Mosquito season in Whatcom County, WA: when it starts and ends
Mosquito months in Whatcom County
In a normal year mosquito season in Whatcom County, WA starts around April 25 and ends around October 14, about 25 weeks.
- Jan
- Feb
- Mar
- Apr
- May
- Jun
- Jul
- Aug
- Sep
- Oct
- Nov
- Dec
What to do in Whatcom County, and when
- Start tipping out standing water weekly by April 11.
- Book any yard treatment for late April, ahead of the August peak.
- Repellent can go away after October 14.
- 1 West Nile case since 2016 (last 2018): you should use repellent at dusk August, more so if over 60.
Whatcom County against the rest of WA
Whatcom County has the 18th longest season of 39 WA counties, at 172 days against 205 in Walla Walla County and 132 in Asotin County. Its mean temperature is 49.2°F.
- Thurston County: April 23 to October 18 (178 days, 1 WNV)
- Adams County: April 23 to October 16 (176 days, 0 WNV)
- Pierce County: April 28 to October 17 (172 days, 2 WNV)
- Grays Harbor County: April 29 to October 18 (172 days, 0 WNV)
West Nile in Whatcom County
From 1999 to 2025, CDC ArboNET counted 4 human West Nile cases in Whatcom County (1 neuroinvasive) across 3 years. The worst was 2015 with 2; the latest was 2018.
More: how the dates are worked out · when the season starts across the US.