Mosquito season in LaSalle County, IL: when it starts and ends
Mosquito months in LaSalle County
In a normal year mosquito season in LaSalle County, IL starts around April 14 and ends around October 22, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in LaSalle County, and when
- Start tipping out standing water weekly by March 31.
- Book any yard treatment for mid April, ahead of the July peak.
- Repellent can go away after October 22.
- 4 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
LaSalle County against the rest of IL
LaSalle County has the 82nd longest season of 102 IL counties, at 191 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 50.1°F.
- Grundy County: April 15 to October 23 (192 days, 2 WNV)
- Henry County: April 14 to October 22 (191 days, 0 WNV)
- Knox County: April 14 to October 21 (190 days, 1 WNV)
- Will County: April 16 to October 23 (190 days, 34 WNV)
West Nile in LaSalle County
From 1999 to 2025, CDC ArboNET counted 27 human West Nile cases in LaSalle County (16 neuroinvasive) across 12 years. The worst was 2002 with 11; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.