Mosquito season in Madison County, IL: when it starts and ends
Mosquito months in Madison County
In a normal year mosquito season in Madison County, IL starts around March 27 and ends around November 6, about 32 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Madison County, and when
- Start tipping out standing water weekly by March 13.
- Book any yard treatment for late March, ahead of the July peak.
- Repellent can go away after November 6.
- 9 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to September, more so if over 60.
Madison County against the rest of IL
Madison County has the 4th longest season of 102 IL counties, at 224 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 56.7°F.
- Saline County: March 26 to November 6 (225 days, 0 WNV)
- Gallatin County: March 26 to November 5 (224 days, 0 WNV)
- Union County: March 29 to November 8 (224 days, 0 WNV)
- Alexander County: March 26 to November 4 (223 days, 0 WNV)
West Nile in Madison County
From 1999 to 2025, CDC ArboNET counted 32 human West Nile cases in Madison County (21 neuroinvasive) across 10 years. The worst was 2002 with 14; the latest was 2025.
More: how the dates are worked out · reading the West Nile counts.