Mosquito season in Sangamon County, IL: when it starts and ends
Mosquito months in Sangamon County
In a normal year mosquito season in Sangamon County, IL starts around April 7 and ends around October 28, about 29 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Sangamon County, and when
- Start tipping out standing water weekly by March 24.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after October 28.
- 8 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
Sangamon County against the rest of IL
Sangamon County has the 49th longest season of 102 IL counties, at 204 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 53.1°F.
- Adams County: April 6 to October 27 (204 days, 4 WNV)
- Greene County: April 7 to October 28 (204 days, 0 WNV)
- Scott County: April 7 to October 27 (203 days, 1 WNV)
- Vermilion County: April 8 to October 28 (203 days, 1 WNV)
West Nile in Sangamon County
From 1999 to 2025, CDC ArboNET counted 38 human West Nile cases in Sangamon County (26 neuroinvasive) across 15 years. The worst was 2002 with 13; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.