported, there is no information on the relative change of these allele frequencies over time. House flies were collected during the 2003-2004 season from New York and Florida before the first application of permethrin, during the middle of the field season, after the final application, and again the following spring (following months without permethrin use). Bioassay results indicated that homozygous susceptible and extremely resistant flies were rare, while moderately and highly resistant individuals were relatively common at all times in both states. The frequency of resistance alleles at the New York dairy rose during the season and declined over the winter, suggesting an overwintering fitness cost associated with these alleles. The super-kdr allele was detected for the first time in North America at the end of 2003. In Florida the frequency of the resistance alleles did not increase during the spray season or decrease during the winter, suggesting there is substantial immigration of susceptible alleles to the Florida dairy and no overwintering fitness cost associated with resistance alleles in this climate. Resistance to permethrin correlated well with the frequency of the Vssc1 and CYP6D1 resistance alleles in flies from New York, but not as well in the population from Florida. This suggests there may be a new resistance mechanism or allele evolving in Florida.