PEST CONTROL

Cluster Fly Control in Toronto, Oakville & the GTA

The slow flies at your windows, treated on the outside walls

Cluster flies are the big, slow flies that pile up at sunny windows in fall and come back on warm winter days. They don't breed in your house: their young grow inside earthworms in lawns and fields, and the adults only move into the walls and attic for winter. Our cluster fly control treats the sunny outside walls in late summer, before they get in, for homes across Toronto, Oakville and the GTA.

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AT A GLANCE

Cluster Flies at a Glance

Cluster fly (adult)

Size: About 8 to 10 mm long
Colour: Dark grey, black and silver checkered abdomen
Body: House fly shape; wings overlap over the back at rest
How to tell: Golden hairs on the thorax, slow clumsy flight
Active: Gathers late summer and fall; out on warm winter days
Where found: Sunny windows, attics, wall voids, closets and spare rooms
Bites or stings: Doesn't bite

WHAT TO LOOK FOR

Signs of Cluster Flies in the House

If you’ve noticed any of these, call and a licensed technician will tell you what it points to.

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Large, slow flies at sunny windows on warm days in winter and early spring, often in a room you don't use much
Flies bumping into lamps and windows, or lying on their backs on the sill, spinning
Flies gathering on the sunny south or west walls of the house in late summer and fall
More flies at the windows the day after you cleared them all
Small dark spots on windows and walls, and stains on the curtains
A sickly sweet smell when a group of them is disturbed or squashed

IDENTIFICATION

How to Identify Cluster Flies

To most people, cluster flies look like ordinary house flies. Up close, a cluster fly is a little bigger, about 8 to 10 mm, dark grey, with a black and silver checkered abdomen and short golden hairs on the thorax, the middle part that carries the legs and wings. The hairs are thickest underneath, between the legs. At rest the wings overlap over the back. They fly slowly and clumsily, bump into lamps and windows, and often drop onto their backs. Ontario has six cluster fly species; for a homeowner, and for treatment, they are all cluster flies.

Cluster fly or house fly: a house fly has four dark stripes down the thorax, where a cluster fly has golden hairs, and it peaks in summer, not fall to spring.
Cluster fly or blow fly: cluster flies belong to the blow fly family but are dull, not shiny. Metallic blue or green flies are bluebottles and greenbottles.
Cluster fly or face fly: only an expert can tell a face fly from a house fly. Near pasture or cattle, the slow flies at the windows may be face flies.

TYPES

Cluster Flies and Look-Alikes in Ontario Homes

Cluster flies
Pollenia spp.
A little bigger than a house fly at 8 to 10 mm, dark grey with golden hairs on the thorax and wings that overlap at rest. Slow and clumsy in flight.
Where found: Lawns, fields and gardens in summer. In fall, sunny south and west walls, then attics, wall voids, closets and empty rooms.
Six species occur in Ontario. None of them breed, feed or lay eggs inside a house.
Face fly
Musca autumnalis
Looks almost exactly like a house fly; only an expert can tell the two apart. In summer the adults feed on the eye and nose secretions of cattle and horses.
Where found: Farm homes and houses near pastures or where cattle are kept, because the larvae grow in fresh cattle manure.
Spends the winter in homes the way cluster flies do, and is handled in much the same way.
Blow flies (bluebottles and greenbottles)
Phormia and Lucilia spp.
Stout flies with shiny metallic blue or green bodies. Strong, noisy fliers drawn to lamps and lights.
Where found: Breed in manure, dead animals, dog waste and garbage. A few turn up indoors in winter and early spring.
A sudden swarm indoors in summer usually points to a dead animal in a wall void or attic.
Fly Control
House fly
Musca domestica
Under 10 mm, with four dark stripes down the thorax.
Where found: Breeds outside in garbage, manure, lawn clippings and rotting produce, then comes in through doors and windows. Peaks in the hottest summer months.
Fly Control

Sluggish flies in the house in late winter are usually cluster flies, face flies or blow flies. For flies that breed indoors, such as fruit flies and drain flies, see fly control. Dead cluster flies in the attic can feed carpet and larder beetles. Other bugs on your sunny walls in fall? See boxelder bugs and other fall invaders.

