Mosquito season in Grundy County, IL: when it starts and ends
Mosquito months in Grundy County
In a normal year mosquito season in Grundy County, IL starts around April 15 and ends around October 23, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Grundy County, and when
- Start tipping out standing water weekly by April 1.
- Book any yard treatment for mid April, ahead of the July peak.
- Repellent can go away after October 23.
- 2 West Nile cases since 2016 (last 2020): you should use repellent at dusk June to August, more so if over 60.
Grundy County against the rest of IL
Grundy County has the 80th longest season of 102 IL counties, at 192 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 50.7°F.
- Warren County: April 13 to October 22 (192 days, 1 WNV)
- McLean County: April 14 to October 23 (192 days, 4 WNV)
- Henry County: April 14 to October 22 (191 days, 0 WNV)
- LaSalle County: April 14 to October 22 (191 days, 4 WNV)
West Nile in Grundy County
From 1999 to 2025, CDC ArboNET counted 2 human West Nile cases in Grundy County (2 neuroinvasive) across 2 years. The worst was 2020 with 1; the latest was 2020.
See how the dates are worked out, and when mosquitoes go away for the winter.