Cluster Flys

Background Information

Cluster flies were given their name due to their tendency to ‘cluster’ on the exterior of a building prior to crawling into the harbourages to hibernate over winter.

Unlike other flies, their presence in a building does not indicate poor cleanliness standards or a nearby breeding source. Control is extremely difficult; often an ongoing maintenance programme with regularly scheduled insecticide treatments is the only way to provide control.

Why are they a problem?

Unlike most pest fly species, the breeding source of cluster flies can be located up to a mile away. The eggs are laid in soil and the larvae emerge and develop within earthworms.

They become a pest when the adults seek hibernation sites over the colder autumn/winter months. They cluster together in large population in attics and ceiling voids of all types of buildings. If they are woken up before the spring (when they venture outdoors) inside heated buildings they can prove a big nuisance inside homes and offices. They can often be found around windowsills.

Unfortunately, the new generation of flies can find their way to the same building as their parents, meaning issues are ongoing. As the breeding source cannot be removed, and proofing a building against flies is seldom effective, often a regular insecticide treatment in the autumn/winter is the only way to keep numbers in check.

What does treatment involve?

Generally a treatment will involve several visits of the affected areas over the autumn and winter months. These can be at set times, or whenever the tenants start to notice an increase in the fly populations.

Water activated insecticidal smoke generators can be placed in the ceiling voids and loft spaces to kill off any flies here, while the other affected areas can be fogged with an ultra-low volume insecticide. Floors and window sills can be sprayed with a residual insecticide if they are congregating here.
You will need to be out of the property for up to 3 hours following the treatment to allow time for the insecticides to dry.

Once dry the chemicals we use are completely safe. Following a fogging treatment such as this we would recommend opening windows to allow the rooms to ventilate once you re-enter the property. Any dead flies found following our treatment can simply be hoovered up and disposed of.

Please bear in mind that smoke alarms will need to be turned off while we treat.

A tailored plan is needed to successfully control cluster flies; please contact our office using the details below.


Pollenia Rudis (pictured above) has the common name Cluster Fly, but there are several other species that exhibit the same biology & behaviour, including the Autumn Fly, Green Cluster Fly & Yellow Swarming Fly.

Appearance of the Common Cluster Fly: 6mm long, distinctive bristly yellowish/ black abdomen; thorax is covered in golden hairs, large red eyes.

If you are unsure of what type of fly you have then we can identify them for you in our office; please send a sample of at least 4 undamaged flies in a sturdy container to our office in Merton.

Contact MICROBEE Environmental

If you have a problem with cluster flies and are based in London then contact MicroBee to discuss how we can help remove this unwanted pest.

Featured image: Alvesgaspar [CC BY-SA 3.0