Mosquito season in Clermont County, OH: when it starts and ends

Mosquito months in Clermont County

In a normal year mosquito season in Clermont County, OH starts around April 11 and ends around October 27, about 28 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 Clermont County, and when

• Start tipping out standing water weekly by March 28.
• Book any yard treatment for mid April, ahead of the July peak.
• Repellent can go away after October 27.
• 6 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.

Clermont County against the rest of OH

Clermont County has the 16th longest season of 88 OH counties, at 199 days against 209 in Gallia County and 177 in Ashtabula County. Its mean temperature is 54.4°F.

• Delaware County: April 9 to October 28 (202 days, 0 WNV)
• Pike County: April 9 to October 28 (202 days, 0 WNV)
• Hancock County: April 12 to October 27 (199 days, 4 WNV)
• Jackson County: April 9 to October 24 (198 days, 0 WNV)

West Nile in Clermont County

From 1999 to 2025, CDC ArboNET counted 22 human West Nile cases in Clermont County (13 neuroinvasive) across 11 years. The worst was 2012 with 5; the latest was 2025.

See how the dates are worked out, and reading the West Nile counts.