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Washington · Mosquito Season Guide

Is it mosquito season in Washington?

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II. Washington Mosquito Season Month by month

Washington is divided by the Cascades into two climatic zones with meaningfully different mosquito profiles. Western Washington — the rainy Puget Sound lowlands — runs a moderate June–September season. Eastern Washington's Columbia Basin is a different story: warm summers, extensive irrigation agriculture, and the Columbia River system create conditions where West Nile Virus circulates actively each year.

Western WA / Puget Sound
Seattle · Tacoma · Olympia · Bellingham
Eastern WA / Columbia Basin
Yakima · Spokane · Tri-Cities · Wenatchee
Very Low Low Moderate High Very High taller = more active · color = severity
~40
mosquito species recorded in Washington
2
distinct seasons divided by the Cascades
June–Aug
peak window statewide; longer in eastern WA
39
counties with seasonal exposure

Western Washington: the Puget Sound lowlands

Seattle, Tacoma, Bellevue, and the Puget Sound metro run a moderate season by national standards, typically June through September. The marine climate limits extreme heat and keeps standing water cooler, which slows larval development. The most active local areas are around wetlands, salmon streams in spawning season, and urban storm drainage.

Eastern Washington: the West Nile hotspot

Spokane, the Tri-Cities (Kennewick/Richland/Pasco), Yakima, and the Columbia Basin agricultural corridor run a meaningfully different season than the west side. Warm, dry summers and extensive irrigation agriculture create conditions highly favorable for Culex tarsalis — the primary WNV vector. Washington logs consistent WNV human cases each year, heavily concentrated east of the Cascades.

The Columbia River corridor

The Columbia River from the Tri-Cities to the Oregon border creates significant mosquito habitat year-round in its irrigation-fed agricultural lowlands. Quincy, Moses Lake, and the Columbia Basin Project area are among the most WNV-active zones in the western US. Residents near agricultural fields and irrigation canals should treat June–August protection as essential.

III. Washington's Three Species to Know
Culex tarsalis mosquito photograph
Culex tarsalis
Western Encephalitis Mosquito

The primary West Nile Virus vector in eastern Washington. Thrives in warm irrigated agriculture and breeds in nutrient-rich standing water — irrigation return flows, stock ponds, roadside ditches. WNV transmission risk is concentrated east of the Cascades, particularly in the Columbia Basin.

Culex pipiens northern house mosquito photograph
Culex pipiens
Northern House Mosquito

The dominant urban mosquito west of the Cascades. Breeds in stagnant water in gutters, catch basins, and urban drainage systems. The primary nuisance species in Seattle, Tacoma, and Bellevue — active June through September in residential neighborhoods.

Western floodwater mosquito photograph
Aedes increpitus
Western Floodwater Mosquito

A native floodwater species that hatches from snowmelt-flooded lowlands and river margins in spring. Responsible for the early-season surge in western Washington river valleys and some Puget Sound lowland areas. Less medically significant than Culex tarsalis but the dominant early-season nuisance species west of the Cascades.

