When does mosquito season start in NH?

Mosquito season in New Hampshire begins as temperatures rise in April, when overwintering eggs hatch and dormant adult females become active again. Activity builds through May and reaches its most active stretch from June through October, according to NH DHHS. Early-season mosquitoes are mostly a nuisance, but disease risk climbs as the season progresses — Jamestown Canyon virus circulates earliest, in spring and early summer, while EEE and West Nile virus risk build through summer as the viruses amplify in local bird populations. The season runs until the first hard frost, typically mid-to-late October in the Manchester area.

When is EEE/West Nile risk highest in NH?

EEE and West Nile virus risk in New Hampshire is highest from August until the first hard frost, according to NH DHHS. Both viruses circulate all summer in a bird-to-mosquito cycle, intensifying as more birds and mosquitoes become infected. By August, 'bridge' mosquito species that feed on both birds and mammals start spilling the viruses over to people and horses — exactly when NH's 2024 fatal human EEE case in Hampstead was diagnosed. Former state epidemiologist Dr. Benjamin Chan has noted the risk continues into fall until a hard frost kills the mosquito population outright.

When does mosquito season end in NH?

Mosquito season in New Hampshire ends at the first hard frost, which kills off the adult mosquito population outright. In Manchester, the average first hard frost falls around October 19, based on NOAA's 1991–2020 climate normals. That date is only an average — about a 30% chance of frost arrives before it, so a cooler-than-usual autumn can end mosquito activity earlier, while a warm fall can push biting and disease risk later into October or even November in some years.

What happened with EEE in NH in 2024?

In 2024, New Hampshire recorded a serious Eastern equine encephalitis event: 2 human cases and 1 death. A Hampstead adult died after contracting EEE and a second adult from Kensington was hospitalized but survived — the state's first human cases since 2014, according to NH DHHS and local news reporting. The same season also produced one confirmed horse case and seven mosquito batches that tested positive for the virus statewide. The fatal case was hospitalized in mid-August and died later that month, squarely inside the August-to-frost window when EEE risk peaks in New Hampshire. It stands as the clearest recent proof that late-summer mosquito-borne disease risk in southern NH is real, not theoretical.

Eastern equine encephalitis is one of the most severe mosquito-borne diseases in the United States. According to the CDC, approximately 30% of people who develop severe EEE die, and many survivors are left with ongoing neurologic problems. There is no human vaccine and no specific antiviral treatment for EEE — care is limited to supportive treatment of symptoms. Because of this severity, anyone with high fever, severe headache, confusion, or neck stiffness after mosquito exposure in an EEE-active area should contact a physician immediately rather than waiting to see if symptoms resolve.

What time of day do mosquitoes bite most?

Mosquitoes in New Hampshire bite most during dawn and dusk, the low-light hours when many species are most active in search of a blood meal, according to NH DHHS. This pattern means the highest-risk times for outdoor activities like yard work, cookouts, or evening walks are the hour or two around sunrise and sunset. Some species will bite throughout the day in shaded, humid areas, especially in wooded yards, but dawn and dusk consistently see the heaviest activity. Wearing long sleeves and an EPA-registered repellent during these windows meaningfully cuts exposure during peak season.

How little water do mosquitoes need to breed?

Mosquitoes need surprisingly little water to complete their life cycle. According to NH DHHS and the CDC, any puddle or container that holds standing water for more than about four days is enough for eggs to hatch and develop into biting adults — even a bottle cap, a clogged gutter, or a forgotten toy in the yard. The full cycle from egg to adult takes roughly 7 to 10 days in warm weather. That's why a weekly 'water walk' to dump birdbaths, buckets, tarps, and other containers is one of the most effective things a homeowner can do.

When should I start mosquito treatment?

The most effective time to start mosquito treatment in New Hampshire is spring — April into May — before populations build toward their June–October peak. This is when homeowners should do a weekly water walk to eliminate standing water and, if desired, begin a professional barrier treatment, which typically suppresses adult mosquitoes for roughly three to six weeks per application (industry-survey estimate). Starting early keeps the yard protected through the season's biggest population growth rather than playing catch-up once mosquitoes — and disease risk — are already climbing in July and August.

Are mosquitoes still dangerous in the fall?

Yes. Cooling fall weather does not mean mosquito-borne disease risk has passed. EEE and West Nile virus risk actually remains highest from August through the first hard frost, which averages around October 19 in Manchester, according to NH DHHS. Mosquitoes stay active on mild fall days even as nighttime temperatures drop, and the viruses continue circulating in birds and mosquitoes until a true hard frost kills the population. Homeowners should keep up dawn/dusk precautions and repellent use through September and October rather than assuming the risk ends once summer calendar dates pass.

