Mosquito season in Mercer County, IL: when it starts and ends
Mosquito months in Mercer County
In a normal year mosquito season in Mercer County, IL starts around April 15 and ends around October 21, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Mercer 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 21.
- 1 West Nile case since 2016 (last 2016): you should use repellent at dusk June to August, more so if over 60.
Mercer County against the rest of IL
Mercer County has the 85th longest season of 102 IL counties, at 189 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 50°F.
- Knox County: April 14 to October 21 (190 days, 1 WNV)
- Will County: April 16 to October 23 (190 days, 34 WNV)
- Stark County: April 15 to October 21 (189 days, 0 WNV)
- Cook County: April 20 to October 26 (189 days, 532 WNV)
West Nile in Mercer County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Mercer County (0 neuroinvasive) across 1 year. The worst was 2016 with 1; the latest was 2016.
Background: when the season starts across the US; how the dates are worked out.