Why Do I Have Cluster Flies in My House? How to Get Rid of Them in Edmonton

Written By Jun Bukht, ACE, CWCO, Founder and Owner of Major Pest Control

Last updated: September 2026

Why do I have cluster flies in my house? If you’re finding slow, sluggish black flies parked on your windowsills or bumbling around your attic in Edmonton, I can tell you exactly what’s going on.

I’ve walked into Edmonton homes where the owner swears they just deep-cleaned everything and the flies are still there. Here’s the short version: your house didn’t attract them because of anything you did wrong. It attracted them because of what’s growing in your yard.

Key Takeaways

• Cluster flies don’t come from anything inside your house; they’re parasites of earthworms that lived outside in your lawn or garden all summer.
• They aren’t breeding in your walls. Every fly you’re seeing right now arrived back in September or October.
• Homes with big lawns, gardens, or land near Edmonton’s river valley are the biggest targets.
• More flies on a sunny day doesn’t mean a growing infestation; it means more of them are waking up, not multiplying.
• Fixing this happens outside your house, not with a fly swatter.

Why Do I Have Cluster Flies in My House?

Cluster flies get into Edmonton homes mainly to escape the cold. As summer winds down and the days get shorter, adult cluster flies start looking for a warm, sheltered place to ride out winter.

They gather on sun-warmed exterior walls first, then squeeze through tiny gaps around your windows, siding, soffits, and roofline. Once they’re in, they settle into your attic or wall voids and mostly sit still until the weather warms up again.

Why do cluster flies suddenly appear?

They don’t really appear suddenly — they’ve usually been in your house for weeks or months already. I’ve pulled flies out of attics that had clearly been there since fall, just waiting. A warm spell, a sunny window, or your furnace kicking on can wake a whole batch of them at once, which is why it feels sudden even though it isn’t.

Why are cluster flies around my windows?

Light and warmth. Once they wake up, cluster flies head straight for the brightest spot in the room, and that’s almost always a window. That’s why you’ll find them clustered on the glass instead of scattered around the house.

Why are they worse on sunny days?

Sun-warmed walls and windows push their body temperature up enough to snap them out of that sluggish winter state. This shows up most on south- and west-facing walls, which catch Alberta’s low winter sun directly — it’s why a January chinook can bring out a wave of flies that were invisible the week before.

Where Do Cluster Flies Come From?

Here’s the part that surprises most homeowners: cluster flies come from your lawn, not your house. As larvae, they’re parasites of earthworms — they burrow into earthworms in the soil, feed for a couple of weeks, then emerge as adult flies by late summer. Those adults are the ones that eventually end up on your windowsill.

Do cluster flies breed inside houses?

No, and this is worth repeating because it’s the biggest worry I hear from homeowners. Cluster flies don’t lay eggs, feed, or reproduce indoors. Every fly inside your house arrived from outside — they aren’t multiplying behind your drywall.

What do cluster fly larvae live on?

Earthworms. It sounds strange, but a healthy earthworm population is actually a sign of good soil. Don’t try to get rid of earthworms to control cluster flies — it won’t work, and it’ll wreck your lawn for nothing.

Where Do Cluster Flies Hide in a House?

Once they’re inside, cluster flies aren’t looking for food or a place to breed — they’re looking for somewhere quiet and undisturbed to sit still until the weather warms up. That’s an important distinction, because it means they gravitate toward the parts of your home you visit the least, not the parts you use every day.

Here’s where they tend to settle in, and why each spot works so well for them:

• Attics: Their favourite spot in Edmonton homes. Attics stay dark, stay relatively stable in temperature, and rarely get disturbed once the insulation is down. A single Edmonton attic can shelter hundreds of flies without you ever knowing, since they don’t move around much once they’ve settled in.
• Wall voids: Especially in older construction with more gaps. The hollow space between your interior and exterior walls is essentially a built-in hiding spot. Older Edmonton homes with looser construction tend to have more of these small gaps, which gives flies more ways in and more places to tuck themselves away.
• Around window and door frames: These spots aren’t just entry points — they’re also where flies often stay put, right behind the trim, until sunlight through the glass warms them up enough to move.
• Closets and unused rooms: Anywhere that doesn’t get opened, cleaned, or walked through often is appealing to a fly that just wants to be left alone all winter. A spare bedroom or a rarely used closet upstairs is a common place to find a cluster you didn’t know was there.
• Soffits, vents, and eaves: These are usually the actual entry point rather than the final hiding spot. Flies squeeze through gaps here in the fall, then work their way further into the attic or wall cavity behind them.

Are Cluster Flies Dangerous?

Here’s the reassuring part: no, not really. Cluster flies don’t bite, don’t sting, and don’t spread disease. They’re a nuisance, not a health risk. The downside is that a big cluster can stain walls and curtains, and if enough of them die inside your walls, they can attract secondary pests like larder beetles that feed on the leftovers.

