Cluster Flies

Flies that gather in windows, buzz around lights and fall spinning to the floor are cluster flies.

For the areas where we provide cluster fly control, see our service area

As many people living in the country have learned the hard way, flies that gather in windows, buzz around the lights and fall spinning to the floor are cluster flies. Their presence solicits anything from mild annoyance to putting a for sale sign on the front lawn!

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Image of cluster flies on window

Cluster Fly Habits

To understand the problem, it helps to know a little bit about the habits of these flies. Cluster flies are different from most of the other flies that we encounter. First, they are parasitic on earthworms, their life cycle taking place in the soil of fields and lawns. Second, they are long lived, over wintering in various places or structures such as homes and then emerging in the spring. Females lay their eggs in the soil and the larvae (maggots) seek out and penetrate earthworms. After 15 to 20 days, the maggots leave the worm, move near the soil surface and pupate. The cycle can be completed 3 or 4 times in a season. The adult flies feed on the nectar of flowers.


To answer some of the most common questions:

Cluster flies do not -

  • breed inside homes
  • feed inside homes
  • lay eggs inside homes
  • present a health hazard in the same way that house flies and other filth flies do
  • enter through the windows (in most cases)
  • develop due to the presence of farm animals

Cluster flies can -

  • cause allergies if in large numbers
  • attract secondary pests such as larder beetles
  • provide food for mice in attics
  • leave marks on windows and sills from their droppings
  • gather, buzzing on lawns on the first warm days of spring
  • congregate in huge numbers on sunny walls in the fall

Cluster Flies in Winter

Cluster flies enter structures in the fall in order to try and over winter. Once inside, while it is cold they remain dormant. Unfortunately on sunny days, even in winter, they will become active and end up coming out inside the house. They are often found at windows because the light from outside is attracting them. Late fall and early spring are when the flies tend to be most numerous in homes. If you think your problem is bad, one structure was reported to have had a quarter of a million flies. That's a lot of vacuuming!

What types of structures have the worst problem? I have treated all styles of homes, from old log cabins, brick Victorian farm houses and Century Stone homes to brand new vinyl sided Cape Cod construction and have found equally heavy infestations in all of them. Cluster fly problems are not confined to older homes.

Image of Cluster Flies

Cluster Fly Control

The most important question of course is how to get rid of these flies, or better yet, prevent them from entering your home or building. The main areas that cluster flies enter are often up high. These include soffits and facia, gable ends, dormers, vents and flashing. They will also enter through cracks and crevices between bricks, stones, wood siding, around windows and doors, under battens or other tiny openings in the structure. Sealing and caulking of these areas will help to reduce the number of entry points.

Cluster Fly Treatment

Effective chemical treatments for cluster flies can be performed with pyrethroid residual insecticides called Permethrin or Lambda-Cyhalothrin. Both have a low toxicity rating for people and pets but are highly effective against a wide variety of pests. Formulations of Permethrin are used in the head louse shampoo called Nix and also in dog & cat flea shampoos. Although they have a residual activity, they do not damage building surfaces. During the treatment, some over-spraying or dripping on windows may occur. If windows are to be cleaned, it is best to wait until after the treatment. Treatments can be done in either the spring or fall or both. In the spring, flies are eliminated and population numbers the next fall will be decreased. Applications are made between late March and the end of April, depending on weather. Treatments in the fall are preventative, stopping flies from entering the house so that you will not have them coming out on sunny winter days or the following spring. Fall applications are carried out between the end of August and early November. The procedure involves treating the outside of the house, concentrating on areas where the flies enter. As well, attics if present are fogged as part of the treatment. Permethrin is also very effective against wasps and spiders and other pests which over winter in homes.

In 1987, I was involved in some of the initial field trials using Permethrin for cluster fly control. Since that time, I have successfully treated thousands of homes as well as such diverse structures as churches, schools, museum buildings, country inns, offices and farm buildings. Cluster flies are one of the 'Joys of Country Living' that you don't have to live with.

A cluster fly on an orchid
If Noah truly was so wise,
He would have swatted
those two flies