Mosquito season in Washington County, ID: when it starts and ends

Mosquito months in Washington County

In a normal year mosquito season in Washington County, ID starts around April 28 and ends around October 7, about 23 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 Washington County, and when

• Start tipping out standing water weekly by April 14.
• Book any yard treatment for late April, ahead of the July peak.
• Repellent can go away after October 7.
• 19 West Nile cases since 2016: you should wear EPA-registered repellent at dusk July to August, and check ID spray notices.

Washington County against the rest of ID

Washington County has the 12th longest season of 44 ID counties, at 162 days against 194 in Canyon County and 85 in Fremont County. Its mean temperature is 49.2°F.

• Bingham County: April 28 to October 9 (164 days, 2 WNV)
• Franklin County: April 30 to October 10 (163 days, 2 WNV)
• Minidoka County: May 2 to October 11 (162 days, 0 WNV)
• Cassia County: May 3 to October 11 (161 days, 1 WNV)

West Nile in Washington County

From 1999 to 2025, CDC ArboNET counted 63 human West Nile cases in Washington County (18 neuroinvasive) across 15 years. The worst was 2006 with 21; the latest was 2025.

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