MOSQUITO SPECIES COMPOSITION IN CHICAGO AREA CATCH BASINS FROM FIVE YEARS OF OPERATIONAL DATA.

During 2020 to 2021 and 2023 to 2025, inspections of more than 1,300 unique storm sewer basins across an approximately 270 sq mi (699.3 sq km) area of Cook County were inspected. During inspections Culex pipiens and Cx. restuans made up 79.0% and 20.8%, respectively, of 11,999 larvae collected during inspections within epidemiological weeks 22 to 43, or June through October. Culex pipiens was the dominant species in basins when average weekly West Nile virus (WNV) percent positive pools were the highest, generally through the months of July through September. This finding suggests that, at a minimum, levels of larvicide control remain near or at 100% mortality in basins during this time of both high Cx. pipiens and high WNV. Ensuring closer to maximum in control in basins during times when Cx. pipiens can be found in basins may also minimize potential for larvicide resistance of that species as sublethal doses would be less common.