A Disconnect Between Operational Evaluations of Larvicide Treatments to Storm Sewer Catch Basins and Findings of Larvicide Resistance in the Northwest Suburbs of Chicago, IL, USA

Regular evaluations of larvicide resistance and operational control effectiveness are two basic components of the Integrated Mosquito Management approach. It is therefore important for mosquito control programs to assess both the efficacy and the effectiveness of larvicide treatments. Recent observations in 2023 and 2024 within areas served by the Northwest Mosquito Abatement District (NWMAD) indicated that methoprene resistance categorized as high and extreme exist in populations of Culex pipiens (L). Despite more than 20 years of exclusive use of Altosid® XR (2.1% methoprene) larvicide in NWMAD catch basins and associated expectations of reduced effectiveness, NWMAD’s evaluations of Altosid® XR-treated catch basins in 2022, 2023, and 2025 with found levels of control (emergence inhibition [EI]) to be similar to those reported in studies over the past 30 years of the same or similar larvicide formulation. An extreme abundance of caution is needed when interpreting findings of NWMAD’s control evaluations as these observational findings were collected over different years and areas served by NWMAD. These findings also lack inferential statistical analyses and were not collected in a scientifically rigorous and systematic manner with a true experimental design. The reported results should not be interpreted as demonstrating that methoprene resistance does not affect operational control.