Is there a mosquito vaccine for EEE?

No. There is currently no vaccine approved for human use against Eastern equine encephalitis, and no specific antiviral treatment exists once someone is infected, according to the CDC. Care for EEE is limited to supportive treatment of symptoms in a hospital setting. This makes prevention — eliminating standing water, using EPA-registered repellent, wearing protective clothing at dawn and dusk, and reducing mosquito populations around the home — the only real defense against the virus. Anyone with symptoms suggestive of EEE after mosquito exposure should see a physician immediately rather than waiting.

Daniel Brady · NH-Licensed Pesticide Applicator · License #782664

|Reviewed by Daniel Brady

When Is Mosquito Season in NH? Timing & Risk Guide

Seasonal Biters & Stingers

Mosquitoes become active in NH as temperatures rise in April and are most active June through October (NH DHHS). They bite most at dawn and dusk. Disease risk from Eastern equine encephalitis (EEE) and West Nile virus (WNV) amplifies through the summer in birds and peaks from August until the first hard frost, which averages October 19 in Manchester (NOAA). The season ends when frost kills adult mosquitoes; warm autumns push that later.

At a glance

NH mosquito season: April–OctoberMost active June–October; begins as temperatures rise in April — NH DHHS
Peak disease window: August until first frostEEE/WNV amplify in birds all summer, then spill over — NH DHHS; Dr. Chan, 2024
Season-ending frost: ~Oct 19 (Manchester)Hard frost kills adult mosquitoes; warm falls extend the season — NOAA 1991–2020
Breeding minimum: Any puddle standing more than 4 daysEgg-to-adult cycle runs 7–10 days in warm weather — NH DHHS / CDC

NH mosquito season
April–October
Most active June–October; begins as temperatures rise in April — NH DHHS
Peak disease window
August until first frost
EEE/WNV amplify in birds all summer, then spill over — NH DHHS; Dr. Chan, 2024
Season-ending frost
~Oct 19 (Manchester)
Hard frost kills adult mosquitoes; warm falls extend the season — NOAA 1991–2020
Breeding minimum
Any puddle standing more than 4 days
Egg-to-adult cycle runs 7–10 days in warm weather — NH DHHS / CDC

The timing answer

The short of it

The verdict · seasonal biters & stingers

Peak season now — biting + disease risk

Mosquito season in NH runs April–October; you're in the thick of it now, and EEE/West Nile risk climbs from August until the first hard frost (~Oct 19 in Manchester).

NH activity calendar

When are they active — the whole year at a glance

Mosquitoes in southern New Hampshire go from fully dormant in winter to peak biting and peak disease risk by mid-summer, collapsing again at the first hard frost.

12-month activity · New Hampshire

Peak · Jul, Aug, SepYou are here · Aug

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

PeakActiveBuildingDecliningDormantCurrent season · Summer

Best window to act · April–May — do a weekly property 'water walk' (dump birdbaths, buckets, tires, clogged gutters, tarps) and start a professional barrier treatment before populations peak. A barrier spray suppresses adults roughly 3–6 weeks per application (industry-survey estimate).

What else peaks thenJul Aug Sep

Peak months reflect peak biting AND peak EEE/WNV risk, ending at Manchester's first hard frost (Oct 19, 50% probability — NOAA 1991–2020).

Season by season

A year in the life

Mosquito risk in New Hampshire follows a clear arc from dormant winter to a dangerous late-summer peak.

01
Spring
Overwintering eggs and adults activate as temperatures rise in April. This is the best window to eliminate standing water and start barrier treatments before populations build. Jamestown Canyon virus (JCV) risk begins to rise earliest, in spring and early summer, ahead of EEE/WNV.
02
Summer
Biting pressure peaks, with dawn and dusk the highest-activity periods. EEE and West Nile virus amplify in bird populations all summer, and by August spillover risk to people and horses is real — NH's fatal 2024 Hampstead case was hospitalized in mid-August.
03
Fall
The most dangerous stretch for EEE/WNV persists until the first hard frost, typically mid-to-late October in Manchester. Cooling weather does not mean the danger has passed — keep dawn/dusk precautions through September and October.
04
Winter
Mosquitoes are fully dormant. Culex species overwinter as mated adults in sheltered spots like culverts and cellars, while floodwater species overwinter as cold-hardy eggs that hatch with spring flooding.

