Cluster Flies: What They Are and Why They Gather in Lofts
Cluster flies are slow, sluggish flies that pack into lofts and wall voids to spend winter. Here is what they are and why they pick your house.
Every autumn, a certain kind of fly turns up at the windows of otherwise clean homes. It moves slowly, bumps against the glass on sunny afternoons, and seems to have no interest in your kitchen. That is almost certainly a cluster fly.
Understanding where they come from and what they are after makes the problem a lot less mysterious, and explains why the same house is often visited year after year.
Where cluster flies actually come from
Cluster flies spend the summer outdoors, and their young develop in soil as parasites of earthworms. They have nothing to do with rubbish, food or drains, which is why they are different from the flies you swat in a kitchen.
In late summer and autumn the new adults go looking for somewhere dry, sheltered and undisturbed to wait out the cold. Your roof space fits the bill perfectly.
Why they gather in lofts
Warmth rises, and the sun-facing walls and roof slopes of a building soak up heat on bright autumn days. Flies land on those surfaces, crawl up into gaps, and settle in the highest, quietest part of the house.
Once one fly finds a good spot, more tend to follow, which is where the name comes from: they cluster, sometimes in large numbers, in the same corner of a loft or behind a fascia board or wall cavity.
- Gaps under roof tiles and eaves
- Loose or cracked soffits and fascias
- Small openings around pipes and cables
- Unsealed gaps around window frames
How to recognise one
Compared with a house fly, a cluster fly is a little larger and darker, with a dull, slightly mottled look and fine golden hairs that you can see when a fly is close up or in strong light. At rest, its wings overlap over its back like a closed pair of scissors.
The behaviour is the biggest clue. Cluster flies are lethargic and clumsy, often found crawling on window sills, curtains and the upper corners of rooms, and they are most active when the sun warms the house.
Are they harmful?
Cluster flies do not bite and do not breed in your home, and they are not linked to food contamination in the way that house flies can be. They are a nuisance rather than a hygiene risk.
The practical problems are the numbers, the buzzing, and the fact that dead flies can build up in lofts and voids, which can attract other insects that feed on dead insect remains. Some people also find the smell when many flies are crushed unpleasant. If anyone in the home has a health concern they think is linked to the flies, speak to a pharmacist or GP.
What this means for your home
Because the flies are drawn to the building itself rather than to anything you have done, a tidy house is just as likely to be targeted as a messy one. Houses that are tall, sunny-faced, or have older roof details often see more of them.
If you find a cluster in the loft, a professional inspection can confirm what the insects are and whether treating the roof space makes sense. Essex Pest Solutions carries out cluster fly loft fumigation across Essex, and the right approach depends on what is found on inspection.
In short: Cluster flies are outdoor insects that treat your loft as a winter shelter. They are a nuisance rather than a hygiene threat, and they tend to return to the same buildings.
Frequently asked questions
Do cluster flies lay eggs in the house?
No. Cluster flies breed outdoors, with their young developing in soil. Indoors they are only looking for a place to shelter, so you will not find them breeding in your kitchen or bins.
Why are there so many in one room?
They gather in the warmest, quietest part of a building and then drift into rooms through gaps and light fittings. A bedroom or spare room below the loft is often where you see them first.
Will they leave on their own?
Some may emerge in spring, but many die in the roof space, and the same building tends to be chosen again. If you see them again each year, it is worth asking for an inspection.
Need help?
Essex Pest Solutions covers all of Essex with local, fully insured technicians. Use the links below, or call or message us.
More guides
How to Get Rid of Cluster Flies in the Home
Cluster flies are tough to shift if you only deal with the ones in view. Here is what helps, what does not, and when to call in a professional.
Read the guide →Cluster Fly Loft FumigationCluster Flies or House Flies? How to Tell the Difference
Cluster flies and house flies look alike at a glance but behave very differently. A few simple checks will tell you which one you have.
Read the guide →Cluster Fly Loft FumigationWhy Cluster Flies Return Every Autumn
If cluster flies keep coming back to your house each year, there are reasons. Here is what draws them in and how to break the pattern.
Read the guide →Cluster Fly Loft FumigationWhen to Book Cluster Fly Loft Fumigation
Cluster fly loft fumigation works well when timed sensibly. Here are the signs it is time to book and how to prepare for a visit.
Read the guide →Book Cluster Fly Loft Fumigation
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