Mosquito season in Jefferson County, IL: when it starts and ends
Mosquito months in Jefferson County
In a normal year mosquito season in Jefferson County, IL starts around April 2 and ends around October 31, about 30 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Jefferson 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 October 31.
- 1 West Nile case since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
Jefferson County against the rest of IL
Jefferson County has the 26th longest season of 102 IL counties, at 212 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 54.6°F.
- Marion County: April 2 to November 1 (213 days, 3 WNV)
- Hamilton County: April 1 to October 30 (212 days, 0 WNV)
- Johnson County: April 2 to October 31 (212 days, 0 WNV)
- Pope County: April 2 to October 31 (212 days, 0 WNV)
West Nile in Jefferson County
From 1999 to 2025, CDC ArboNET counted 6 human West Nile cases in Jefferson County (3 neuroinvasive) across 5 years. The worst was 2006 with 2; the latest was 2025.
See how the dates are worked out, and when the season starts across the US.