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Pennsylvania · Mosquito Season Guide

Is it mosquito season in Pennsylvania?

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II. Pennsylvania Mosquito Season Month by month

Pennsylvania mosquitoes build through May, peak in July and August, and taper through September. The state straddles two distinct risk zones — the dense Philadelphia metro where Culex pipiens drives West Nile activity, and rural western and central counties where Eastern equine encephalitis is detected in surveillance traps most seasons.

Very Low Low Moderate High Very High taller = more active · color = severity

Chart reflects Philadelphia / SE Pennsylvania baseline. Mountain counties (Centre, Clearfield, Potter) typically have a shorter season — starting 2 weeks later and ending 2–3 weeks earlier — with lower overall peak.

~60
mosquito species recorded in Pennsylvania
May–Sep
typical active season for SE Pennsylvania
2
disease threats: West Nile virus and Eastern equine encephalitis
67
counties — all with seasonal mosquito surveillance

Philadelphia and the Delaware Valley: West Nile country

The Philadelphia metro — spanning Philadelphia, Montgomery, Bucks, Delaware, and Chester counties — is the densest Culex pipiens habitat in the state. Storm drains, catch basins, and container water across dense suburban and urban landscapes support enormous breeding populations. The Pennsylvania Department of Environmental Protection (DEP) and county vector control programs run surveillance from May through October; in high-activity years, aerial spray programs are activated in heavily impacted townships. WNV is detected in mosquito pools in the Philadelphia region annually.

Eastern equine encephalitis in the rural counties

EEE is a more serious concern west and north of the Appalachian ridges. The disease is rare in humans but has a fatality rate above 30% and can cause permanent neurological damage in survivors. Pennsylvania DEP surveillance has detected EEE in mosquito pools in rural counties — particularly in areas with forested wetlands and swamps where the primary vector, Culiseta melanura, breeds in tree holes and soil cavities. EEE-positive pools are reported most seasons in counties including Clinton, Centre, and Clearfield. When EEE activity is elevated, the state recommends avoiding outdoor activities around dawn and dusk, especially near wooded wetland areas.

The floodwater factor along the Susquehanna

Aedes vexans, the inland floodwater mosquito, is the most abundant species in rural and suburban Pennsylvania and responsible for most of the nuisance biting following heavy rain events. The Susquehanna, Delaware, and Lehigh river corridors support large floodwater populations that can spike dramatically within 5–7 days of flooding. After significant summer storms, residents throughout central and eastern Pennsylvania should expect elevated outdoor biting activity.

III. Pennsylvania's Three Species to Know
Culex pipiens (Northern house mosquito) macro photograph
Culex pipiens
Northern House Mosquito · ⚠ West Nile Vector

The dominant mosquito across the Philadelphia metro and other urban/suburban areas. Breeds in stagnant water — storm drains, catch basins, birdbaths, and neglected containers. Feeds primarily at dusk and overnight, preferring birds but shifting to humans in late summer. The primary WNV vector statewide; DEP monitors trap counts weekly from Philadelphia to Allegheny County as an early-warning system. Eliminating standing water in your yard reduces local populations measurably.

Aedes albopictus (Asian tiger mosquito) macro photograph
Aedes albopictus
Asian Tiger Mosquito

Well established across southeastern Pennsylvania and continuing to expand northward and westward. Visually distinctive: black with bold white stripes, smaller than Culex. A daytime biter — most aggressive in the morning and late afternoon — making it the species responsible for most of the daylight nuisance biting in Philadelphia, Pittsburgh, and their suburbs. Breeds in tiny volumes of water: bottle caps, saucers, rain gutters, any container larger than a teaspoon. The single most effective control: eliminate all standing water weekly.

Aedes vexans (inland floodwater mosquito) macro photograph
Aedes vexans
Inland Floodwater Mosquito

The most abundant mosquito in rural Pennsylvania and a significant nuisance across the state after rainfall. Eggs survive dry conditions in low-lying soil for years, hatching in massive numbers when flooded. Unlike Culex, it bites aggressively during daylight. Populations along the Susquehanna, Delaware, and Lehigh river corridors can explode within 5–7 days of a significant storm. Not a primary disease vector, but responsible for most of the miserable post-storm outdoor biting across central and eastern PA.

EEE is detected in Pennsylvania surveillance traps most seasons. While human cases are rare, the fatality rate exceeds 30%. When your county issues an EEE advisory — particularly in rural western and central PA — take dusk/dawn precautions seriously. Check current PA DEP mosquito surveillance reports →

IV. City-by-City Reference
City Active Season Off-Season Notes Live data
Philadelphia May – Sep Near-zero Oct–Apr Largest WNV surveillance program in the state; dense Culex pipiens and established Ae. albopictus populations; Fairmount Park and Wissahickon Creek corridor create significant breeding habitat; county vector control operates year-round trap monitoring Check live
Pittsburgh May – Sep Off Oct–Apr Three rivers confluence creates extensive floodwater habitat; Ae. vexans spikes significantly after heavy rain; Ae. albopictus expanding but less dense than Philadelphia; Allegheny County mosquito control runs active larviciding program Check live
Harrisburg May – Sep Off Oct–Apr Susquehanna River floodplain drives floodwater mosquito spikes; state capital — DEP surveillance data is benchmarked here; WNV detected in Dauphin County traps regularly; Culex pipiens and Ae. albopictus both present Check live
Allentown May – Sep Off Oct–Apr Lehigh Valley — river corridor habitat; Ae. albopictus well established in suburban neighborhoods; Lehigh River floodplain amplifies Ae. vexans numbers after storms; urban density favors container breeders Check live
Erie Jun – Aug Off Sep–May Shortest season in PA — Great Lakes influence moderates temperatures; Lake Erie shoreline wetlands provide spring breeding habitat; season peaks hard in July but is over by early September; Ae. albopictus present but less established than southern PA Check live
Reading May – Sep Off Oct–Apr Schuylkill River corridor; similar profile to Allentown; Berks County has active vector control program; Ae. albopictus and Culex pipiens dominant in residential areas; floodwater species spike after summer thunderstorms Check live
Scranton Jun – Sep Off Oct–May Higher elevation and latitude — season runs slightly shorter than Philadelphia; Lackawanna River corridor; WNV detected in Lackawanna County but human case rate historically lower than Philadelphia metro; Ae. albopictus present but expanding margin 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 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 Washington Western WA stays mild and damp year-round; the Yakima Valley and Columbia Basin face the highest WNV risk in the Pacific Northwest — peak July through September 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 →