IV. Washington Cities at a Glance
CityPeak SeasonOff-SeasonNotes
Seattle Jun – Sep Off Oct–May Puget Sound marine climate; moderate by national standards; urban storm drainage sustains Culex Check live
Tacoma Jun – Sep Off Oct–May Commencement Bay wetlands; similar to Seattle; Puyallup River valley adds floodwater habitat Check live
Spokane Jun – Sep Off Oct–May East side — warmer and drier; WNV detected in Spokane County most years; Spokane River corridor Check live
Yakima Jun – Aug Off Sep–May Columbia Basin agriculture; irrigation canals; WNV risk zone; shorter intense window Check live
Vancouver Jun – Sep Off Oct–May Columbia River corridor; similar to Portland; wetland habitat along the river drive local activity Check live
More State Guides
Florida Year-round activity in the south — 80+ species including invasive Ae. aegypti and the Asian tiger mosquito across all 67 counties Texas ~85 species across four climate zones — Gulf Coast active March through November, with year-round pressure in the Rio Grande Valley California Central Valley is the #1 West Nile corridor in the US — invasive Ae. aegypti and Ae. albopictus now established in 20+ counties Georgia Atlanta metro leads the Southeast in Asian tiger mosquito density — active statewide March through November New York Ground zero for West Nile in the US (1999 Queens outbreak) — NYC metro and Long Island see the highest Northeast seasonal pressure Illinois Home of the 2002 Cook County West Nile outbreak — the deadliest urban WNV event in US history — peak season June through September Ohio Lake Erie to the Ohio River — WNV active in Franklin and Cuyahoga counties, with floodwater species surging after summer storms Pennsylvania Philadelphia WNV corridor meets rural EEE risk in western PA — the densest human population in the mid-Atlantic mosquito belt Michigan Site of the 2019 EEE outbreak — the deadliest in decades — 10 human cases and 6 deaths concentrated in SW Michigan Great Lakes wetlands New Jersey Oldest organized county mosquito control in the US (1914) — Pine Barrens EEE corridor and fierce salt marsh Shore species Virginia Tidewater season runs April–October — the longest mid-Atlantic window — with the Great Dismal Swamp as an active EEE hotspot North Carolina The coastal plain runs one of the longest seasons in the Southeast — NC leads the nation in La Crosse encephalitis cases from the Appalachian foothills Maryland Chesapeake Bay watershed creates extraordinary mosquito habitat — tidal marshes, Eastern Shore wetlands, and the DC-Baltimore corridor combine for dense seasonal pressure Tennessee Memphis and the Mississippi River corridor historically record the highest WNV case counts in the mid-South — Ae. albopictus has colonized all 95 counties Indiana Indiana sits at the crossroads of the Great Lakes EEE belt — Lake Michigan wetlands in the north and the Wabash River floodplain drive intense seasonal pressure Louisiana The Gulf Coast's most intense pressure — coastal marshes and summer rains keep Cx. quinquefasciatus and Ae. aegypti active from March through November Alabama Mobile Bay to the Tennessee Valley — Ae. albopictus has reached all 67 counties and peak WNV season runs June through September South Carolina Low Country marshes produce some of the Southeast's densest Anopheles and Culex populations — season runs March through November Minnesota 10,000 lakes means 10,000 breeding sites — the Northwoods sees intense but short July–August peaks, while the Twin Cities face a longer June–September window Wisconsin Great Lakes wetlands and the Mississippi River backwaters drive strong WNV pressure in the Milwaukee corridor — peak season July through August Missouri At the confluence of the Missouri and Mississippi rivers — St. Louis historically ranks among the highest WNV cities in the US — season May through October Arkansas Delta rice-field flooding and the Arkansas River bottomlands create some of the highest rural mosquito pressure in the mid-South — active April through October Arizona Monsoon season (July–September) drives sudden mosquito explosions across the Valley of the Sun — WNV detected annually Colorado Front Range WNV corridor stretches Denver to Fort Collins — eastern plains carry the state's highest risk Connecticut Connecticut River valley WNV pressure meets EEE risk in inland freshwater wetlands — active May through October Delaware Chesapeake and Delaware Bay marshes create outsized pressure for the nation's second-smallest state Hawaii Year-round activity at sea level — no native species, but Ae. aegypti and tiger mosquitoes established statewide Idaho Snake River Plain irrigation districts fuel summer WNV pressure — season June through September Iowa Mississippi River corridor and prairie wetlands drive intense June–September pressure — WNV active statewide Kansas Great Plains WNV corridor — Wichita and Kansas City see peak pressure July–August; floodwater species surge after storms Kentucky Ohio River corridor and Kentucky Lake wetlands sustain a long April–October season across all 120 counties Maine Short but intense July–August season — EEE detected in some years in the southern coastal plain Massachusetts EEE fatality rate exceeds 30% — the Plymouth County swamp complex is the active epicenter — season May through October Mississippi Gulf Coast near-year-round pressure meets Delta rice-field flooding for one of the South's most intense mosquito environments Montana Yellowstone and Missouri River corridors drive a compressed June–August season — WNV detected most years in eastern MT Nebraska Platte River valley and eastern Nebraska are among the Great Plains' most active WNV zones — peak July–August Nevada Las Vegas monsoon season drives July–September activity; Reno irrigation districts fuel a parallel northern peak New Hampshire EEE detected most years — the southern lakes and coastal plain are in the active Northeast EEE corridor New Mexico Monsoon season (July–September) creates sudden breeding habitat; WNV circulates in the Rio Grande valley annually North Dakota Red River Valley flooding produces some of the Great Plains' most extreme floodwater mosquito events — peak June–August Oklahoma One of the South-Central's most intense seasons — the Arkansas and Red River corridors sustain populations April through October Oregon Eastern Oregon Columbia Basin is the state's WNV hotspot; western Oregon runs a mild coastal season June through September Rhode Island New England's densest coastal pressure — Narragansett Bay salt marshes and WNV active in all five counties South Dakota Missouri River and James River valleys drive summer WNV activity — eastern SD sees the most intense pressure Utah Salt Lake Valley WNV corridor and irrigation districts sustain a July–September peak — drier areas see limited activity Vermont Champlain Valley runs Vermont's longest season — EEE detected in the Lake Champlain lowlands in some years West Virginia Kanawha and Ohio River valleys sustain a May–October season — Ae. albopictus now established statewide Wyoming The shortest meaningful season of any contiguous US state — North Platte valley WNV window is just 6–8 weeks
West Nile, dengue, and EEE: US disease risk by region → Asian tiger mosquitoes: why you're getting bitten during the day → Eliminating standing water: the highest-leverage yard move →