Mosquito season in Hamilton County, IL: when it starts and ends
Mosquito months in Hamilton County
In a normal year mosquito season in Hamilton County, IL starts around April 1 and ends around October 30, about 30 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Hamilton County, and when
- Start tipping out standing water weekly by March 18.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after October 30.
- No West Nile cases since 2016, so bites in Hamilton County are a nuisance; check window screens.
Hamilton County against the rest of IL
Hamilton County has the 25th longest season of 102 IL counties, at 212 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 55.4°F.
- St. Clair County: March 31 to October 30 (213 days, 18 WNV)
- Marion County: April 2 to November 1 (213 days, 3 WNV)
- Jefferson County: April 2 to October 31 (212 days, 1 WNV)
- Johnson County: April 2 to October 31 (212 days, 0 WNV)
West Nile in Hamilton County
CDC ArboNET lists no human West Nile cases for Hamilton County from 1999 to 2025, though virus turned up in birds, mosquitoes or horses in 5 of those years.
More: how the dates are worked out · when the season starts across the US.