Mosquito season in Marion County, IL: when it starts and ends
Mosquito months in Marion County
In a normal year mosquito season in Marion County, IL starts around April 2 and ends around November 1, about 30 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Marion County, and when
- Start tipping out standing water weekly by March 19.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after November 1.
- 3 West Nile cases since 2016 (last 2023): you should use repellent at dusk June to August, more so if over 60.
Marion County against the rest of IL
Marion County has the 24th longest season of 102 IL counties, at 213 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 54.9°F.
- Montgomery County: April 2 to November 2 (214 days, 0 WNV)
- St. Clair County: March 31 to October 30 (213 days, 18 WNV)
- Hamilton County: April 1 to October 30 (212 days, 0 WNV)
- Jefferson County: April 2 to October 31 (212 days, 1 WNV)
West Nile in Marion County
From 1999 to 2025, CDC ArboNET counted 9 human West Nile cases in Marion County (6 neuroinvasive) across 6 years. The worst was 2011 with 3; the latest was 2023.
Background: when the season starts across the US; how the dates are worked out.