OUR APPROACH

How Our Cluster Fly Control Works

Step 1
Confirm the Fly
Slow flies at the windows in winter can be cluster flies, face flies or blow flies, while a sudden swarm of blow flies indoors in summer usually points to a dead animal in a wall or attic. So the first step is naming the fly from the ones on your sills, which also tells us whether an outside treatment will help.
Step 2
Treat the Sunny Walls Early
Cluster flies look for winter shelter as the days shorten in late summer, and they land on the warm south and west walls first. An exterior treatment goes on those walls, under the eaves and around the windows before they gather, using a product with a PCP number as its label directs. Sunlight breaks it down within days to about a week, so timing matters.
Step 3
Point Out the Gaps
They crawl in through the smallest cracks around window and door frames, gaps in siding and under the fascia, behind chimneys, around utility pipes and through unscreened soffit vents. We show you where those openings are on your house so they can be caulked or screened before late summer. Sealing is the step extension services rank first.
Step 4
Advice for Flies Inside
Once they're in the walls and attic, treating inside isn't advised. Indoor sprays and foggers don't stop flies coming out of the wall voids, and piles of dead flies feed carpet and larder beetles. Instead, vacuum the ones that come out, and seal the gaps around baseboards, window trim and ceiling lights where they reach the rooms.

BITES AND STINGS

Do Cluster Flies Bite?

No. University of Guelph researchers note that cluster flies neither bite nor contaminate food, and Health Canada says they are not usually a health hazard. Unlike house flies, which breed in garbage and manure, cluster flies have nothing to do with either: their larvae grow inside earthworms in the soil. Penn State Extension adds that they aren't known to carry any disease of medical importance to people.

RISKS AND DAMAGE

Are Cluster Flies Harmful?

Cluster flies don't damage the building, the furniture or the people in it. The problem is how many there are and what they leave behind: clusters of thousands in an attic, small dark spots on windows and walls, stains on curtains, and a sickly sweet smell when they're disturbed or squashed. The bigger risk comes later. Flies that die in the walls and attic feed larder and carpet beetles, which then spread into the house and can go on to woollens and stored dry food.

Cluster flies are a nuisance, not a danger; dozens at the windows every winter, or an attic full of them year after year, is the point where a late-summer exterior treatment is worth booking.

CAUSES

Why You Have Cluster Flies in the House

Cluster flies come from the lawn, not the kitchen. Their young feed on earthworms, and Health Canada notes there tend to be more of them in houses surrounded by large lawns or backing onto open parks. In late summer the adults pick the warm south and west walls, especially on buildings in open, exposed spots. Window screens don't stop them, because they crawl in through small gaps in the walls. Summer soil conditions, moisture above all, decide how many earthworm hosts there are, and so how many flies arrive.

What Draws Them In

Large lawns, fields, gardens or a park behind the house, where they breed in earthworms
South- and west-facing walls in full sun
A house standing in an open, exposed spot
A summer with soil conditions that favour earthworms
Lawns and fields next door: they can travel a few hundred metres to find shelter

How They Get In

Cracks around window and door frames
Gaps in siding, under the fascia and behind chimneys
Openings around utility pipes
Soffit and attic vents without insect screening

HABITS

Cluster Fly Life Cycle and Habits

Habitat

In summer, cluster flies live outdoors in fields, gardens and lawns, and the adults can be among the most common insects on flowers in towns and on farms, where they help pollinate. In fall they hide anywhere sheltered: hedgerows, under loose bark, in rock crevices, in sheds and barns, and in the wall voids, attics, closets and empty rooms of houses.

Diet

The larvae are parasites of earthworms: they hatch in the soil, find an earthworm and feed inside it. A few cluster fly species have been recorded on other hosts, including caterpillars and bees. The adults visit flowers. Indoors they don't feed at all.

Life Cycle

Females lay eggs in cracks in the soil. The larvae hatch in about 3 days and spend 11 to 14 days feeding inside earthworms, then pupate in the ground, with adults out a few days later. The whole cycle takes 27 to 39 days, depending on temperature, so there are two or more generations each summer, and up to four. Only the last generation of the year moves into houses.

Seasonal Behaviour in Ontario

As the days shorten in late summer and the air cools to about 12 to 13°C, cluster flies gather on sunny walls and crawl inside. They rest there all winter, waking whenever it warms past that point. On warm spring days the survivors head back out, often covering the sunny side of the house, and return to the lawns and fields to lay eggs.

