Mosquito season in Cass County, IL: when it starts and ends
Mosquito months in Cass County
In a normal year mosquito season in Cass County, IL starts around April 8 and ends around October 27, about 29 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Cass County, and when
- Start tipping out standing water weekly by March 25.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after October 27.
- No West Nile cases since 2016, so bites in Cass County are a nuisance; check window screens.
Cass County against the rest of IL
Cass County has the 54th longest season of 102 IL counties, at 201 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 52.5°F.
- Brown County: April 8 to October 27 (202 days, 0 WNV)
- Morgan County: April 8 to October 27 (202 days, 1 WNV)
- Douglas County: April 8 to October 27 (201 days, 1 WNV)
- Hancock County: April 8 to October 27 (201 days, 0 WNV)
West Nile in Cass County
From 1999 to 2025, CDC ArboNET counted 1 human West Nile case in Cass County (0 neuroinvasive) across 1 year. The worst was 2015 with 1; the latest was 2015.
Background: when the season starts across the US; how the dates are worked out.