Why You Have Flies Around Your Windows (And How To Stop Them)

extreme macro shot of a disgusting black fly

If you’re noticing groups of flies hanging around your window sills, you might be wondering why they decided to conjugate there as opposed to the bowl of fruit on your kitchen counter or your trash bin. Flies are always attracted to garbage and rotting food, right? Well, not always.

House flies, fruit flies, and blow flies are typically the species associated with trash as they love to feed on garbage and stagnant organic matter. In addition to this, flies can also be attracted by light, open windows, and cracks in your door frames.

So, what can you do to remove these swarms of lazy flies from your home? Let’s first take a look at what makes cluster flies different from other common house flies.

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How To Tell What Kind If Fly Is Near Your Windows

To be able to effectively treat your fly problem, it’s important to confirm whether or not the bugs flying around your windows are indeed cluster flies.

Other flying insects such as gnats and mosquitoes may also hang around your window, which is a whole other story! If you suspect a gnat problem, check out our article the things to so if you find gnats in your house.

So – if it IS those two, read those guide. If it’s not – enjoy the ride!

Let’s begin by discussing the physical attributes of a typical cluster fly. 

Cluster flies are around 8-10 mm long (slightly larger than a house fly) and are dark gray in color. To tell the difference between a cluster fly and a house fly, take a look at the hairs on its back. Are they gold or yellow in color? If so, you’re most likely looking at a cluster fly!

Another unique feature is its wings – cluster flies overlap across the abdomen when at rest. They are also not metallic and shiny in color like other flies are.

Another unique trait of the cluster fly is its flying patterns. While indoors, cluster flies typically move slowly compared to other common fly species.

With these characteristics in mind, ask yourself these questions to determine whether or not you have a cluster fly infestation:

  • Are the flies most commonly found in swarms or groups?
  • Is the temperature cold where you’re living?
  • Do they appear dark gray in color? 
  • Do their wings overlap when at rest? 

If you were able to answer yes to all of these questions, there is a good chance you’re dealing with cluster flies! The good news is these flies are generally harmless and will leave you alone.

The bad news? They can be sneaky little pests. Cluster flies usually hide out between the cavities of your walls and only exit when they spot a warm window they can latch onto, especially once the winter temperatures get a tad warmer.

If you do notice cluster flies hanging around your windows, there’s a good chance there are more inside your home that you just can’t see right away.

And, this can become a big problem when they lay eggs inside your home.

So, what can you do to remove these warmth-loving insects from your home?

Before we answer that question, let’s first examine how they chose your particular windows in the first place.

What Attracts Cluster Flies To Your Home (And Windows) In The First Place?

Yellow house fly on a red tree leaf

You would think that like most insects, cluster flies would prefer to spend their time outdoors. Well, similar to humans who would rather cozy up to a fireplace in the winter, cluster flies prefer warm, sunny areas and may find your home the perfect winter getaway.

During the summer, you’ll usually see cluster flies outdoors, hovering around grass and shrubbery. This is around the time they will lay their eggs in soil. Upon hatching, the larvae will feed on earthworms until they mature.

Once an adult, cluster flies will emerge from the soil and attempt to seek out warm places to live, including the sun-exposed sides of homes and other buildings.

When the air becomes cooler in the early fall months, cluster flies will do anything to enter your home to escape the cold. According to research by Iowa State University, cluster flies will typically enter your home around mid to late September.

Flies are able to detect warm air leaking from buildings during cold weather, and will enter a building or home wherever they detect leaks. 

So, what can you do to prevent these pesky insects from entering your home?

On top of these sources, there are a lot of other things that can attract flies. Read more in our article about things that attract flies to your home if you’d like!

How Cluster Flies Enter Your Home 

Cluster flies will do anything they can to escape the incoming cold, including searching for ways to enter your house. Let’s examine the various entrances cluster flies use to squeeze their way into your home or apartment.

Cracks and Crevices Near Windows and Door Frames

Closely inspect the exterior of your home for cracks, crevices, or gaps that cluster flies can get into. Though larger than a common house fly, cluster flies are still on the small side, averaging 3/8 of an inch long.

Cluster flies are also able to detect any and all temperature leaks coming from inside your home. This means they can easily infiltrate your home if you have any small, untreated openings that are leaking warm air. 

To prevent cluster flies from getting indoors, try using Gorilla Waterproof Caulk & Seal to properly seal any crevices or gaps around the exterior of your home. Pay special attention to gaps around your windows and door frames, as these are easy entrances for cluster flies and other insects. 

Damaged Window or Door Screens

Closeup of a damaged mosquito screen.

Mesh window and door screens are great for bringing in a nice breeze during the summer time, but when damaged, these screens can act as the perfect entrance for flies.

