Mosquito season in St. Clair County, IL: when it starts and ends
Mosquito months in St. Clair County
In a normal year mosquito season in St. Clair County, IL starts around March 31 and ends around October 30, about 30 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in St. Clair County, and when
- Start tipping out standing water weekly by March 17.
- Book any yard treatment for late March, ahead of the July peak.
- Repellent can go away after October 30.
- 18 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to August, and check IL spray notices.
St. Clair County against the rest of IL
St. Clair County has the 23rd longest season of 102 IL counties, at 213 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 57.4°F.
- Lawrence County: April 2 to November 1 (214 days, 0 WNV)
- Montgomery County: April 2 to November 2 (214 days, 0 WNV)
- Marion County: April 2 to November 1 (213 days, 3 WNV)
- Hamilton County: April 1 to October 30 (212 days, 0 WNV)
West Nile in St. Clair County
From 1999 to 2025, CDC ArboNET counted 61 human West Nile cases in St. Clair County (43 neuroinvasive) across 18 years. The worst was 2002 with 15; the latest was 2025.
See how the dates are worked out, and reading the West Nile counts.