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

Mosquito months in DeKalb County

In a normal year mosquito season in DeKalb County, IL starts around April 20 and ends around October 18, about 26 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 DeKalb County, and when

• Start tipping out standing water weekly by April 6.
• Book any yard treatment for mid April, ahead of the July peak.
• Repellent can go away after October 18.
• 6 West Nile cases since 2016 (last 2025): you should use repellent at dusk July to August, more so if over 60.

DeKalb County against the rest of IL

DeKalb County has the 97th longest season of 102 IL counties, at 180 days against 232 in Pulaski County and 174 in Boone County. Its mean temperature is 48.4°F.

• McHenry County: April 21 to October 19 (182 days, 11 WNV)
• Whiteside County: April 19 to October 18 (181 days, 1 WNV)
• Jo Daviess County: April 22 to October 16 (177 days, 1 WNV)
• Stephenson County: April 22 to October 16 (177 days, 3 WNV)

West Nile in DeKalb County

From 1999 to 2025, CDC ArboNET counted 12 human West Nile cases in DeKalb County (5 neuroinvasive) across 7 years. The worst was 2025 with 3; the latest was 2025.

Background: reading the West Nile counts; how the dates are worked out.