New Hampshire context

Mosquito season in southern New Hampshire

Southern NH's ponds, wetlands, and hardwood swamps make mosquitoes a fixture from April through the first frost. The state saw a severe 2024 season — 2 human EEE cases and 1 death (a fatal Hampstead case, with a Kensington case who survived), its first human cases since 2014 — a reminder that late-summer risk is real even in a small state.

Key local data

Mosquito season ends at Manchester's first hard frost, averaging October 19 (50% probability); warm autumns push it later — NOAA 1991–2020.

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Southern New Hampshire

When to act

Timing your treatment

The cheapest, most effective protection is pre-season: drain standing water and start barrier treatment in spring, before populations build. Escalate at the late-summer disease peak.

The right window

April–May — do a weekly property 'water walk' (dump birdbaths, buckets, tires, clogged gutters, tarps) and start a professional barrier treatment before populations peak. A barrier spray suppresses adults roughly 3–6 weeks per application (industry-survey estimate).

Persistent breeding sites you can't drain, such as low wet areas or poor drainage

Property backing onto a freshwater hardwood swamp — prime EEE habitat

Your town issues an elevated EEE/WNV advisory or begins spraying

Biting pressure makes the yard unusable at dawn or dusk

Suspected mosquito-borne illness symptoms — contact a physician immediately, not a pest control company

Mosquito Control

Free inspection and same-day mosquito service across southern New Hampshire. NH-licensed #782664.

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Learn more about this pest

Common Questions

Frequently asked

Sources & references

NH DHHS — Mosquito Surveillance & Control
most active June–October; breeding cycle; dawn/dusk biting
NH DHHS — Mosquito-Borne Illness risk tips
EEE/WNV transmission window; prevention guidance
CDC — Eastern Equine Encephalitis (About)
~30% lethality of severe EEE; no vaccine or specific antiviral
CDC — Mosquito life cycle
egg-to-adult cycle 7–10 days; container breeding
NHPR — Hampstead EEE death (2024)
2024 fatal NH human EEE case; first since 2014
UNH Extension — EEE & Mosquito Management in NH
EEE vs WNV vectors; container-source nuance
NOAA/NWS Gray — Frost/Freeze Program
Merrimack-Valley growing-season end Oct 11 corroborates frost anchor

Editorial disclaimer: This content is provided for informational purposes only and does not constitute pest control advice. Every property is unique — consult a licensed professional for guidance specific to your situation. Anchor Pest Services is licensed in New Hampshire (#782664).

Written & reviewed by

Verified

Daniel Brady

NH-Licensed Pesticide Applicator · License #782664 (NHDAMF, RSA 430)

Daniel Brady is the New Hampshire–licensed applicator behind Anchor Pest Services, a family-owned pest control company serving the Manchester area since 2017.

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Last updated Jul 2026

Anchor Pest Services

Whatever the season — we handle the timing for you.

Same-day service across Southern New Hampshire. NH-licensed #782664, family-owned since 2017. We track the local calendar so you don't have to guess the right window.

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Keep Exploring

When Is Mosquito Season in NH? Timing & Risk Guide

The short of it

When are they active — the whole year at a glance

A year in the life

Spring

Summer

Fall

Winter

Mosquito season in southern New Hampshire

Timing your treatment

Frequently asked

Sources & references

Whatever the season — we handle the timing for you.

Your next step

How to Get Rid of Mosquitoes

When Does Mosquito Season Start in New Hampshire?

Eastern Equine Encephalitis (EEE) in New Hampshire

Mosquito — Quick ID for New Hampshire Homeowners

Are Mosquitoes Dangerous in New Hampshire?

Where Do Wasps Go in the Winter? NH Wasp Timing Guide

When Is Mosquito Season in NH? Timing & Risk Guide

The short of it

When are they active — the whole year at a glance

A year in the life

Spring

Summer

Fall

Winter

Mosquito season in southern New Hampshire

Timing your treatment

Frequently asked

When does mosquito season start in NH?

When is EEE/West Nile risk highest in NH?

When does mosquito season end in NH?

What happened with EEE in NH in 2024?

How deadly is EEE?

What time of day do mosquitoes bite most?

How little water do mosquitoes need to breed?

When should I start mosquito treatment?

Are mosquitoes still dangerous in the fall?

Is there a mosquito vaccine for EEE?

Sources & references

Whatever the season — we handle the timing for you.

Your next step

How to Get Rid of Mosquitoes

When Does Mosquito Season Start in New Hampshire?

Eastern Equine Encephalitis (EEE) in New Hampshire

Mosquito — Quick ID for New Hampshire Homeowners

Are Mosquitoes Dangerous in New Hampshire?

Where Do Wasps Go in the Winter? NH Wasp Timing Guide