Hundreds of sluggish flies around your loft, sash windows or an unused bedroom every autumn - and again on the first warm days of spring? Those are cluster flies, and the same building gets chosen year after year until it is treated and proofed. JG Pest Control treats cluster fly infestations across the UK. We are open every day except Christmas Day, early until late.
They are not breeding in your house
Cluster flies breed outdoors in grassland. The flies in your loft came in from outside to hibernate.
The same house, year after year
A building that has had cluster flies once tends to be chosen again every autumn until it is treated and proofed.
A nuisance, low on health risk
They do not breed indoors, bite or go for your food. The problem is the numbers, the staining and the smell.
Autumn is the time to act
Treating as the flies arrive, with proofing, is what cuts next year’s numbers.
What are cluster flies?
Cluster flies are slightly larger and slower than house flies, with short golden hairs on the thorax and wings that overlap when they rest. They spend summer outdoors (their larvae parasitise earthworms in grassland, which is why rural and edge-of-town properties get them worst), then as temperatures drop in autumn they gather, or cluster, in the warm upper parts of buildings to hibernate: lofts, roof voids, sash window boxes, and south or west facing wall cavities.
A building that has hosted cluster flies once tends to be chosen again every autumn, regardless of how clean it is. Scent markers left by earlier clusters are thought to be part of the reason.
Cluster fly or house fly?
Most people first notice cluster flies because the “house flies” at the window behave strangely. These are the quickest ways to tell them apart:
How they move
Cluster flies are sluggish and dozy, often crawling or buzzing slowly at the glass. House flies are fast and quick to take off.
When you see them
Cluster flies turn up in autumn, on warm winter days and in early spring. House flies are a summer fly.
Where you see them
Cluster flies gather at upstairs windows, in lofts and in rooms that are rarely used. House flies go for kitchens, bins and food.
What they look like
Cluster flies are slightly bigger, with golden hairs on the thorax and wings folded over each other at rest. House flies have four dark stripes on a grey thorax and hold their wings apart.
Why cluster flies pick the same houses
Two houses on the same street can have very different years. The ones that suffer usually share some of these:
- Sunny walls. South and west facing walls warm up in the afternoon, and flies settle on them as the air cools.
- Height and exposure. Tall or isolated buildings, and the upper floors of any building, are favoured.
- Grassland nearby. Fields, paddocks, parks and large lawns support the earthworms the larvae feed on.
- Small gaps at the roofline. Gaps around soffits, fascias, vents, tiles and window frames let them into voids.
- A history of clusters. Once a loft or wall void has been used, flies tend to return to it.
The cluster fly year
- Summer, outdoors. Eggs are laid in soil and the larvae feed on earthworms. Adults feed on nectar and sap, unnoticed.
- Late summer to autumn. Adults gather on warm, sunny walls and work their way in through gaps at the roofline and window frames.
- Winter. The flies hibernate in lofts, roof voids and wall cavities, sometimes in thousands.
- Warm days. Heating or a sunny spell wakes some of them, and they head for the light of the windows indoors.
- Spring. The survivors try to leave the building, and many end up in rooms instead.
Signs you have cluster flies
- Large numbers of dozy flies at upstairs windows in autumn, especially on sunny afternoons.
- A buzzing loft or roof void - sometimes thousands strong in a bad year.
- Flies reappearing on warm winter and early spring days as the hibernating mass wakes up.
- The same rooms affected every year - classically one corner bedroom or one end of the loft.
- A sweetish, unpleasant smell where large clusters have formed or died off.
Are cluster flies harmful?
They are a much lower hygiene risk than house flies or blowflies: they do not breed indoors, bite, or target food. The problems are sheer numbers, the smell of large die-offs, staining around windows, and the secondary pests (like larder beetles and carpet beetles) that feed on the dead flies. For homes with solar panels, the warm void beneath panels is an increasingly common clustering spot.
Professional cluster fly treatment
Our technicians treat the voids and surfaces where the flies cluster - lofts, window boxes and roof spaces - using professional products and equipment that reach the hibernating mass, not just the visible stragglers. Timing matters: autumn treatment intercepts the flies as they arrive, and we advise on proofing (sealing entry gaps around fascias, vents and window frames) to cut down next year’s invasion. Persistent annual problems are usually best handled with a planned visit each autumn. See our fly pest control service for related fly problems. Our technicians hold the RSPH (BPCA) Level 2 Award, and trainees work alongside qualified colleagues.
What you can do in the meantime
Vacuuming the flies at the windows and emptying the vacuum straight away gives quick relief. It will not reach the cluster in the loft or the walls, which is why the flies keep coming back on warm days. If you are clearing a loft, leave the cluster undisturbed until it has been treated, so the flies are not driven down into the house.
Frequently asked questions
Cluster flies are thought to leave scent markers that draw new flies back to the same building and often the same void. Until the harbourage is treated and entry points proofed, the pattern repeats every year.
They are not breeding indoors - they came in through gaps around the roofline, vents and window frames in autumn and have been hibernating in the loft or wall void. Warm days wake them and they head for the light of windows.
Sprays kill the flies you can see, but the hibernating mass in the void carries on regardless. Professional treatment reaches the cluster itself, which is what actually breaks the cycle.
Early autumn, as the flies arrive, with proofing advice to reduce next year. Spring treatments deal with wakers, but autumn is where the war is won.
They stain around windows and large die-offs can smell and attract other insects, but they do not damage the building itself or spread disease like bin flies do.
The flies that move into your loft in autumn can live through the winter: they spend it hibernating, appear at windows on warm days, and leave in spring. Outdoors there can be two or more generations over a summer, more in a hot year.
They are hibernating or just waking up. Cold makes them sluggish, which is why they crawl at the glass instead of flying off like a house fly.
No. Cluster flies breed outdoors in soil, where their larvae feed on earthworms, and they come indoors only to hibernate. House flies breed in rubbish and food waste and are active in summer.
Usually, yes. A building that has had them tends to be chosen again each autumn. Treatment combined with proofing the gaps they use is what reduces the numbers.