Cluster Fly

- Cluster flies enter buildings in the fall -

Summary: Cluster flies look like house flies but that's where the similarity ends. They can be nuisance in the fall when they enter homes in search of shelter for the winter. They cause no real damage but steps should be taken to prevent their entry and to control any flies that do gain access to walls voids and attics.

Jack DeAngelis, PhD

OSU Ext. Entomologist (ret.)

Identification of cluster flies

This page will help you identify and get rid of cluster flies that have invaded your home. Cluster flies enter homes in the fall, through cracks or broken screens, in search of a protected place to spend the winter months. They will leave in the spring to resume their normal life cycle (see below). They cause no damage and will not reproduce indoors but can be a real nuisance if they gain entry to the living-space.

Cluster flies look like house flies except for some golden hairs behind the head (see photo right). These hairs rub off easily so they may not always be present.

Cluster fly - about the size of a house fly, note golden hairs behind head.

Unlike many other similar flies, cluster flies do not develop in manure or garbage but rather are parasites of earthworms. Cluster fly larvae develop inside earthworms so they tend to be more of a nuisance in homes that are surrounded by healthy turf areas that support large numbers of earthworms like pastures, golf courses, well-managed lawns, and cemeteries. Cluster flies do not harm earthworm populations.

Flies emerge from their earthworm hosts in the fall and congregate, often in large numbers, on nearby homes. Flies that gain entry into wall voids and attic spaces can be a nuisance all winter when they migrate into the living-space. Control efforts should concentrate first on excluding flies from entering homes in the fall and second on controlling any that do get in. Do not attempt to control earthworms as this may actually damage the soil and turf.

Control of cluster flies in homes

First, exclude flies by repairing window screens, soffit vents, and sealing cracks around windows and doors; even a small opening can allow flies to enter the home. Flies that do manage to get into the attic or ceiling can be safely controlled with one of the new, low toxicity botanical insecticides. If you have access to fogging equipment a good choice would be EcoExempt IC (see Using Botanical Insecticides).

Flies also congregate in exterior walls and get into the voids between exterior wall studs. These spaces can be treated with natural dust insecticides (see Using Natural Dust Insecticides) such as borate and silica dusts or the botanical dust EcoPCO DX. Wall void treatments can be tricky and usually involve drilling holes from the outside, similar to treating for carpenter ants. This application can be done by a pest control operator if you are not able to do it yourself.

Supplies for controlling Cluster Flies are available here from DoMyOwn.com, our affiliate

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