Why Are These Large, Black, Slow-Moving Flies In My Home?

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If you’re wondering Why are these large, black, slow-moving flies in my home, don’t worry; we completely understand your predicament. Especially because finding pests in your tranquil dwelling is one of the most frustrating, unpleasant, and unsanitary experiences imaginable—and enormous, black, slow-moving flies invading homes are no exception.

Therefore in this article, we will discover everything you need to know about large black slow-moving flies in your home!

Why are these large, black, slow-moving flies in my home?

Cluster flies are the most likely suspects when you characterize the pest entering your home as a large, slow-moving, black-hued house fly.

cluster flies

As a result, it’s these flies that you’ve most likely observed!

Naturally, this will lead you to wonder what cluster flies are? Well, Cluster flies (synonyms for the grass or attic flies) are members of the Polleniidae family of flies.

These flies are fundamentally distinguished from other types of flies mainly because of their slow and labored movement. They are larger and have a dark brown or black body. They also have golden hairs on their thorax.

Further characterization includes attraction to warmth and thereby the tendency to congregate ( in large numbers) near windows and doors; that is, cluster flies can be found inside and outside but are most commonly seen indoors during the winter months.

While they do not bite or sting, they can be a nuisance. Control of cluster flies is not easy and usually requires multiple treatments.

Take all the necessary steps to make sure these pests do not become a part of your house.

How do cluster flies gain entry to your home?

Cluster flies enter homes through open doors and windows, loose electrical outlets, roof eaves, openings around cables and pipes, and vents.

Identifying Cluster Flies

slow-moving flies

Cluster flies are a common household pest in North America. They are often mistaken for houseflies but can be easily identified by their large size and slow flying speed.

They are large, black, and slow-moving pests that can be found in homes during the winter months. They enter through any cracks or openings around doors and windows, and once inside, they can be difficult to get rid of.

Furthermore, although these flying pests are not harmful to humans, cluster flies can be a nuisance because they often return year after year.

Therefore, it is important to seal all entry points around your home to keep them out.

What’s the most effective cluster fly treatment?

Smoke bombs and foggers are the most efficient cluster fly treatment, but it’s best to hire a specialist because the fumes can damage human respiratory systems.

Professional pest specialists will have all of the necessary tools to provide safe and successful cluster fly treatment, so you won’t have to worry about your health and can relax in a fly-free environment.

What should you do if you find flies in your attic or loft?

Cluster flies tend to congregate in attics and lofts and can be difficult to get rid of.

If you find cluster flies in your home, there are a few things you can do to try and deter them from returning. One option is to use a ‘Citronella’ fragrance wick, which emits a smell that these flies don’t like. You can also try vacuuming them up or using a bug zapper to kill them.

Furthermore, you can also contact a pest controller to help get rid of them. The pest controllers will be able to use more effective methods, like bait traps and fly papers, that will eliminate your pest problem.

Cluster flies: How concerned should you be?

Cluster flies are not dangerous to humans and do not breed in the dwellings they infest, but they are often considered a nuisance, especially because they hibernate in large groups.

When they all emerge at the same time in the spring, they form swarms that congregate around windows.

In addition, indoor warmth and unusually mild weather can disrupt hibernation, causing the bulk of cluster flies stashed away to reawaken. On warmer winter days, homeowners may notice developing pests flying toward windows and other outlets of light.

Besides that, Hibernating cluster flies’ feces can also discolor curtains and walls. During hibernation, cluster flies can attract kitchen/pantry beetles, which feed on the fly carcass and then stay in the house. A skilled pest control service can help you stop an infestation.

How long do cluster flies stay?

The cluster flies that have been hibernating inside your home during the winter will start to come out as the weather starts to warm up in the spring.

They will make their way back outside, and you will no longer see them around your house until they return during the next cycle.

How do you get rid of cluster flies?

Why are these large black slow moving flies in my home

You can eliminate the cluster flies and prevent them from returning by undertaking the following:

Debris removal in and around the yard

Cluster flies are attracted to piles of debris in and around the yard, such as leaves and compost. These pesky pests can be difficult to get rid of because they lay their eggs in these areas.

By removing the debris, homeowners can reduce the number of cluster flies in their yard and help keep them out of their homes.


Large black slow-moving flies can be a nuisance in any home. Another common way to remove them is by vacuuming. Vacuuming can help remove the flies as well as the larvae they may have left behind.

The suction will pull them into the vacuum cleaner, where they will be killed. Make sure to vacuum thoroughly and dispose of the contents appropriately.

Sealing up the house 

One of the primary things you can do to keep cluster flies out of your home is to make sure your roof is in good condition.

If there are any leaks or holes, it will allow flies and other pests easy access to your home. Make sure to seal up any cracks or holes and check your roof for damage regularly.

Additionally, make sure all the doors and windows close tightly and that there is no damage around these areas that could provide entry points for these flies.

Food should be sealed.

A food source that is not sealed properly can attract flies. You can keep them away by sealing your food in airtight containers.

Remove any standing water.

An ideal way to get rid of large black slow-moving flies is to remove any standing water. This will deprive them of a place to live and breed. Standing water can be found anywhere, so it is important to be vigilant in looking for it and getting rid of it.

Sprays and Traps

Another option for getting rid of these flies is to use a contact pyrethrum spray. CB-80 is a product that has been approved for indoor use, and it will quickly kill the flies. Be sure to follow all the instructions on the label, and take care not to get the spray in your eyes.

You may also want to consider using some type of fly trap in your home. Fly strips work well, as do jars with sweetened water and a perforated lid.

Plan and Prepare ahead of the invasion

When it comes to cluster flies, prevention is crucial. In late August or early September, just before adult cluster flies appear, spray the exterior of the building with a residual pyrethroid-based pesticide like Suspend SC or Demon WP.

If they do infiltrate your home, be sure to catch them quickly and dispose of them, so they don’t lay eggs and create another infestation next year!


Hopefully, this guide helped you identify and distinguish the enormous black slow-moving insects in your home.

While the Cluster flies are not particularly dangerous to humans, we now know that they may be a real nightmare if they infest your home in large numbers.

To get rid of these pesky pests, you can use any of the methods listed above. It is also advisable to seek professional assistance.

About the author

A biotechnologist by profession and a passionate pest researcher. I have been one of those people who used to run away from cockroaches and rats due to their pesky features, but then we all get that turn in life when we have to face something.