Mosquito season in Hamilton County, OH: when it starts and ends
Mosquito months in Hamilton County
In a normal year mosquito season in Hamilton County, OH starts around April 5 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 22.
- Book any yard treatment for early April, ahead of the July peak.
- Repellent can go away after October 30.
- 13 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to August, and check OH spray notices.
Hamilton County against the rest of OH
Hamilton County has the 4th longest season of 88 OH counties, at 208 days against 209 in Gallia County and 177 in Ashtabula County. Its mean temperature is 53.8°F.
- Pickaway County: April 5 to October 31 (209 days, 1 WNV)
- Greene County: April 5 to October 30 (208 days, 1 WNV)
- Washington County: April 6 to October 29 (207 days, 0 WNV)
- Scioto County: April 7 to October 31 (207 days, 0 WNV)
West Nile in Hamilton County
From 1999 to 2025, CDC ArboNET counted 76 human West Nile cases in Hamilton County (61 neuroinvasive) across 16 years. The worst was 2002 with 30; the latest was 2025.
See how the dates are worked out, and reading the West Nile counts.