What Are Cluster Flies?

Cluster Flies Defined

Cluster flies are common pests that invade homes, particularly when temperatures start to drop. They gain entry through small openings, cracks, and crevices. This is to seek shelter from wind and frost during the winter. 

Once flies are inside, these flies hibernate the winter away, ‘clustering’ together to stay warm. 

As the seasons change and the weather starts getting warmer, they wake up and try to get out. This leads to the sudden appearance of cluster flies. They are all clamoring to get out after hibernation. 

If it’s still cold outside, these flies will keep buzzing around, eventually dying of dehydration, leaving behind a swarm of dead flies. 

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Here’s all the information you need to know about cluster flies, including how to get rid of them.

What are cluster flies?

Commonly referred to as ‘cluster’ or ‘attic’ flies, they derive their name from their clustering behavior. They are part of the Pollenia Genus of flies which are in turn part of the Calliphoridae family. 

Differences between a cluster fly and a housefly

Though cluster flies resemble houseflies, they are larger and characterized by distinct stripes behind the head, yellowish/golden hairs on the thorax and overlapping wings. They make slow and sluggish movements and often look confused. 

Cluster flies are a common problem in rural areas, with vast stretches of land or fields. Because cluster fly larvae are parasitic to earthworms, they breed outside in warm weather during the late spring into early summer. 

They prefer places with rich, damp soil and a heavy population of earthworms. These flies often go unnoticed during the summer as they search for earthworm hosts. 


An adult female cluster fly lays up to 130 eggs at a time in earthworm burrows. When the cluster fly larvae hatch, they find and follow underground tunnels made by worms until they find a live earthworm which they then infest and use as food sources.  

The larvae enter the cocoon stage and transform into adult flies. When temperatures drop, cluster flies can remain dormant for long periods without eating much or reproducing. 

Where do cluster flies live?

Cluster flies are primarily outdoor flies.

They breed outside, feed on nectar, and sunbathe to stay warm. However, as soon as temperatures start to drop, they begin looking for overwintering sites where they can stay out of the wind and frost. (This is similar to stink bugs, which hide in your home and typically appear at the changing of seasons.)

The warmth of the home’s interior draws them in since, without heat, flies can’t survive freezing temperatures. 

Once inside, they gather together in an isolated safe place, including areas around window frames, door frames, or eaves and sidings. The entry points tend to be on the sunny side of the residence, primarily the west or southern side, which is the same place they will emerge from during warm periods. 

It is common for these flies to stow away in attics, and then appear suddenly through cracks and vents.

Identifying a cluster fly infestation

Check both indoors and outdoors to find these flies and their hiding spots. On the inside, check around doors and windows, electric outlets, in and under eaves and sidings, inside wall voids, pipe wire, and cable opening, the roof or attic, storage room, around vents. 

On the outside, check the exterior of your home, inspect for any possible cracks, crevices, slits, or openings that they may use as an entry point.

break in slab

Checking the places mentioned above will indicate whether or not you have a cluster fly infestation.

How to get rid of cluster flies?

Once you have identified a cluster fly infestation, you can get rid of them using any of the following methods.

DIY fly traps: Create DIY traps using sweetened water inside a glass jar. Cluster flies are attracted to sweets. Perforate the lid with holes big enough for the flies and to trap them inside. Change the water daily.  

fly swatter

Vacuuming: Once cluster flies come inside, the best course of action is removal. Use a vacuum to remove a large gathering of flies and release them outside. Unfortunately, using this method may not get rid of the flies entirely. More might emerge, causing an exasperating unending process of vacuuming. 

Electronic fly killers and fly boxes: You can use electronic fly killers such as the electronic bug zapper or electric fly swatter racket to eliminate cluster flies. Fly boxes work to eliminate flies before they enter the building. They target the flies’ external breeding sites. If used together with the fly screens, they can be pivotal in reducing the risk of flies entering your property.

Fly control lamps: These are innovative lamps such as the Vectothor Falcon, which emits continuous UV-A light, optimized for attracting flies. The temperature-optimized glue boards retain the flies it captures.

Use insecticides: Use aerosols sprays containing pyrethrums to kill cluster flies on contact. Aim directly at the flies. Pyrethrum is non-toxic to humans and pets. However, it is toxic to fish and amphibians.

Avoid spraying in places where they can come into contact with it. Also, make sure you buy products listed as pyrethrum and not pyrethroid, a synthetic version that is more toxic. 

Treat your home with residual insecticides both indoors and outdoors and use dust for cracks and crevices.

Professional pest controllers: For more extreme cases. Exterminators can use treatments to target both the adult flies and their larvae, completely getting rid of them.

Seal cracks: this is a good approach if you can identify a clear area where pests might be entering your home. 
Examples of products you can use to eliminate cluster flies

For outdoor treatment, you can use Reclaim IT to create a barrier around your home. Or, spray recommended residual insecticides such as LamdaStar UltraCap 9.7 (Amazon), Cyper WSP, Suspend SC (Amazon), or Demon WP.

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For indoor treatment, use recommended pyrethrin aerosols which include CB-80 or PT 565 contact aerosols. You can also apply BorActin Dust to cracks, crevices, and wall voids.

Preventing cluster flies

The most effective way to control cluster flies is to prevent them from getting inside the house. You can do this by fly proofing your home.

  • Fully seal off your house by applying caulk to cracks and crevices along the foundation, windows, and doors. Ensure the exterior molding is well-caulked.
  • Close or install fly screens on windows, doors, or any other ventilation or utility openings.
  • Use residual insecticides on the exterior walls, window and door frames, and other areas where cluster flies are likely to land before entering the house. Consider applying insecticides on the surrounding soil to help reduce the cluster fly populations by eliminating their breeding ground.


Final Thoughts

Prevention is the best way to control a cluster fly infestation. So take time to seal up cracks and fly proof your house. You can also, apply residual pyrethroid-based insecticide on the exterior of your home to prevent flies from coming in.

Article Reviewed By:

Dakota S. MS in Biology

Dakota holds a Masters of Science in biology/biological sciences, with an emphasis on entomology and parasitology. An accomplished researcher and writer, Dakota has completed numerous research papers and published peer-reviewed literature. Notable accomplishments include the study and documentation of new structures not previously known to be fluorescent in Ixodida (ticks).

Last update on 2024-07-22 / Affiliate links / Images from Amazon Product Advertising API