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

Mosquito months in Winnebago County

In a normal year mosquito season in Winnebago County, IL starts around April 23 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 Winnebago County, and when

• Start tipping out standing water weekly by April 9.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 16.
• 11 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check IL spray notices.

Winnebago County against the rest of IL

Winnebago County has the 100th 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.

• Jo Daviess County: April 22 to October 16 (177 days, 1 WNV)
• Stephenson County: April 22 to October 16 (177 days, 3 WNV)
• Carroll County: April 23 to October 16 (176 days, 2 WNV)
• Boone County: April 24 to October 15 (174 days, 2 WNV)

West Nile in Winnebago County

From 1999 to 2025, CDC ArboNET counted 22 human West Nile cases in Winnebago County (14 neuroinvasive) across 12 years. The worst was 2024 with 4; the latest was 2025.

Background: reading the West Nile counts; how the dates are worked out.