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New Hampshire · Mosquito Season Guide

Is it mosquito season in New Hampshire?

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II. New Hampshire Mosquito Season Month by month

New Hampshire is small but carries real mosquito-borne disease risk — particularly from Eastern Equine Encephalitis in the southeastern swamp complexes shared with Massachusetts. The state's season is compressed into a June–August window by its northern latitude, but within that window, the pressure in the Manchester-Nashua corridor and the seacoast communities can be significant.

Southern NH / Manchester-Nashua
Manchester · Nashua · Salem
Seacoast / Lakes Region
Portsmouth · Dover · Laconia
Very Low Low Moderate High Very High taller = more active · color = severity
~40
mosquito species recorded in New Hampshire
Jun–Aug
peak season statewide
EEE
detected in NH most years in high-risk swamps
10
counties — Rockingham and Hillsborough carry highest risk

Southern New Hampshire: the EEE and WNV zone

Rockingham and Hillsborough counties share the EEE belt that runs through southeastern Massachusetts. The Nashua River and Merrimack River corridors, along with the extensive swamp complexes of southern Rockingham County, provide habitat for Culiseta melanura and the Aedes species that bridge EEE to humans. West Nile Virus is detected in mosquito pools across the Manchester-Nashua metro most summers.

The seacoast: coastal pressure

The Great Bay Estuary and the seacoast communities — Portsmouth, Dover, Durham — experience moderate mosquito pressure driven by the tidal marshes and freshwater wetlands of the Piscataqua River watershed. EEE has been detected in Strafford County. The University of New Hampshire campus in Durham sits within a high-risk zone for both EEE bridge species.

The Lakes Region and north country: brief intensity

Lake Winnipesaukee and the Lakes Region see intense but very short pressure centered on July, when snowmelt-fed wetlands and warm summer temperatures briefly create ideal conditions. Above the White Mountains, the season is compressed to 4–6 weeks of meaningful activity, primarily from Aedes vexans and Aedes canadensis in boreal wetlands.

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

Manchester and Nashua sit at the southern end of a genuine WNV corridor that runs up the Merrimack River valley. Culex pipiens breeds in the storm drain systems and urban standing water of New Hampshire's two largest cities, with surveillance detections in most active years. Hillsborough County's human WNV case totals are modest in absolute numbers but consistent — the Merrimack River wetlands and suburban catch basins provide reliable breeding habitat through July and September.

Aedes japonicus mosquito photograph
Aedes japonicus
Asian Rock Pool Mosquito

New Hampshire's most epidemiologically significant mosquito — not for WNV but for EEE. Aedes japonicus breeds in water-filled tree holes, rock pools, and shaded containers in exactly the woodland edge habitat that defines southern New Hampshire's landscape. Unlike Culex species, it bites throughout the day, increasing human exposure in the forested settings where EEE circulates. Its presence in Rockingham County swamp complexes shared with the Massachusetts EEE belt makes it the key bridge vector in the state's highest-consequence disease risk.

Aedes vexans mosquito photograph
Aedes vexans
Floodwater Mosquito

The reason New Hampshire's June feels like it has more mosquitoes than the season warrants. Aedes vexans eggs overwinter in the Merrimack, Soucook, and Contoocook River floodplains and hatch in synchronized surges following spring flooding. The Lakes Region sees the most intense events — Lake Winnipesaukee's shallow marsh edges and the wetlands north of Concord produce peaks in late June and early July that drive the state's highest nuisance complaints before populations stabilize.

IV. New Hampshire Cities at a Glance
CityPeak SeasonOff-SeasonNotes
Manchester May – Sep Off Oct–Apr Merrimack River; highest WNV pressure in state; Hillsborough County EEE zone Check live
Nashua May – Sep Off Oct–Apr Nashua River; Merrimack confluence; shares EEE corridor with Massachusetts border Check live
Concord May – Aug Off Sep–Apr Merrimack River; capital area; shorter effective season than southern NH Check live
Dover May – Sep Off Oct–Apr Cochecho River; Great Bay wetlands; Strafford County EEE detected some years Check live
Portsmouth May – Sep Off Oct–Apr Piscataqua River; Great Bay Estuary; tidal marsh pressure; EEE risk area 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 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 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 →