First, be assured that the presence of cluster flies is not an indication of poor hygiene. One can neither expect to control nor predict where a cluster may hibernate. Studies and research have shed light on their life cycle and habits, which help us deal intelligently with swarms.

In summer months, female cluster flies mate. They lay their eggs in the soil. The larvae hatch 3 or 4 days later and, they look for a host, usually earthworms. They enter body cavities of earthworms for anywhere from 13 – 22 days until they return to the soil to moult and pupate. That takes about 11 – 14 days. Upon reaching maturity, they emerge from the soil as cluster flies. They go about their business in the great outdoors but as the weather cools, they start to seek warmth and shelter.

In the fields, they hibernate in hollow trees or underneath loose bark. But with our rural homes set in grassy fields, they tend to make use of buildings. However, they never lay eggs in homes and they don’t bite. Cluster flies do not pose a direct health risk for humans as their larvae never feed on dead animal carcasses, or garbage or manure, so they do not contaminate food. When the summer comes, they leave their hibernation destination and head back into the fields.

The first issue for homeowners is their sheer number – hence the ‘cluster’ designation and the swarm tag. The second problem is that they use pheromones to locate old roosts and so return to the same property every season. The third matter is that mice and rodents love eating cluster flies. With food sources dwindling in the wild, a cluster fly swarm can feed house mice the whole winter long. Lastly, if a cluster fly dies in a building, it is likely to attract larder beetles.

What Is The Difference Between House Flies And Cluster Flies?

Starting with the less obvious, one of the most interesting facts about cluster flies is their longevity. Most other flies have a less than three-month life cycle. Cluster flies, on the other hand, live for two years or more! There are often 3 or 4 generations in a swarm. Here is the list of the differentiation between house flies and cluster flies:


House flies are dark grey with four black stripes on their bodies, whereas cluster flies have a silvery chequered pattern on a dark grey body and golden hairs on their thorax.


House flies are between 3-6 mm in length. Cluster flies, on the other hand, are larger at about 7mm.


While housefly wings remain apart at rest, cluster fly wings will overlap.


House flies are faster and more frenetic., but cluster flies are considerably more sedate.


House flies go solo, minding their own business. Cluster flies, on the other hand, well…they cluster together, especially at windows.


House flies seek food and water inside buildings. Cluster flies prefer to live, feed and breed outdoors.


House flies feed on garbage and faeces and therefore spread germs. Cluster flies, clearly the clean cousins, are not known to spread disease at all.

Dealing With Cluster Flies UK's Swarming Scourge 2

How Do I Know If I Have An Infestation?

Cluster flies are also known as attic flies and are not that easy to spot once they’ve snuggled in somewhere to hibernate. It really isn’t that easy to spot a swarm cleverly hiding in tiny cracks and crevices. They favour south and west-facing buildings because of the chance of more sunlight and warmth. They are, however, not harmful to structures at all.

  • They do however give off a kind of sweet, buckwheat honey odour if disturbed. So, if there is a strange smell fitting that description in your house, that could be a giveaway.
  •  Should one see little dark-coloured spots on windows or walls that prove difficult to remove, you may be looking at the excrement of a swarm of cluster flies hidden nearby.

The Best Way To Get Rid Of Cluster Flies In The UK

A DIY pest control job may work if the swarm is small and readily visible.  Fly sprays will kill whichever flies it actually comes into contact with, but it won’t do a thing for any underneath the top layer of cluster flies or those in the cracks and crevices.

  • Preventative measures – mechanically closing up all gaps and filling all cracks and openings is laborious but it does discourage cluster flies. Check for cracks around the windows and doors, utility pipes, behind chimneys and especially in the facia boards or eaves of any building they may favour.

There are also exterior insecticide applications that may help if sealing an area proves impossible. But these must be applied by a licensed pest controller in early autumn. Unfortunately, sunlight affects these pesticides, reducing their efficacy down to several days or a couple of weeks only.

  • Post infestation measures – aerosol-type foggers do kill cluster flies that have massed together in attic rooms but, any swarms in wall voids escape those measures. Continual use of insecticides inside the home is not recommended.

A professional pest controller, on the other hand, will use better treatments that actually do work. They achieve results using ULV (Ultra Low Volume) generators and foggers.

With the onset of winter, it may well be tricky to spot a cluster fly swarm as they may already have bedded down for the season. However, it is also a great time to treat your home if you think that you have a problem.

Please feel free to call on Warren and his team for advice or assistance.