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Cluster Fly — species photo
Daniel Brady · NH-Licensed Pesticide Applicator · License #782664

How to Get Rid of Cluster Flies

DIY-friendly
To get rid of cluster flies, seal exterior entry points before they seek shelter in fall — UNH Extension advises sealing by September 10, well ahead of Manchester's average first hard frost — and vacuum up the ones that get inside, without crushing them. Caulk gaps around windows, doors, soffits, fascia boards, and utility penetrations, and screen attic vents. Once flies are already inside wall voids, sealing won't evict them for the season; they'll keep emerging on warm days throughout winter, so vacuuming is your ongoing tool. Yard insecticides don't reduce home entry and aren't worth applying. Cluster flies are harmless — they don't bite, don't breed indoors, and don't damage anything — so this is a nuisance-control job, not a pest emergency.

At a glance

  • Difficulty: Beginner
  • Time: A day of sealing in late summer; ongoing vacuuming through winter
  • Cost: Low — caulk, screen, and a vacuum
  • Success: High for reducing numbers with well-timed exclusion; rarely 100% on an older home
The honest answer

Can you DIY Cluster Fly?

The verdict on Cluster Fly DIY

DIY-friendlyBeginner

A beginner-friendly DIY — timing matters more than difficulty.

Cluster flies are larger, sluggish, golden-haired flies that gather on sunny walls in fall and slip inside to overwinter — they don't breed indoors, don't bite, and cause no structural or food damage. Their larvae are earthworm parasites outdoors, so an indoor infestation is purely a seasonal nuisance, not a health issue. The whole job is sealing entry points before they arrive (UNH Extension recommends before September 10) and vacuuming the ones that get in. No exterminator needed for a typical home.

The playbook

The 5 DIY methods — honestly graded

Every approach graded for this species, in IPM order — prevention first, chemistry late. Pick a method to see how it works, the steps, and what it actually takes.

Prevention / Exclusion

Works well

The key fix — must be done BEFORE flies seek shelter in fall, not after.

Cluster flies gather on sunny exterior walls in fall looking for a way inside to overwinter. Physically sealing that access before they arrive stops next year's invasion rather than reacting to this one.

Best for: Stopping the annual fall invasion for good

  1. Seal before September 10

    Caulk cracks around windows, doors, soffits, fascia boards, and utility/pipe penetrations well ahead of NH's fall cooldown.

    Tip
    UNH Extension specifically recommends completing sealing before September 10 in New Hampshire.
    Caution
    Sealing after flies have entered wall voids does nothing for the ones already inside.

  2. Screen attic vents

    Screen attic and soffit vents, which are common overwintering entry points.

    Tip
    Focus on sunny south- and west-facing walls, where flies congregate most.

Tools & materials

Caulk and caulking gunExpanding foamReplacement screenLadder
Before you start
Timing is everything — this is a late-summer job, not a November one.
Reality check

Why DIY falls short here

Not scare tactics — the specific, proportionate reasons a home attempt can stall for this species.

Sealing after flies have already entered

Once cluster flies are inside wall voids, they keep emerging on warm winter days regardless of sealing — exclusion only works in the pre-entry window.

Treating the yard for earthworms

Cluster fly larvae parasitize earthworms outdoors, but killing earthworms with yard insecticide has not been shown to reduce indoor fly entry.

Straight talk

How risky is it, really?

An honest read — most nuisance species are genuinely low risk, and we say so.

Low risk

Cluster flies are confirmed harmless: no biting, no disease transmission, no indoor breeding, and no structural or food damage. The only real issue is the seasonal nuisance of clustering at windows and walls through fall and winter.

  • Indoor clustering nuisance at windows and wallsCommon

    Occurs in fall and winter as flies emerge on warm days

    UNH Cooperative Extension

  • Bites, disease, or structural damageRare

    None confirmed — cluster flies pose no meaningful health or property risk

    UNH Cooperative Extension

When to call a pro

Tired of the Annual Cluster Fly Invasion?

Sealing an old multi-story home is a big job to do alone. Anchor Pest Services handles exclusion and correctly timed exterior treatment across southern and central New Hampshire. Family-owned since 2017, NH license #782664.

Call a pro when…

  • Year after year of heavy fall invasions despite your own sealing
  • Entry points you can't safely reach (high soffits, multi-story fascia)
  • You want a timed exterior barrier applied correctly in the pre-entry window

General Pest Control

You almost never need a pro for cluster flies — this is a seal-and-vacuum job. The exception is a home under heavy annual pressure where full exclusion and a correctly timed exterior barrier are worth handing off.

Get a Free inspection(603) 785-0118

Keep them out

  • Seal exterior gaps before September 10 in New Hampshire
  • Prioritize sunny south- and west-facing walls where flies congregate
  • Screen attic and soffit vents
New Hampshire context

Cluster flies in New Hampshire

In New Hampshire, cluster flies gather on sunny walls and seek indoor shelter as temperatures drop — UNH Extension recommends sealing exterior gaps before September 10, well ahead of Manchester's first hard frost, about October 19–29 (NOAA). Once inside, they emerge on any unseasonably warm day through the winter, which is why homeowners often notice them again in late winter or early spring near windows.

Peak season

Fall entry, with warm-day emergences through winter

First-frost anchor

Manchester first hard frost ~Oct 19 (50%) / Oct 29 (80%), NOAA 1991–2020; UNH recommends sealing before Sept 10

Before you treat

Make sure it's a Cluster Fly

Two quick detours worth taking — confirm the ID, then learn what drew it in so it doesn't come back.

Not sure it's this?

Identify Cluster Fly in the Pest Library

Why is it here?

What actually attracts Cluster Fly

Common questions

Frequently asked

Sources & further reading(3)
  1. [1]
    UNH Cooperative Extension — Cluster Flies Fact Sheet

    Sealing timing (before Sept 10), biology, harmless-indoors confirmation

    extensionverified 2026-07-13
  2. [2]
    Michigan State University Extension — Cluster Fly

    Earthworm-treatment myth, biology

    extensionverified 2026-07-13
  3. [3]
    Northeast IPM Center — Cluster Fly (School IPM)

    Exclusion and vacuum guidance

    academicverified 2026-07-13

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 July 2026

More in Flies & Gnats

Other species you can tackle

Same category — the dot shows each one's DIY verdict at a glance (green = friendly, amber = doable with care, red = call a pro).

Professional Pest Control

Tired of the Annual Cluster Fly Invasion?

Sealing an old multi-story home is a big job to do alone. Anchor Pest Services handles exclusion and correctly timed exterior treatment across southern and central New Hampshire. Family-owned since 2017, NH license #782664.

Get a Free inspection(603) 785-0118
  • NH-licensed applicators (#782664)
  • Free inspection — same-day service across southern NH

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