Mosquito season in Johnson County, IA: when it starts and ends
Mosquito months in Johnson County
In a normal year mosquito season in Johnson County, IA starts around April 13 and ends around October 21, about 27 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Johnson County, and when
- Start tipping out standing water weekly by March 30.
- Book any yard treatment for mid April, ahead of the July peak.
- Repellent can go away after October 21.
- 7 West Nile cases since 2016 (last 2025): you should use repellent at dusk June to August, more so if over 60.
Johnson County against the rest of IA
Johnson County has the 6th longest season of 99 IA counties, at 191 days against 198 in Des Moines County and 160 in Worth County. Its mean temperature is 50.3°F.
- Fremont County: April 13 to October 23 (193 days, 1 WNV)
- Wapello County: April 13 to October 22 (192 days, 0 WNV)
- Mills County: April 13 to October 21 (191 days, 3 WNV)
- Muscatine County: April 14 to October 22 (191 days, 0 WNV)
West Nile in Johnson County
From 1999 to 2025, CDC ArboNET counted 9 human West Nile cases in Johnson County (6 neuroinvasive) across 7 years. The worst was 2018 with 2; the latest was 2025.
Background: reading the West Nile counts; how the dates are worked out.