If you notice any small holes in your window or door screens, keep your windows and doors shut at all times until you can replace the damaged screen. Since cluster flies tend to seek shelter around mid September, you’ll want to have replacements ready to go by late summer.

For the best protection against cluster flies, try to repair these particular areas of your home no later than Labor Day weekend – as this is when they’ll start to come inside.

How to Keep Flies Away From Your Window

If you’ve taken all of the necessary precautions but are dealing with a current cluster fly issue, there are ways to safely remove these dormant insects from your home. 

We do recommend trying a variety of removal strategies to increase your chance of success, especially if you suspect that many of the flies are still living within cavities behind your walls, as these will be harder to eliminate. 

Use a Window Fly Trap

Purple with electronic mosquito traps blurred

If the large swarms of cluster flies around your windows are giving you the creeps, start by installing a Garsum Window Fly Trap to catch incoming flies during all hours of the day. 

Simply place the adhesive onto your window and watch as unsuspecting cluster flies land right onto the sticky surface. Best of all, these fly traps are non-toxic and odorless, meaning they can be safely used around children and pets. 

If cluster flies are not your only pest concern, you’ll be pleased to know that these window traps work against other flying insects including gnats, mosquitos, fruit flies, and house flies.

You can also place some of the specific scents that flies hate near your window to keep them out!

Set up a UV Light Trap

Since it typically provides them with some level of warmth, cluster flies are attracted to light.

Try setting up a UV light trap where you frequently see swarms of cluster flies. The trap works by luring cluster flies into a small fan located inside the device. The fan then traps the cluster flies onto a glue board, preventing them from escaping. A great UV light trap is the Katchy Indoor Insect Trap.

Similar to window traps, light traps can capture all sorts of flying insects including gnats and mosquitos. When catching cluster flies, you might even be surprised to find other flying critters sticking to the glue board. 

Tips for Using a UV Light Trap:

  • Place the UV light in a dim area to attract more cluster flies. 
  • Replace the glue board on your trap regularly, especially if you’re trapping a lot of flies.
  • Use a light trap along with a window trap to increase effectiveness. 
  • Be patient. If you’re not seeing the results you’d like, try moving your light trap to a different area of the house. 
  • Install at least 5 feet away from any food processing areas. The last thing you want to do is attract insects to your food! 

Fly Swatters and Vacuums

flyswatter Bamboo

It might seem like a no-brainer, but a good old fashioned fly swatter or hand vacuum can do wonders for removing cluster flies from your home. These objects are great to have on hand if you’d prefer not to use an old sneaker or rolled up magazine to smack down one of these nasty insects. 

Additionally, cluster flies are fairly dormant and fly around lazily, meaning they are fairly easy to target with a simple fly swatter or vacuum. You won’t have to spend too much time running around your bedroom or kitchen chasing after this particular species of fly (unless your aim isn’t the best. In that case, they might just take off!) 

Be aware that using a fly swatter on a cluster fly may leave a nasty brown stain. These spots can usually be removed easily with a mixture of vinegar and water.

Contact a Professional

If you’re concerned that your cluster fly problem is larger than it seems, contacting a pest control company might be the way to go. If it’s hard to determine where the flies came from in the first place, they will be able to do a detailed sweep of your property to pinpoint potential entrances.

Additionally, they may spray insecticides around the perimeter of your property, a task that should be handled by a professional pest control business.

You can use our pest control finder service to find an exterminator near you. We’ll get you connected with a professional in just a few seconds!

That’s a Wrap!

Like we mentioned, cluster flies are harmless creatures, but they can still be a major nuisance during the winter time.

Though they won’t be lured by bowls of apples or garbage bins like other nasty little flies, they are still unsettling to discover. Who enjoys having flies as a house guest? (Besides a venus flytrap, maybe.)

With just a little bit of knowledge about why cluster flies take up space by your windows and the strategies you can use against them, you’ll be able to efficiently tackle your fly problem. Now that you’ve read this article, we hope you feel more confident in your ability to remove these nasty little creatures from your home!

Good luck and happy fly trapping!


Cerretti, P., Stireman III, J. O., Badano, D., Gisondi, S., Rognes, K., Giudice, G. L., & Pape, T. (2019). Reclustering the cluster flies (Diptera: Oestroidea, Polleniidae). Systematic Entomology, 44(4), 957-972.

Cervenka, V., & Hahn, J. (2000). Fall Nuisance Flies.

Cranshaw, W. (2018). Cluster flies and other” winter flies” (Doctoral dissertation, Colorado State University. Libraries).

Vezsenyi, K. A., Langer, S. V., Samkari, B. A., & Beresford, D. V. (2022). The history and current state of cluster flies (Diptera: Polleniidae: Pollenia) in North America, with new Canadian provincial records. The Canadian Entomologist, 154(1).

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