Getting Rid of Cluster Flies in Window Seal: Do Dead Flies Attract More Flies?

Georgette Kilgore headshot, wearing 8 Billion Trees shirt with forest in the background.Written by Georgette Kilgore

Pest Control | April 9, 2024

Grossed out woman wonders do dead flies attract more flies and how do you get rid of cluster flies in window seal areas, and what attracts flies and dead flies in the house?

Many people wonder, do dead flies attract more flies, especially after seeing tons of cluster flies in window seal areas.

Unlike typical summer flies, cluster flies sneak into homes as the temperatures drop, living up to their name by clustering together for warmth in attics and wall cavities.

Known for their loud buzzing, they are more of a nuisance than a danger, but that doesn’t stop most homeowners from trying to get rid of them as quickly as possible.

Understanding the answers to the question, do dead flies attract more flies in window seal areas, can help you realize how to solve this common pest control problem. 

Dead Flies in House: Do Dead Flies Attract More Flies?

When you see fly carcasses piling up in the window sill, you may be asking yourself “Do dead flies attract more flies?” When it comes to cluster flies, there’s no evidence that leaving those dead flies on the window sill will attract more flies.

Laboratory studies of fruit flies have found just the opposite.5 When scientists exposed live fruit flies to dead ones, the neurons changed in the brains of the live flies, and they were more likely to die sooner.

Fruit flies exposed to dead flies for just 48 hours had an average life of 45 days, compared to 60 days for flies who had not been exposed to the dead bugs. While more study is needed, don’t feel pressured to quickly remove dead flies in your window sill.

Graphics that shows how to get rid of flies in window seal and how to seal windows from bugs.

They may be helping to reduce your pest problems rather than making them worse.

Why Do Dead Flies Attract More Flies?

Some people believe that dead flies attract more flies, but that isn’t necessarily the case with cluster flies.

Some theories suggest that dead flies are simply organic matter, which is what flies feed on, but cluster flies feed on nectar and are not attracted to rotting matter like summer flies would be.

Another theory suggests that dead flies give off a hormone that attracts flies, but this is yet to be proven.6 As a general rule, it’s more likely that hormones generated by a group of live flies would be more attractive to other flies than hormones given off by dead ones.

Cluster Fly Identification

Cluster fly identification can help you to know your enemy, which is the first step to getting rid of these pests.

Appearance of Cluster Flies

The appearance of cluster flies is not all that different from that of regular blow flies or house flies. These insects are slightly larger than the flies you might see during the summer and are slow fliers with an annoyingly loud buzz.1

They are ⅜ inch long and don’t have that metallic shine you may associate with flies.2 Instead, cluster flies are dark gray with golden hairs on their body and a lighter patch on their abdomen.

How Long Do Flies Live?

How long do flies live? It varies by individual, but the life cycle of these insects often lasts more than a year.

It may be hard to believe when you see them buzzing around your house or hanging out on your window sill, but these flies actually live the majority of their lives outdoors. Female cluster flies lay eggs in the soil.

Within three days, larvae will hatch and will make their way into an earthworm, where they will burrow in and make their home in the worm’s body.1 Eventually, the larvae develop into flies and escape.

As long as it’s warm outside, they will live outdoors and may hang out on the sunny side of your house. As fall approaches, the flies begin seeking shelter and will crawl into your house via any gap they can find.

They’ll do their best to survive inside until spring when they will head back outdoors to lay eggs and continue the cycle.

Flies Nesting in Window Frames

The good news is that flies nesting in window frames don’t pose any harm beyond what they may steal from your sanity. These flies aren’t harmful to your health, don’t spread disease, and won’t damage property or furniture.1,3

Of course, that doesn’t mean you want them buzzing around all winter either, but you can at least take some comfort in knowing they are no more than a nuisance.

Flies in Window Seal

You’ve seen these flies in window seal areas, but have you wondered where else they may hang out in the house? The reason cluster flies in window seal areas are so common is that these insects love warmth.

You are most likely to see them hanging around windows or lamps because of the warmth and light these objects provide.2 There are often many more cluster flies you don’t see hiding in your home, however.

Macro photo of a fly on a wood surface.

