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Cluster flies: Identification and Information

Cluster flies are named as such because they assemble together in large swarms. There are several species in the UK, and they often travel in mixed swarms. During summer months, the swarms reside outdoors and do not present as a problem. However, when the temperatures drops, they search for a sheltered, warm place to overwinter. They especially like quiet spaces where they will not be disturbed, which can make your loft space an ideal location.  It is because of this that you may also hear Cluster flies referred to as, “attic flies”.


Cluster flies are dark grey in colour, with yellow hairs on the back. They measure roughly 8mm in length and have wings which overlap.  They fly more slowly than a house fly, looking rather sluggish in flight.

The Yellow swarming fly is a species of Cluster fly, and, as the name suggests, is yellow in colour with black stripes on the back. Adults measure about 3mm in length.

How did Cluster Flies get into My Home?

Flies are small, and can enter houses through openings around pipes, roof tiles, doors, and windows. They can also use vents and enter under roof eaves. If you have noticed any cracks or small openings around doors and windows, it is advisable to seal them off with caulk

Signs of a Cluster Flies Infestation

Cluster flies leave an unpleasant, sweet smell produced from the pheromone in their saliva. This smell attracts other cluster flies to the overwintering site and can signpost the flies back to your home the following autumn, leading to an annual infestation. The saliva is yellow in colour; finding small yellow stains on your walls, is a common sign of a cluster fly infestation. You may also see the flies themselves, usually congregating in large numbers in attic spaces, unused rooms or around sunny windows.

DIY Control

Whilst aerosol fly sprays can be effective on flies you can see, it will not reach into the cracks in your attic where flies tend to prefer to hibernate. Products such as fly papers may offer minimal control for small infestations, but are not suitable to eradicate the problem.

Time to Call the Professionals

Unlike Blow flies who land on faeces and dead animals, rendering them harbourers of parasites and bacteria, Cluster flies lay their eggs in soil and are not known to carry disease. Whilst they are harmless to humans, they swarm in large numbers and leave an unpleasant smell, making them a significant nuisance in your home. As these pests can return to the same hibernation site each year, it is advisable to contact a professional pest control technician to minimise the risk of the same problem returning next autumn and re-emerging next spring. Pest control companies have access to high quality, effective insecticides, and equipment, and will assess the best course of action for you and your pest problem.

Concerned cluster flies are gathering in your attic?  Call Avon Pest Control today and let us deal with your pest problem for you.

Merry Christmas holidays greetings from Avon Pest Control


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