Mosquito season in McHenry County, IL: when it starts and ends
Mosquito months in McHenry County
In a normal year mosquito season in McHenry County, IL starts around April 21 and ends around October 19, about 26 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in McHenry County, and when
- Start tipping out standing water weekly by April 7.
- Book any yard treatment for late April, ahead of the July peak.
- Repellent can go away after October 19.
- 11 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check IL spray notices.
McHenry County against the rest of IL
McHenry County has the 95th longest season of 102 IL counties, at 182 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 48.9°F.
- Lake County: April 22 to October 22 (183 days, 41 WNV)
- Lee County: April 20 to October 19 (182 days, 2 WNV)
- Whiteside County: April 19 to October 18 (181 days, 1 WNV)
- DeKalb County: April 20 to October 18 (180 days, 6 WNV)
West Nile in McHenry County
From 1999 to 2025, CDC ArboNET counted 40 human West Nile cases in McHenry County (23 neuroinvasive) across 16 years. The worst was 2012 with 6; the latest was 2024.
Background: reading the West Nile counts; how the dates are worked out.