PREVENTION

How to Keep Cluster Flies Out

Caulk cracks around window and door frames, siding, utility pipes and chimneys before late summer.
Fit insect screening over soffit and attic vents.
Weather-strip windows and doors.
Repair or replace damaged window and door screens.
On a sunny fall afternoon, watch the sunny wall to see where the flies are going in, and mark those spots for sealing.
Inside, caulk the gaps where flies reach the rooms: behind baseboards, at window and door trim, and around ceiling lights and exhaust fans.
Vacuum dead flies from the sills and the attic so they don't feed carpet and larder beetles.
Leave the lawn and the earthworms alone: spraying the lawn or killing worms doesn't cut cluster fly numbers.
Book the exterior wall treatment for late summer, before the flies gather.

How to Get Rid of Cluster Flies in the House

Cluster flies in the house are hard to clear out from the inside, because more keep crawling out of the walls until spring. What works is picking off the ones you see and closing the way into the rooms. They're slow and make little attempt to escape, so a vacuum is the easiest tool: take the bag out, or empty the canister, outdoors right away, or the flies can crawl back out. A swatter works too, but squashed cluster flies smell and can stain.

Sticky flypaper strips can catch them in rooms with little air movement.
If you use an insecticide, choose one labelled for flies with a PCP number, and follow the label.
Skip foggers and bug bombs: they don't stop flies coming out of the walls.
Call when dozens show up every winter, and book before late summer.

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Inspection first, then targeted treatment with registered products used as the label directs. Your technician will advise on any precautions.

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Treatments are applied by technicians licensed under Ontario pesticide rules.

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COMMON QUESTIONS

Cluster Fly Control: Questions We Get

Why do cluster flies come back every year?

Because nothing that drew them has changed. Each summer a new crop grows in the earthworms in nearby lawns and fields, and the last generation looks for shelter on the same warm south and west walls, through the same gaps. They can travel a few hundred metres to find it, so the lawns next door count too. Sealing the gaps and treating the walls each late summer cuts the numbers.

Why are there more cluster flies on warm winter days?

They wake up. Cluster flies rest in the wall voids and attic all winter and become active whenever it warms past about 12 to 13°C, from sun on the wall or heat from the furnace. Then they crawl out through gaps around trim, baseboards and light fixtures and head for the bright windows, as if trying to get back outside.

Are cluster flies breeding in my house?

No. Health Canada notes that cluster flies don't breed, feed or lay eggs inside homes; their larvae grow in earthworms outdoors. Every cluster fly at your windows in March got into the walls the fall before. New ones appear the day after you clear them, which looks like breeding, but they're coming out of the walls, not hatching.

Do foggers or bug bombs work on cluster flies?

Not for long. A fogger can kill the flies sitting in an open attic, but it doesn't reach the ones in the wall voids, so they keep coming out. Penn State Extension advises against insecticide inside a building once the flies are in the walls or attic, partly because the dead flies can feed carpet beetles that later attack woollens and stored food. Vacuum instead.

Will treating the lawn or getting rid of earthworms help?

No. UConn's extension service says spraying lawns and gardens doesn't work, because the adult flies never stay on treated ground long enough, and cutting earthworm numbers rarely works either. Health Canada advises against trying: earthworms build fertile soil and aerate the lawn for free.

Why are the cluster flies in one room?

They choose quiet, out-of-the-way spaces. Health Canada says they spend the winter in secluded parts of a house, like wall voids, attics, closets and empty rooms, and the University of Kentucky notes they often gather at the windows of rooms that aren't used much.

When should I book cluster fly treatment?

In late summer, before the flies gather on the walls. Health Canada says they start looking for shelter as the days shorten in late summer and early fall, and Maryland's extension service says they usually enter homes in late August. If they're already collecting on your outside walls, call anyway: Kentucky's extension service says an exterior treatment can still help while they gather there.

Is cluster fly treatment part of a plan?

Yes. Cluster flies are covered by our Deluxe Protection plan: 6 scheduled visits that also cover carpenter ants, cockroaches, crawling pests, spiders and stinging insects, with rodent monitoring. If cluster flies are your only problem, you can book a one-time treatment instead, with no plan and no contract.

Sources

Last reviewed October 2026

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Cluster Fly Control Across Toronto, Oakville & the GTA

We treat cluster flies in homes and businesses throughout Toronto and Southern Ontario.

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