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How to Get Rid of Cluster Flies: 4 Easy Methods

These flies aren't harmful, but they often become a nuisance

The sudden appearance of big flies on house walls or windowsills, especially in winter, is likely cluster flies. These oversized swarming black flies are not ordinary house flies. Cluster flies overwinter in the warmth between the inside and outside walls of your home, the attic, or the basement. They generally stay in secluded areas until spring's warming and lengthening days bring them out of hiding. Cluster flies enter your house by finding a passage through cracks and openings.

The Spruce / Catherine Song

Cluster flies are an indoor nuisance, but they are not breeding in your walls, and unlike horseflies, they do not bite. Cluster flies feed on plant nectar, and larvae feed on earthworms. At the end of winter, they emerge to find a way out of your home. The following tips will help you identify and learn how to eliminate cluster flies in your home.

What Do Cluster Flies Look Like?

How do you know if the winter fly is a cluster fly (Pollenia rudis) or another large fly? The cluster fly can be distinguished from the house fly in several ways:

  • Body characteristics: A cluster fly is larger than a housefly and has a black/silvery-black checkered body. Additionally, they have short golden or yellowish hairs on their lower bodies that you would not find on a house fly.
  • Sluggish movement: The cluster fly will fly around the home but at a less frantic pace than the house fly.
  • Overlapped wings: When at rest, the cluster fly will overlap its wings; the house fly's wings remain separate.
  • Clustering at windows: If there is a large population of cluster flies, they cluster along windows or inside attics and usually in little-used areas on warm, sunny days.

Signs of Cluster Flies

These flies typically live outdoors and are most likely to appear in the spring and fall. But overwintering cluster flies can emerge into homes throughout the winter.

Cluster flies don't cause much of a problem beyond annoyance. They do not cause home damage, mate inside the home, or eat anything indoors; they prefer plant nectar as their food of choice and lay eggs in the soil near earthworm burrows. However, they can leave tiny dots of excrement where they cluster, and large numbers can be a significant nuisance within a home.

4 Ways to Get Rid of Cluster Flies

Swat or Vacuum the Visible Flies

Cluster flies are sluggish, so it is relatively easy to swat or vacuum them. This method is a more natural way to get rid of cluster flies. Insecticides can kill the flies harboring within walls. Still, it's best to avoid insecticides because many dead or dying flies can attract secondary pests, such as beetles and rodents.

As you get rid of them, more are likely to emerge, causing an ongoing process of swatting and vacuuming. It may be exasperating, but this is one of the more effective natural ways to stop cluster flies from returning to your home.

Cluster fly on window being swatted with orange swatter

The Spruce / Nelly Cuanalo

Use Traps or Flypaper

Cluster flies are relatively easy to trap using flypaper or sticky traps or mixing a sweet water solution in a jar with a lid opening large enough for the flies to enter. Like any flies, cluster flies are attracted to sweets (and rotting food). Make sure to empty the jar regularly.

Flypaper being hung from ceiling to trap cluster flies near window

The Spruce / Nelly Cuanalo

Spray an Approved Indoor Pesticide

Insect sprays are a fast solution for getting rid of bugs. If thinking about an indoor insect spray, consider pyrethrins, an extract from chrysanthemum flowers that kills cluster flies quickly. Use it lightly and aim it directly at the flies. Pyrethrins are found in several commercial products but are not completely harmless—they are toxic to fish, so keep them out of water supplies. Pyrethrins are low in toxicity to humans and other mammals. Also, get "pyrethrins," not pyrethroids, a synthetic version.

Warning

Pyrethrins are less toxic pesticides, but they will kill all insects, including beneficial pollinators. Use them to target specific visible pests, not as a broadcast spray. Read labels carefully, as some products include additional ingredients with differing toxicity levels.

Cluster fly on window being sprayed from bottle with approved indoor pesticide

The Spruce / Nelly Cuanalo

Seal Interior Cracks and Gaps

Fill interior and exterior cracks and find the source where flies get into the house or the walls. Flies can't become an indoor annoyance unless there are crevices for them to get in and out. Remember that flies can fit through minor cracks along windows, door frames, and baseboards.

Stop their access by caulking or filling all cracks and crevices, including:

  • Around doors and windows
  • Around electrical outlets
  • Around vent registers
  • At the joints between baseboards and flooring
Interior crack near window being sealed with caulk gun

The Spruce / Nelly Cuanalo

What Causes Cluster Flies?

Cluster flies enter your home for a straightforward reason: They seek warmth to survive the cold winter months. These are primarily outdoor insects that lay eggs in the soil, which hatch into larvae that feed on earthworms. They enter homes and other warm buildings as fall turns cold to survive the winter. These insects don't reproduce in your walls or your home, nor are they explicitly seeking food, as are standard houseflies.

Homes that are tightly weather-sealed with good insect screens on windows and screen doors have minimal trouble with cluster flies, and these insects are generally not an indoor problem in warm southern climates, where the flies remain outdoors year-round.

How to Prevent Cluster Flies

Preventing cluster flies is essentially a matter of sealing gaps and cracks in the walls and foundation of your home so that the flies won't have access to warm hiding places. The west and south sides of a building are where cluster flies most often enter. Seal gaps around exterior vents, rooflines, water spigots, pipe entries, and foundation sill plates.

Ensure the windows you use for ventilation are fitted with insect screens in good condition, that the windows and doors are tight-fitting, and that exterior moldings are well caulked.

If your infestations are severe, you can have a residual pyrethroid-based insecticide applied on the outside walls in late summer and early fall as cluster flies prepare to hibernate. This is usually a job for a professional exterminator.

Cluster Flies vs. House Flies

Though similar in appearance, cluster flies (Pollenia rudis) are slightly larger than standard houseflies (Musca domestica)—up to 1/2 inch in length vs. the 1/4 inch for a housefly. Cluster flies will usually be louder insects with an audible buzzing sound, and they very often cluster on glass windows or walls heated by the sun. Compared to a housefly, which flies much of the time, cluster flies are relatively sluggish, often buzzing as they creep along wall or window surfaces.

Cluster flies are most likely to appear in the early spring as the sun begins to heat western and southern walls and tend to disappear to the outdoors as warm summer weather appears, and they seek plants to feed on and outdoor soil in which to lay eggs. On the other hand, houseflies reach their peak of indoor activity during the warmest part of summer.

FAQ
  • Do cluster flies carry disease?

    Cluster flies are not known to carry disease.

  • How long do cluster flies live?

    Cluster flies are a longer-living species, with adults living as long as two years under ideal circumstances. On the other hand, houseflies typically go through their entire lifecycle in about 15 to 30 days, depending on temperature and living conditions.

  • Does killing cluster flies attract more?

    Some evidence suggests that dead flies give off a hormone to attract more flies to them; however, a trap full of live flies' hormones is more of a fly magnet than dead ones.

  • Do cluster flies cause damage?

    Cluster flies do not damage the structure of homes. Instead, they leave behind a poop trail, and their carcasses become a mess that needs cleaning.

The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Cluster Flies. Penn State Extension. 2017.

  2. Cluster Fly. Plant & Pest Diagnostics, Michigan State University.

  3. Cluster flies. Utah State University.

  4. Pyrethrins General Fact Sheet. National Pesticide Information Center, Oregon State University Extension Services.