SEASONAL MOSQUITO PRODUCTION IN STORM SEWER CATCH BASINS OF THE NORTH SHORE SUBURBS OF CHICAGO, 2021 TO 2024

A fundamental aspect of mosquito surveillance and larvicide control efforts is an understanding of when standing water sites may be producing mosquitoes (AMCA 2021). Despite almost a century of seasonal larvicide applications to the tens of thousands of storm sewer catch basins in the Chicago metropolitan area, seasonal mosquito production in these structures has yet to be fully characterized. Recent routine quality control efforts of the North Shore Mosquito Abatement District (NSMAD) offered an opportunity to observe seasonal occurrence of mosquito pupae in these structures in the North Shore suburbs of Chicago. These efforts included sampling up to 20 basins with 5 dips of a 350-ml dipper each within a sq mi (∼1.6 km) site and counting all observed pupae. The presence of larvae was also noted as present/absent for each basin. However, the intent of this current study was to compare the number of pupae found in these structures across calendar months. This may provide some insight as to when during a given year adult mosquitoes may be emerging from basins, particularly if left untreated or when holding sublethal doses of larvicides.

During the 4 years 2021–2024, a subset of 1,993 different basins were sampled 1 to 68 times across approximately 70 sq mi (∼1.6 km) of the NSMAD service area. Sampling effort varied across months, but a minimum of 100 basins were sampled during each calendar month (Fig. 1). Pupae were found during the months of April through December, with the highest levels seen during June through November. Larvae were also observed to be present for all months that pupae were found and were absent during months when pupae were absent (January, February, and March). This assessment represents a thorough examination of the length of a seasonal catch basin: the earliest in the calendar year any pupae were found in a basin was April 26, 2024, and the latest was December 21, 2021. Larvae were also present in at least 1 basin between these 2 dates.

Although mosquitoes were not identified to species in this work, catch basins in the Chicago metropolitan area overwhelmingly harbor Culex mosquitoes, particularly Cx. pipiens (L.) and Cx. restuans (Theobald) (Hamer et al. 2011; Gardner et al. 2012, 2013). These data suggest that basins may remain productive a month or more after NSMAD and other abatement programs in the Chicago metropolitan area end their routine Culex adult surveillance with gravid traps in late September and early October each year (CCDPH 2024). This assessment represents the first time the length of seasonal catch basin production was observed in the North Shore suburbs and may help with determining the timing and intensity of larvicide treatments in Chicago area catch basins in the future. Finally, these observations may also serve as a baseline for comparison to future assessments.

This manuscript was supported by Cooperative Agreement Number U01CK000505, funded by the Centers for Disease Control and Prevention (CDC). Its contents are solely the responsibility of the author and do not necessarily represent the official views of the CDC or the Department of Health and Human Services.