Mosquito season in Cook County, IL: when it starts and ends
Mosquito months in Cook County
In a normal year mosquito season in Cook County, IL starts around April 20 and ends around October 26, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Cook County, and when
- Start tipping out standing water weekly by April 6.
- Book any yard treatment for mid April, ahead of the July peak.
- Repellent can go away after October 26.
- 532 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check IL spray notices.
Cook County against the rest of IL
Cook County has the 87th longest season of 102 IL counties, at 189 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 51°F.
- Mercer County: April 15 to October 21 (189 days, 1 WNV)
- Stark County: April 15 to October 21 (189 days, 0 WNV)
- Bureau County: April 15 to October 20 (188 days, 0 WNV)
- DuPage County: April 18 to October 22 (187 days, 99 WNV)
West Nile in Cook County
From 1999 to 2025, CDC ArboNET counted 1,811 human West Nile cases in Cook County (1,245 neuroinvasive) across 24 years. The worst was 2002 with 635; the latest was 2025.
See how the dates are worked out, and reading the West Nile counts.