Mosquito season in Stephenson County, IL: when it starts and ends

Mosquito months in Stephenson County

In a normal year mosquito season in Stephenson County, IL starts around April 22 and ends around October 16, about 25 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Stephenson County, and when

• Start tipping out standing water weekly by April 8.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 16.
• 3 West Nile cases since 2016 (last 2023): you should use repellent at dusk July to August, more so if over 60.

Stephenson County against the rest of IL

Stephenson County has the 99th longest season of 102 IL counties, at 177 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 47.7°F.

• DeKalb County: April 20 to October 18 (180 days, 6 WNV)
• Jo Daviess County: April 22 to October 16 (177 days, 1 WNV)
• Winnebago County: April 23 to October 16 (177 days, 11 WNV)
• Carroll County: April 23 to October 16 (176 days, 2 WNV)

West Nile in Stephenson County

From 1999 to 2025, CDC ArboNET counted 7 human West Nile cases in Stephenson County (5 neuroinvasive) across 6 years. The worst was 2018 with 2; the latest was 2023.

More: how the dates are worked out · when mosquitoes go away for the winter.