(Image: Jin Yeong Kim10)

True to their name, these flies cluster together for warmth and are often in places where they won’t be disturbed. This may include the attic, inside of wall cavities, in vents, cracks or voids in walls, or even in empty or seldom-used rooms.

Fly Infestation in House

Once you have one cluster fly, do you have to worry about a fly infestation in the house growing worse and worse? The good news is that cluster flies don’t breed indoors.3

That means that once winter has started, the fly population in your house isn’t going to grow. It may seem that you are seeing more flies, but that is only because flies already inside are coming out to seek warmth and light around your windows and lamps.

They may also be confused by warm or sunny days, thinking that spring has arrived and it’s time to head back outside.4 If you can put up with a cluster fly infestation for the winter, the problem will resolve itself by the next season.

As warmer days appear, the flies will move back outside, and you might spot them hanging out on the exterior siding on the sunny side of your house.3

Tiny Bugs on Window Sill

Tiny bugs on window sill areas in the winter are typically just cluster flies seeking a warm spot to hang out, but what else attracts these insects?

Cluster Flies on Window Sill

Cluster flies on window sill areas will only appear if certain conditions are met, and are not found all over the globe:

  1. While they are native to Europe, these insects are now common all over North America as well.4
  2. They are only active when the temperature is over 54 degrees F. This means you won’t see them in very cold parts of Europe or North America.1
  3. Because they develop within earthworms, cluster flies are most common in areas with rich, worm-filled soil. This means they are common around pastures, turfgrass, and near rivers and streams.1
  4. There is some evidence that cluster flies are more common at higher elevations.7

What Attracts Flies?

What attracts flies? In simple terms, earthworms.

Cluster flies have a parasitic relationship with earthworms and cannot reproduce without them. This creates a huge problem when it comes to eliminating these flies, however.

Earthworms are a critical part of the ecosystem, and experts recommend against taking any action that could harm them.1 This means that trying to kill cluster fly larvae with pesticides is not a safe option, because it would also harm their earthworm hosts and pose a serious risk to the local environment.

Dirty Flies

When many people think of flies, they think of dirty flies, also known as filth flies.

These insects are attracted to garbage and manure, and they transport bacteria and other pathogens from these areas everywhere they go.

Photo of a dirty area which is one of the things what attracts flies.

(Image: Bart Everson11)

Cluster flies are different, and are not generally included in the dirty fly or filth fly categories. They feed on nectar, not decaying matter, which separates them from the typical summer flies.

While no one wants flies taking up residence in their home, a cluster fly infestation is less of a cause for concern than an infestation of summer flies.

Do Flies in Window Seal Impact Health?

Flies in window seal areas are typically cluster flies, which pose no risk to human health. To test this, scientists closely studied a cluster fly infestation at a German hospital.8

They found that even though the flies carried bacterial pathogens, they could not be spread without doing a laboratory culture, concluding that the risk of health effects is incredibly low, even in a sensitive setting.

What Are Average Exterminator Prices for Treating Flies in Window Seal Areas?

Exterminator prices vary widely by area and by the size of an infestation. Terminix pricing generally ranges from $40 per month to $1,700 per year.9

If you’re looking for an exterminator to come out one time to treat flies in window seal areas, expect to pay about $250.9

Getting Rid of Cluster Flies

Getting rid of cluster flies is largely a matter of keeping these flies out, and not letting them move into your home in the first place.

Get Rid of Flies in Window Seal Areas

To get rid of flies in window seal areas, start with your trusty vacuum cleaner. Vacuum all the flies – alive and dead – and take the bag outside right away so the flies don’t escape back into the home.

Flypaper can also help with getting rid of these insects and keeping them away from your windows. Better still, grab an old-fashioned flyswatter.

Because cluster flies are so slow, they are fairly easy to swat.7 If you have a major infestation within the home, light traps have shown some effectiveness in getting rid of cluster flies.7

They are attracted to light, so light combined with sticky traps can be quite effective at reducing their numbers. Chemical pesticides should only be considered when all other alternatives have been exhausted.

