When it's not a House Fly, It May Be a Cluster Fly

Photo of a cluster fly on a leaf

TristramBrelstaff (Own work)/CC BY-SA 3.0/Wikimedia Commons

We usually expect to have to swat a fly or two in the house during the summer months - as family members constantly file in and out; doors are propped to carry in groceries, and windows area opened on screens that need mending. 

But what is an expected annoyance in the summer can be exasperating in the winter when doors and windows are sealed tight, and one wouldn't think that any flies exist in the cold outdoors to come inside - especially big, fat flies that sit on walls and windows. What are these flies and where are they coming from?

Cluster Flies Cluster

In all likelihood that large fly is not a house fly, it is probably a cluster fly that overwinters in the protected areas between the inside and outside walls of your home, or in the attic or basement. Overwintering insects generally stay in their hidden harborage areas until the warming and lengthening days of spring pull them from hiding.

But all too often, that overwintering cluster fly is drawn into the warmth of the home's interior - finding its way out the inside of your home in much the same way it found its way into the outside wall - through cracks and openings.

Cluster fly characteristics
Illustration: Catherine Song. © The Spruce, 2018 

Cluster Fly Identification

How do you know if the winter fly is a cluster fly or some other large fly? The cluster fly can be distinguished from the house fly by its:

  • Body characteristics. It is a bit larger than the house fly with a black/silvery-black checkered body. Young, newly emerged flies have short light-brown/yellowish hairs on their lower bodies.
  • Sluggish movement. The cluster fly will fly around the home but at a less frantic pace than that of the house fly.
  • Overlapped wings. When at rest, the cluster fly will overlap its wings; the house flies will remain separate.
  • Clustering at windows (thus its name). If a large population exists, it will tend to cluster at windows or attics, especially in little-used areas on warm, sunny days.

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

Cluster flies don't cause much of a problem, beyond annoyance. They are not known to transmit disease or damage structures, and they do not reproduce after emerging into the home. However, they can leave tiny dots of excrement where they, and large numbers can be a significant nuisance within a home or other building.

Cluster Fly Control

Once cluster flies enter a building - to harbor within the walls or buzz around the rooms, control methods are limited. Although insecticides can kill the flies harboring within walls, large numbers of dead or dying flies can attract secondary pests such as beetles and rodents.

Because the flies are sluggish, it is usually fairly easy to swat or vacuum those that find their way indoors. But, unfortunately, just as you get rid of these, more are likely to emerge, causing an ongoing process of swatting, vacuuming, and exasperation.

Thus, the keys to cluster fly control are exclusion and prevention. Maintain the home in good condition and implement pest-proofing procedures. Caulk or fill all cracks and crevices in the home's structure, including:

  • Around doors and windows.
  • Under and in eaves and siding.
  • Around electrical outlets.
  • At pipe, wire, and cable openings.
  • Around vents.
  • Along the roof.


  • Ensure all doors and windows are tightly fitted.
  • Check for and repair any holes in screens.
  • Use yellow sodium vapor or halogen lighting outside to reduce insect attraction.

Pesticides have limited effectiveness in control of these flying insects, however some control can be achieved by a professional application of an insecticide to the south sun-warmed side of the structure before cluster begin to cluster on the walls in early fall, and an application of dust in wall voids can help to kill the flies before they can breed.