FIRST EVIDENCE OF LARVAL RESISTANCE TO S-METHOPRENE IN CULEX PIPIENS POPULATIONS IN THE TWIN CITIES METROPOLITAN AREA OF MINNESOTA, USA

The Metropolitan Mosquito Control District, based in St. Paul, MN, has been protecting the residents of the Twin Cities metropolitan area of Minnesota from disease and annoyance from mosquitoes and other pests since 1958. Regarding mosquitoes, this mission is carried out by administering a wide-sweeping control program that is almost entirely larvicidal. For control of the Culex species, residual products containing the active ingredient S-methoprene have been heavily used for decades without product rotation in stormwater catch basins and ditches. In the fall of 2025, a preliminary investigation of S-methoprene and pyriproxyfen resistance was conducted using standardized laboratory bioassays. The preliminary data indicate that a field-collected population of Culex pipiens has developed high resistance to S-methoprene, with a resistance ratio at 50% mortality (RR50) value of 56.29 and were tolerant to pyriproxyfen with an RR50 of 4.35. Field-collected Cx. restuans remain susceptible to S-methoprene with an RR50 of 1.79. The preliminary data laid the groundwork needed to warrant further testing to determine where else this resistance is occurring and to what extent.