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Massachusetts · Mosquito Season Guide

Is it mosquito season in Massachusetts?

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II. Massachusetts Mosquito Season Month by month

Massachusetts carries more mosquito-borne disease risk per square mile than almost any other northeastern state. The southeastern corner — Plymouth, Bristol, and Norfolk counties — is ground zero for the EEE corridor that extends from Florida to Maine. The 2019 outbreak was devastating: Massachusetts alone recorded 12 human cases and 6 deaths, prompting aerial spraying and school closures. At the same time, West Nile Virus circulates annually in the Boston metro.

Boston Metro / North Shore
Boston · Cambridge · Lowell
Southeast MA / Cape Cod
Plymouth · Brockton · New Bedford
Very Low Low Moderate High Very High taller = more active · color = severity
EEE
case fatality rate exceeds 30% — deadliest US mosquito-borne disease
~50
mosquito species recorded in Massachusetts
2019
worst EEE outbreak in decades — 12 MA cases, 6 deaths
May–Oct
active season in the Boston metro

The EEE corridor: southeastern Massachusetts

Plymouth County is consistently identified as the highest-risk area for EEE in Massachusetts and among the highest in the US. The vast freshwater swamp complexes in the Plymouth pine barrens — including the Hockomock Swamp, one of the largest wetlands in New England — provide ideal habitat for Culiseta melanura, the bird-biting EEE amplification vector. Bristol and Norfolk counties are also classified as high-risk zones most years.

Boston metro: West Nile hub

The Boston metro — Suffolk, Middlesex, and Essex counties — experiences consistent West Nile Virus circulation driven by Culex pipiens in urban and suburban storm drain systems. The Charles River, Mystic River, and the network of ponds and reservoirs throughout the metro support significant Culex populations. WNV is detected in mosquito pools in the Boston area virtually every summer.

Western Massachusetts: lower risk, still active

The Connecticut River valley (Springfield, Holyoke, Northampton) and the Berkshires experience a more typical Northeast season — active June through September but with lower WNV case totals and generally lower EEE risk than the southeast. The Pioneer Valley's agricultural areas produce Aedes vexans floodwater surges after spring rains.

III. Massachusetts's Key Mosquito Species
Culex pipiens mosquito photograph
Culex pipiens
Northern House Mosquito

Boston's dense urban drainage network — catch basins, storm drain outfalls, and the slow-moving ditches throughout the metro — functions as a Culex pipiens nursery through July and August. The species drives WNV transmission in eastern Massachusetts, with Boston, Worcester, and Middlesex County surveillance zones recording the most positive mosquito pools annually. A strictly nocturnal biter that doesn't range far from its breeding water, high concentrations cluster in neighborhoods with older drainage infrastructure.

Aedes japonicus mosquito photograph
Aedes japonicus
Asian Rock Pool Mosquito

The Japanese rock pool mosquito is Massachusetts' most important EEE bridge vector — the species that carries the virus from its bird reservoir into human territory. It breeds in rock pools, tree holes, and shaded woodland containers throughout the southeastern high-risk corridor and bites during the day. Its presence in the Plymouth County swamp complex and adjacent suburbs is why MDPH issues the earliest EEE risk advisories in the country.

Aedes vexans mosquito photograph
Aedes vexans
Floodwater Mosquito

Spring snowmelt and the heavy June rains that saturate the Connecticut River and Merrimack valleys produce mass emergence events that Ae. vexans is built to exploit — larvae hatch within days of a flood pulse and adults disperse 10+ miles from breeding sites on favorable winds. The flood mosquito accounts for the sudden, statewide biting waves that materialize after wet periods in June, often before WNV season has properly begun. No meaningful disease vector role, but responsible for the most intense nuisance biting of the Massachusetts season.

IV. Massachusetts Cities at a Glance
CityPeak SeasonOff-SeasonNotes
Boston May – Oct Off Nov–Apr Urban WNV corridor; Charles River; Suffolk County WNV active most years Check live
Worcester May – Sep Off Oct–Apr Central MA; Blackstone River watershed; above-average EEE risk vs. Boston Check live
Springfield May – Sep Off Oct–Apr Connecticut River valley; agricultural floodwater pressure; lower EEE risk than SE Check live
Cambridge May – Oct Off Nov–Apr Part of Boston metro; Charles River; dense Culex pipiens urban population Check live
Lowell May – Sep Off Oct–Apr Merrimack River; floodwater surges; Middlesex County WNV pressure 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 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 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 →