Clustering Flies

What are they?

Homeowners are often bothered by sluggish flies in their homes in late winter and early spring. These flies are collectively called "clustering flies," but may actually be any of the three most common types of flies.

  • Cluster flies are about 8 to 10 mm (.31 to .39 inches) long. They are dark grey, with black and silver (non-metallic) checkered stomachs, and many golden hairs on their upper body (these may or may not be present on older flies). Unlike house flies, their wings overlap when they are at rest.
    At first glance, they may look like house flies, but they are larger, darker and slower-moving. There tends to be more of these flies in houses surrounded by large lawns or those backing onto open parks. The name "cluster fly" is used because of their habit of gathering in clusters after entering a house in the fall. They may give off a sickly, sweetish odour if disturbed.
  • Face flies are pests of cattle and may hibernate in homes or invade them during the summer. Hibernating face flies have very similar habits to cluster flies. Since the larvae develop in fresh cattle manure, face flies are most likely to invade farm homes or homes located near pastures or where cattle are kept.
  • Blow flies, also referred to as blue or green bottle flies, are robust flies with shiny metallic bodies. They can often be found in homes during winter and early spring. These "buzzing" insects develop in manure or dead animal carcasses. They are strong fliers and are attracted to lamps or lights. The green bottle fly is also attracted to dog feces and garbage.
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Should I be concerned?

Although blow flies and face flies can present a risk of food contamination because of their breeding and feeding habits, cluster flies are not usually a health hazard. However, cluster flies are a nuisance, and are parasites on our friendly earthworms.

How do I know if I have a problem?

Blow flies or face flies may hibernate in homes that are located near livestock farms, or places where animal wastes or garbage accumulation are not well managed.

Cluster flies hibernate in secluded areas of houses (like in wall voids, attics, closets, and empty rooms). They are a particular nuisance because they leave stains on walls and curtains. Also, if flies die in wall voids, they may attract larder beetles, which will feed on the dead flies and then migrate to other areas of the house.

Adult cluster flies start looking for winter shelter when the days shorten in late summer and early fall. They are often attracted to the warm south and west sides of buildings, particularly those located in open and exposed areas. After sunning themselves during the day, the flies will look for a warmer place as the sun goes down and will eventually crawl into any small opening they can find in the house siding or under eaves. They may then settle within the walls or attic to hibernate.

A fly problem becomes more obvious on days when temperatures rise and the flies become more active, gathering in windows where light attracts them.

How can I get rid of flies?


Clustering flies can enter your home through the tiniest cracks around door and window frames, but also through any other small unsealed opening. To prevent infestation:

  • Make sure that all screens fit properly and that there are no cracks or holes that would allow flies to enter your house.
  • Weather strip windows and doors.
  • Fill any cracks or crevices with caulking around doors, window frames, and other openings in the building structure.
  • Install insect screening over air vents in soffits.

Did you know?

Cluster flies do not breed, feed, or lay eggs inside homes, as do some other common flies.

Physical control

Clustering flies can be controlled by simply killing them with a fly swatter. Be aware that cluster flies will release an unpleasant odour and may stain surfaces when squashed. They can also be released outdoors or vacuumed up and the vacuum bag disposed of.

Devices designed to trap clustering flies are also offered on the market. These traps, placed in windows where flies are a problem, contain a powder that works like quicksand, trapping the insects that fall in.



If you use a pesticide to control your pest problem, read the label to make sure you are choosing the right product for the right pest. Follow all label directions and warnings carefully. Always look for a Pest Control Products (PCP) number on the label so you know the product has been approved by Health Canada. See Use pesticides safely for more information on using pesticides safely

If the infestation is bad enough, pesticide products can be used. Pesticide products can be bought in hardware or grocery stores and garden centres. They can be sprayed around the outside surfaces of window and door frames to prevent entry. Check the label for product-specific directions.

There are also products that can be sprayed on interior window and door frames, baseboards, localized areas of floors or floor coverings, underneath furniture, and in closets and crevices and other places where insects hide or are seen.

You can also choose to contact a professional pest control operator to solve the problem.

You should not try to control cluster flies by attempting to control the earthworm population, because earthworms are the most natural means of producing fertile soil and the cheapest lawn aerators.

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