Mosquito season in O'Brien County, IA: when it starts and ends
Mosquito months in O'Brien County
In a normal year mosquito season in O'Brien County, IA starts around April 18 and ends around October 18, about 26 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in O'Brien County, and when
- Start tipping out standing water weekly by April 4.
- Book any yard treatment for mid April, ahead of the July peak.
- Repellent can go away after October 18.
- 5 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
O'Brien County against the rest of IA
O'Brien County has the 41st longest season of 99 IA counties, at 182 days against 198 in Des Moines County and 160 in Worth County. Its mean temperature is 48.2°F.
- Decatur County: April 19 to October 19 (183 days, 0 WNV)
- Boone County: April 18 to October 18 (182 days, 3 WNV)
- Harrison County: April 19 to October 18 (182 days, 7 WNV)
- Iowa County: April 19 to October 17 (181 days, 0 WNV)
West Nile in O'Brien County
From 1999 to 2025, CDC ArboNET counted 10 human West Nile cases in O'Brien County (2 neuroinvasive) across 7 years. The worst was 2025 with 2; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.