Year-round Observations of Adult Culex Mosquitoes in Storm Sewer Pipes in the Northwestern Chicago Suburbs

The built environment has long been associated with providing winter refuge for adult Culex mosquitoes (Nasci et al. 2001, Bugbee and Forte 2004, Farajollahi et al. 2005, Arsenault-Benoit et al. 2021, Ciota et al. 2011). Because of the buffered temperatures created in belowground and covered environments, networks of storm sewer conveyance pipes can serve as refuge for these mosquitoes. For example, from October 1994 to March 1995, McClellan (2000) found Cx. pipiens L. in central Illinois in a storm drain. Similarly, Siperstein et al. (2023) found Culex spp. in stormwater culverts in central Ohio from September to May during 2019 to 2022.

The Chicago metropolitan area holds an expansive underground storm sewer infrastructure that may harbor mosquitoes beyond the many storm sewer catch basins that are routinely treated with larvicides by local mosquito abatement districts (MADs). Within the jurisdiction of the Northwest MAD, one of four abatement districts within the Chicago metropolitan area, it is estimated that there exists at least 2,200 miles (3,540 km) of belowground storm sewer pipe at a rate of about 14.6 miles of belowground pipe per 1 sq mile (23.5 km per 1 2.6 sq km) of the area that district serves. It was thus hypothesized that Chicago area storm sewer pipes may provide reliable refuge for adult Culex spp. From 2019 to 2022, 9 total excursions were made among 3 larger (>6 ft [1.8 m] diam) belowground stormwater conveyance pipes located within the Northwest suburbs of Chicago, IL. These pipes were chosen for their large diameter that allowed inspectors to easily walk through unimpeded and search for adult mosquitoes along the pipe walls (Fig. 1a). To safely enter and search within pipes, two confined space safety personnel escorted 4 to 6 mosquito inspectors during inspections. A third safety expert remained aboveground in case rescue measures were needed. Permits allowing for entry into pipes were also arranged by the safety team (Amerisafe Group, Greensburg, PA). Each inspection lasted for 30 to 45 minutes by walking approximately 200 ft to 0.75 mi (60 m to 1.2 km) from the pipe outfall into the pipe.

Dates of inspections were determined based on the availability of inspectors and safety personnel with an effort to sample at least once during each of the 4 meteorological seasons. Two pipes were sampled 4 times each, while the 3rd was sampled only once. The latter pipe was the largest and the diameter of some sections exceeded 10 ft [3 m]. This height was beyond the reach of inspectors and made observations and collections of mosquitoes more difficult. Mosquitoes were captured using Nature Bound™ Bug Catcher Toys, Thin Air Brands, LLC (Colorado Springs, CO) (Fig. 1b).

Mosquitoes were found during all 9 inspections and during months of all 4 meteorological seasons (Table 1). Those mosquitoes collected during 2 inspections were identified to species (Darsie and Ward 2005; Ferreira-de-Freitas et al. 2020): December 10, 2020, 200 Cx. pipiens, 3 Cx. restuans Theobald, and 8 Culex spp.; June 7, 2022, 2 Cx. pipiens, 8 Cx. restuans, and 1 Aedes trivittatus Coquillett. In the remaining inspections all other mosquitoes were identified to genus as Culex spp. Of the 556 total mosquitoes collected, 105 (18.9%) were males. A small number of bloodfed females were observed and collected during the August 10, 2021, inspections (5 in the Melas pipe and 3 in the Sunset Meadows pipe) and 1 during the June 7, 2022, inspection. The results of these efforts provide further evidence that Culex spp. may routinely use belowground conveyance pipes as sites for refuge, diapause, and estivation.

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