A licensed professional should be employed to apply a synthetic pyrethroid.4 Keep in mind that sunlight breaks this chemical down quickly, so it is only effective for a few days at best, and should only be applied by someone skilled and experienced.

As a last resort, chemical pesticides should only be used after all mechanical prevention methods have been attempted.

How To Seal Windows From Bugs

When dealing with cluster flies, how to seal windows from bugs should be your very first question. Remember that these bugs breed outside and enter your home through cracks and gaps.

If you can seal these openings, you can keep flies out.

Here’s how:

  1. Use a silicone or silicone-latex caulk.4
  2. Apply caulk at all gaps in your home’s exterior, including around windows and doors, electrical outlets and vents, utility pipes, and chimneys.
  3. Use expanding foam to seal larger openings.
  4. Repair or replace all weather stripping around doors and windows.
  5. Look for loose siding and fix any damaged or missing sections. Caulk gaps or cracks where flies can penetrate the walls.
  6. Add screens to doors, windows, and attic vents, or repair existing screens.
  7. Focus on the south-facing side of the home, where flies are more likely to enter.1

Cluster Fly Prevention

The very best method of cluster fly control is cluster fly prevention.2 That means never letting these flies move into your home in the first place.

For this method to be effective, you have to think a season ahead of the flies and start taking steps toward prevention before it gets cold. In late summer, inspect the exterior of your home and make sure all openings, cracks, gaps, and voids are sealed.

Pay particular attention to the area around the window and door frames, where flies like to enter. Inspect and repair any screens or covers, including the attic vent cover.

Photo of dead flies on a sticky flytrap.

(Image: Sergei Frolov12)

If you’ve taken steps to prevent flies and still find a few sneaking in as temperatures drop, work on sealing the interior of your home as well. Inspect window frames and caulk around them if you spot any gaps or cracks.

Other common areas for fly entry include openings around utility pipes, electrical outlets, or vents. This will keep the flies from crawling out of the walls and buzzing around your home.

“Do dead flies attract more flies” is a legitimate consideration, and whether flies in window seal areas are driving you crazy or you’re tired of the flies slowly buzzing around your home, there are plenty of ways to eliminate your cluster fly issue without resorting to pesticides.


1Oregon State University Extension Service. (2018, July). Cluster flies: Noisy but harmless. OREGONSTATE. Retrieved November 24, 2023, from <>

2University of Maryland Extension. (2023, March 01). Cluster Flies. UMD. Retrieved November 24, 2023, from <>

3Eaton, A. T. (2016, August). Cluster Flies [fact sheet]. UNH. Retrieved November 24, 2023, from <>

4Jacobs, S. (2014, January). Cluster Flies. PSU. Retrieved November 24, 2023, from <>

5Page, M. L. (2023, June 13). Flies die sooner if they see dead flies. NEWSCIENTIST. Retrieved November 24, 2023, from <>

6Lupo, L. J. (2023, June 15). How to Get Rid of Cluster Flies: 4 Easy Methods. THESPRUCE. Retrieved November 24, 2023, from <>

7Cranshaw, W. (2018, February). Cluster Flies and other “Winter Flies” – 5.618. COLOSTATE. Retrieved November 24, 2023, from <>

8Faulde, M., Sobe, D., Burghardt, H., & Wermter, R. (2001, March). Hospital infestation by the cluster fly, Pollenia rudis sensu stricto Fabricius 1794 (Diptera: Calliphoridae), and its possible role in transmission of bacterial pathogens in Germany. NCBI | NLM | NIH. Retrieved November 24, 2023, from <>

9Dotson, R. (2023). How Much Does Terminix Pest Control Cost? STAYSAFE. Retrieved November 24, 2023, from <>

10Photo by Jin Yeong Kim. Cropped, Resized, Changed Format. Unsplash. Retrieved April 9, 2024, from <>

11Flies. Bart Everson. CC BY 2.0 DEED | Attribution 2.0 Generic. Resized, Adjusted Color Balance, and Adjusted Brightness and Contrast. From <>

12File:LENTAMUX.JPG Photo by Sergei Frolov / Public Domain. Cropped, Resized, Changed Format. Wikimedia Commons. Retrieved April 9, 2024, from <>