Mosquito season in Jefferson County, ID: when it starts and ends
Mosquito months in Jefferson County
In a normal year mosquito season in Jefferson County, ID starts around May 14 and ends around September 21, about 19 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Jefferson County, and when
- Start tipping out standing water weekly by April 30.
- Book any yard treatment for mid May, ahead of the July peak.
- Repellent can go away after September 21.
- No West Nile cases since 2016, so bites in Jefferson County are a nuisance; check window screens.
Jefferson County against the rest of ID
Jefferson County has the 29th longest season of 44 ID counties, at 130 days against 194 in Canyon County and 85 in Fremont County. Its mean temperature is 43.7°F.
- Clark County: May 10 to September 30 (143 days, 0 WNV)
- Butte County: May 13 to October 1 (141 days, 0 WNV)
- Boise County: May 21 to September 27 (129 days, 0 WNV)
- Bonneville County: May 19 to September 23 (127 days, 0 WNV)
West Nile in Jefferson County
From 1999 to 2025, CDC ArboNET counted 15 human West Nile cases in Jefferson County (3 neuroinvasive) across 2 years. The worst was 2006 with 14; the latest was 2007.
More: how the dates are worked out · when the season starts across the US.