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Seasonal Pest Watch – Cluster Flies

Cluster flies, also known as grass flies, or attic flies, do not pose a risk to human health. However, they can become a significant pest due to the large numbers in which they congregate in our homes to over-winter; several thousand flies may be found in one building if it has become heavily infested. Such large numbers also create an unpleasant odour.

The earlier you identify and seek help with your pest problem, the easier removing it will be.

A Seasonal Pest

In summertime, cluster flies live outside and lay their eggs in soil, near earthworm burrows. The hatched larvae then feed on the worms. It is when the temperature begins to fall in autumn (around August to November) that the cluster fly can become a pest. As the cold weather comes, cluster flies begin to gather in large numbers in roof spaces, attics, and upper rooms of houses to hibernate in a protected space over winter.

This habit of “clustering” in large numbers for hibernation has given them their name. Cluster flies can also be a nuisance when they re-emerge in spring to mate and find sites to lay their eggs outside; you may see them congregating in clusters on window frames on warm days, looking for an exit.


Cluster flies are predominantly found in rural areas, are around 7mm in length, dark grey in colour with yellow hairs on their back and overlapping wings.

Cluster flies can sometimes be confused with the house fly. One way to tell the two apart is to consider their flight speed; cluster flies look clumsy in flight and fly much slower than the house fly.

House fly not to be confused with cluster flies

A House Fly

How can I prevent Cluster Flies entering My Home?

As cluster flies are small in size, they can enter your property via small gaps, e.g., in walls, around windows or roof tiles. This can make them very difficult to safeguard against. The main means of dealing with cluster flies is through pest control, however, some action you could take to help reduce the risk of a cluster fly infestation are as follows:

  • Close trickle vents and other entry points if you are able to
  • Ensure outside entry points e.g., any gaps around window frames and cracks in walls are sealed
  • Use sealing products on the inside of your property, e.g., around electrical sockets, door, and window frames
  • Kill flies whenever you see them to reduce the amount that may successfully hibernate
  • Investigate attic spaces if there is a mild spell in winter, and dispose of any flies you find; cluster flies may emerge during this time thinking it is spring

As cluster flies can hide out in very small, difficult to penetrate areas, and may return to the same hibernation location annually, Avon Pest Control recommend seeking a professional pest control company to more effectively deal with your cluster fly pest problem.

If you are looking for qualified, reliable, professional assistance dealing with a cluster fly infestation, look no further! Call Avon Pest Control today on 01926 632 929 or 01789 293 463, or click here to request a call back; let us deal with your pest problem for you.

Avon Pest Control was established in 1991.
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