Mosquito season in Williamson County, IL: when it starts and ends

Mosquito months in Williamson County

In a normal year mosquito season in Williamson County, IL starts around March 29 and ends around November 1, about 31 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Williamson County, and when

• Start tipping out standing water weekly by March 15.
• Book any yard treatment for late March, ahead of the July peak.
• Repellent can go away after November 1.
• 3 West Nile cases since 2016 (last 2021): you should use repellent at dusk June to August, more so if over 60.

Williamson County against the rest of IL

Williamson County has the 15th longest season of 102 IL counties, at 217 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 56.2°F.

• Clinton County: March 30 to November 3 (219 days, 2 WNV)
• Monroe County: March 31 to November 4 (218 days, 1 WNV)
• Jackson County: March 30 to November 1 (216 days, 3 WNV)
• Clay County: March 31 to November 2 (216 days, 0 WNV)

West Nile in Williamson County

From 1999 to 2025, CDC ArboNET counted 11 human West Nile cases in Williamson County (7 neuroinvasive) across 8 years. The worst was 2013 with 3; the latest was 2021.

See how the dates are worked out, and when mosquitoes go away for the winter.