Cluster Fly vs. House Fly: What’s the Difference?

Homeowners mix these two up constantly, so here’s the quick way to tell them apart:

Feature	Cluster Fly	House Fly
Movement	Slow, sluggish	Fast, erratic
Size	About 8–10 mm	Smaller
Appearance	Dark grey with golden hairs	Plain grey
Indoor behaviour	Clusters near windows	Drawn to food and garbage
Breeds indoors	No	Yes

If it’s slow, dark, and parked on your window in October or February, it’s a cluster fly.

How to Get Rid of Cluster Flies in Your Edmonton Home

The flies you’re seeing right now just need removing, and vacuuming is the easiest, least messy way — empty the canister outside afterward. For the bigger picture, here’s what actually matters:

• Seal cracks and gaps around siding, trim, and rooflines.
• Check window and door screens for damage.
• Look at weatherstripping on doors and windows.
• Inspect soffits and vents for unscreened openings.
• Book a professional inspection if this happens every year.

We’ve put together a full walkthrough on sealing and treatment timing here: How to Get Rid of Cluster Flies.

Still seeing cluster flies after sealing the obvious spots? That usually means they’re getting in somewhere hard to spot from inside, like behind siding or under the eaves. That’s when it’s worth having someone check the exterior properly.

How to Stop Cluster Flies From Coming Back

Getting rid of the flies you can see is the easy part. Keeping next fall’s batch from moving in is where the real work happens, and it all comes down to closing off the entry points they used this time. Walk the outside of your home and check these areas before the weather cools down again:

Area	What to Check
Windows	Damaged screens, gaps around frames
Doors	Worn weather stripping
Soffits	Unscreened openings
Attic	Entry points around vents
Roofline	Small exterior gaps
Exterior walls	Cracks around trim

When Are Cluster Flies Most Active in Edmonton?

Late summer and early fall are when adult flies leave your lawn and start looking for shelter — this is when they actually get in. Through fall and winter, they mostly stay hidden and dormant in your attic or walls.

Then a Chinook or a sunny stretch can wake them up and send them toward windows, even in the middle of January. By spring, overwintering flies become active again as the days warm, often showing up in noticeably bigger numbers than homeowners expect.

Edmonton’s mix of big yards, river valley green space, and sharp seasonal swings makes this a pretty common call for us. Detached homes with larger lawns, older exterior trim, or a spot near open grassy areas tend to see them the most.

Why do they keep coming back every year?

I’ve seen homeowners clear out every visible fly and still get another wave weeks later. That’s not a new infestation — it’s more flies waking up from the same hiding spots. If the entry points that let them in last fall are still open, next year’s flies will use the same route. That’s why sealing the exterior matters more than chasing flies around the house.

Frequently Asked Questions

Why do I have cluster flies in my house?
Cluster flies get into homes in late summer and early fall looking for a protected place to spend winter. They enter through small cracks around windows, doors, siding, and rooflines, then stay hidden until warmer temperatures wake them up.

Where do cluster flies come from?
They spend the warmer months outdoors. As larvae, they’re parasites of earthworms living in the soil. Once they become adults, they look for buildings to shelter in before winter arrives.

Do cluster flies breed inside houses?
No. Cluster flies don’t lay eggs, feed, or reproduce indoors. They only come in for shelter — they’re not multiplying in your walls.

Why are cluster flies around my windows?
When cluster flies become active again, they’re drawn to light and warmth, which usually means the nearest window. That’s why you’ll see them on the glass instead of around the rest of the room.

Do cluster flies bite?
No. Cluster flies don’t bite people or pets. They’re a nuisance pest, not a biting one.

Are cluster flies dangerous?
Cluster flies are not usually considered a health hazard. However, large numbers can be a nuisance, and dead flies in wall voids may attract larder beetle

How do I get rid of cluster flies?
Vacuum up the ones you see, then seal cracks around windows, doors, and siding so fewer get in next time. Recurring or large infestations usually require a professional to identify entry points and treat them properly.

How do I stop cluster flies from coming back?
Seal exterior gaps, repair screens, weather-strip doors and windows, and check soffits and vents before late summer, since that’s when next year’s flies start looking for a way in.

The Bottom Line

Cluster flies in your house aren’t a sign you did anything wrong. They’re a sign your yard has healthy soil, and your home has a few gaps worth sealing. Deal with the flies you’re seeing now, then focus on the exterior before next fall so this year’s wave is the last one.

Seeing cluster flies every year and tired of handling it yourself? Contact Major Pest Control to inspect your Edmonton property, find where pests’re actually getting in, and put together a plan that stops the cycle, not just the flies you can see today.