The status of resistance in Culex quinquefasciatus say (Diptera: culicidae) populations in Brazos and Harris Counties, Texas
In 2002, West Nile virus was isolated for the first time in Harris County, Texas. The subsequent epidemic led the Harris County Mosquito Control Division to initiate an extensive spraying operation to suppress infected adult mosquitoes. The control program was aimed at the predominate disease-carrying mosquito for the southern United States, Culex quinquefasciatus Say. With the increase of insecticide pressure on the mosquito populations, the possibility of resistance was brought into question. A three year study using a vial bioassay test was conducted in Harris (2004-2005) and Brazos (2005-2006) counties to determine the resistance status of Cx. quinquefasciatus to the six chemicals (malathion, naled, resmethrin, permethrin, sumithrin, and pyrethrum) used most frequently in adult mosquito control programs. The resistance ratios acquired from the vial bioassay tests were mapped onto shapefiles for Harris and Brazos counties, which revealed clustering of areas with pyrethroid resistance mosquito populations in the northeastern, southeastern, and southwestern corners of Loop 610 in Harris County. An additional six-month preliminary study, involving six operational areas in Harris County and three in Brazos County, was conducted, demonstrating only minor fluctuations in the monthly resistance ratios occurring in both counties in 2005. A significant correlation was documented between the two years of resistance ratios for mosquitoes to the three pyrethroids in Harris County and all the insecticides except pyrethrum in Brazos County. A significant relationship was also found between the resmethrin resistance ratios and the number of spray events performed during the previous year and the malathion resistance ratios with the insecticide treatments conducted in the same year. The correlation analyses provide data used to predict areas where resistance can develop in the mosquito population, thus providing the control agency more data to plan future control tactics. The overall analysis indicated that Harris County has localized pockets of resistant mosquitoes; but, on a whole, it does not seem to have widespread resistance in its mosquito populations. The only resistance that was detected was in the mosquitoes tested against the three pyrethroids. Mosquitoes in Brazos County, which has no organized mosquito control, demonstrated county-wide susceptibility to all six insecticides tested.