What Are Cluster Flies? Why They Come Inside, and How to Get Rid of Them
Author:
Mosh Latifi
Co-Owner of Ecocare
Published:
November 12, 2026
If you have ever walked into a spare room or loft space in autumn and found it full of slow-moving flies, you have likely encountered cluster flies. They are one of the most misunderstood pests in the UK, partly because they appear suddenly and in large numbers, and partly because they behave nothing like the flies most people are used to dealing with.
They are not interested in your food. They are not a sign of poor hygiene. And the standard fly spray from the cupboard under the sink is not going to solve the problem.
Table of Content
What are cluster flies?
Cluster flies (Pollenia rudis) are a species of blowfly found throughout the UK. They are slightly larger than a common house fly, around 8 to 10mm in length, and have a distinctive golden or yellowish sheen of fine hairs on the thorax. When resting, their wings overlap flat across their backs, which is one of the easiest ways to tell them apart from other species.
Their life cycle in spring and summer is entirely outdoors. Adult females lay eggs in soil, and the larvae burrow into earthworms, developing as parasites inside them before emerging as adults. By late summer, the adults are feeding on flowers and plant matter. Then, as temperatures begin to fall from September onwards, their behaviour shifts entirely.
They start looking for somewhere to spend the winter.
Why do they come into buildings?
Cluster flies are what entomologists call overwintering insects. They do not die off in winter; they enter a state of reduced activity called diapause and wait out the cold months in a sheltered location.
Buildings are ideal for this. A roof void, loft space, or wall cavity offers exactly what they are looking for: stable temperature, low humidity, and protection from the elements. South-facing and west-facing walls tend to attract them first, because those surfaces warm up most in the autumn sun. Cluster flies congregate on these walls in large numbers before finding a gap to move inside, typically through spaces around roof tiles, gaps in fascia boards, poorly sealed eaves, or openings around cables and pipework entering the building.
Once inside, they move to the highest, darkest point available and cluster together in a mass. Hence the name.
One building that has successfully hosted cluster flies will attract them again the following year. They release a pheromone that draws subsequent generations back to the same site. Without treating the entry points and the harbourage, the same infestation will return each autumn.
What are the signs of cluster flies?
The most obvious sign is the flies themselves, but by the time you see them in numbers, the infestation is usually well established. Here is what to look for earlier in the process.
Flies gathering on exterior walls in autumn. A cluster of flies sitting on a sunny wall in September or October, particularly on the south-facing side of a building, is a strong indicator that they are preparing to enter.
Sluggish flies at windows on warm days. When temperatures rise briefly during winter, cluster flies can become temporarily active and move towards light. Finding slow, disoriented flies on interior windowsills in December or January, in a room you do not use often, is a classic cluster fly sign.
A sweet, sickly smell in the loft or upper rooms. A large hibernating mass produces a detectable odour. It is often described as a faintly sweet or buckwheat-like smell. If you notice it when you open the loft hatch, check carefully.
Dark staining on ceilings or in roof voids. In significant infestations, the mass of flies and their droppings can leave visible staining on surfaces.
Dead flies in window frames and on sills. Some flies die during the winter period. A regular accumulation of dead flies in a room you rarely heat is worth investigating.
How many are we talking about?
This is where people are often caught off guard. A cluster fly infestation is not a few dozen insects. A well-established hibernating mass in a loft or roof void can contain tens of thousands of flies. Because they are dormant and hidden in a void, there is often no sign at all until a warm day brings them to life and they emerge into the living space below.
Properties that back onto farmland, parkland, or open grassland tend to have higher cluster fly pressure, because the surrounding soil provides more opportunity for the spring and summer larval stage. That said, cluster flies are found in urban and suburban properties across the UK too. They are not exclusively a rural problem.
What does not work
Before getting to effective treatment, it is worth being clear about what people commonly try that does not work.
Fly sprays and plug-in devices. Consumer insecticide sprays and electronic fly killers are designed for the flies you encounter in living spaces. They have no meaningful effect on a mass of dormant insects sitting inside a roof void or wall cavity. You are treating the symptom, not the source.
Vacuuming them up. Removing visible flies during a warm spell reduces numbers temporarily but does nothing to address the hibernating mass elsewhere in the building. They will continue to emerge.
Sealing entry points without treating the existing infestation. Proofing the building is the right long-term step, but if there is already a colony inside, sealing them in creates a different problem. Treatment comes first, proofing second.
Waiting until spring. Cluster flies become more active and begin dispersing in March and April. By that point, they have been in the building for six months, have caused staining and odour, and will have already released the aggregation pheromone that attracts next year's population. Treating early in the winter period gives much better results than waiting.
How to get rid of cluster flies properly
Effective cluster fly treatment has two stages: treating the hibernating population and preventing re-entry.
Stage one: insecticidal treatment of the harbourage. This involves applying an appropriate insecticide directly into the voids and spaces where the cluster is located. This is not something that can be done effectively from the room below. Access to the loft or void is required, and the product needs to reach the mass rather than just the periphery. A BPCA-certified technician will identify all harbourage areas and treat them thoroughly in a single visit, with a residual product that continues to work as dormant flies become active.
Stage two: proofing the entry points. Once the existing population has been treated, the building needs to be proofed to prevent the following year's infestation. This means sealing gaps around roof tiles, repairing damaged fascia and soffit boards, filling openings around cables and pipework, and checking any other points where the roofline meets the external wall. This work is best carried out in late summer or early autumn, before cluster flies begin their aggregation behaviour, but it can be done at any point.
Some properties benefit from a residual external spray applied to south-facing walls in September, which deters cluster flies from landing and entering in the first place. This is worth considering for buildings with a persistent annual problem.
Are cluster flies harmful?
Cluster flies do not bite, do not sting, and do not carry or transmit disease to humans. They are not interested in food storage areas or waste. In that sense, they are less of a direct health concern than some other pest species.
However, a large infestation causes real problems. The smell produced by a significant hibernating mass is unpleasant and persistent. Their droppings stain surfaces and are difficult to clean from soft furnishings, insulation, and timber. Dead flies accumulate and attract secondary pests. And the aggregation pheromone they leave behind virtually guarantees a return visit the following autumn if the entry points are not addressed.
For commercial properties, particularly those in hospitality, food service, or healthcare, the presence of a large fly infestation of any kind is not acceptable regardless of species.
When to call us
If you have found cluster flies in your loft, seen them gathering on your exterior walls this autumn, or dealt with the same problem for two or more years running, professional treatment is the most effective and time-efficient solution.
EcoCare provides cluster fly treatment across London and the Midlands. We identify the harbourage, treat it thoroughly in a single visit, and advise on proofing to reduce the likelihood of recurrence. All treatments are carried out by BPCA-certified technicians using eco-friendly methods.
If you are not sure whether what you have found is cluster flies or another species, get in touch, and we can help you identify the problem and recommend the right course of action.
