298 - Reduced susceptibility of S-methoprene in Culex pipiens throughout the Chicago area

Insect growth regulators, like S-methoprene and pyriproxyfen, are heavily relied upon for larval mosquito control due to their target specificity and long-lasting effects. Despite decades of use in the Chicago area to control West Nile virus vectors in catch basins, resistance to these active ingredients has not yet been evaluated. To determine susceptibility of field-collected Culex pipiens to S-methoprene, mosquitoes were evaluated from 16 locations throughout the Chicago area in collaboration with the North Shore MAD, Northwest MAD, Des Plaines Valley MAD, South Cook County MAD, and the City of Chicago. To compare to areas without S-methoprene pressure, Cx. pipiens from Madison WI, Milwaukee WI, and a susceptible colony were also evaluated. Reduced susceptibility to S-methoprene was detected from all locations in the Chicago area, but was not detected in Madison or Milwaukee. This provides evidence that resistance or tolerance to S-methoprene will form with enough selection pressure over an extended period of time, and highlights the recommendation of product rotation.