If you live in an older farmhouse, barn conversion or cottage in the countryside around Newark or Southwell, you may already know the pattern. On a bright October afternoon, dozens or even hundreds of slow, dark flies appear at an upstairs window. You vacuum them up, and the next sunny day there are more. When you go into the loft, you find them clustered in corners, around the chimney and along the roof timbers. These are cluster flies, and they are one of the most common autumn pest calls we receive from rural Nottinghamshire.

The good news is that cluster flies are a nuisance rather than a health hazard. The less good news is that the same buildings tend to be affected year after year. Understanding why helps you manage them sensibly.

What are cluster flies?

Cluster flies belong to the genus Pollenia. They look a little like a house fly but are slightly larger and slower, with a dull, dark body covered in fine golden hairs on the thorax. When they are gathered together in autumn and winter they tend to be sluggish and easy to swat, which is often the first clue that you are not dealing with ordinary house flies.

Their life cycle is unusual. Cluster fly larvae do not develop in rubbish, animal waste or food. Instead, they are parasites of earthworms. Eggs are laid in soil, and the larvae find and feed inside earthworms through the spring and summer. Adult flies emerge from grassland and pasture, which is why homes surrounded by fields, paddocks and large lawns are most affected. The farmland of the Trent valley and the rolling countryside around Southwell provide ideal conditions.

Why they choose your house

As temperatures fall in late summer and early autumn, adult cluster flies look for a sheltered place to spend the winter. They are drawn to the warm, sunlit sides of buildings, usually those facing south and west, and to tall buildings that stand out in the landscape. They crawl into small gaps under roof tiles, around window frames, beneath fascias and soffits, and through cracks in old mortar.

Once inside, they settle in quiet voids such as lofts, roof spaces, wall cavities and unused upstairs rooms. Older properties with traditional roofs, many small gaps and large unheated loft spaces are particularly attractive.

Crucially, the flies seem to be attracted to buildings where cluster flies have overwintered before. It is widely thought that they leave chemical traces behind that help the next generation find the same shelter. This is why some houses have a cluster fly problem every single autumn while a neighbour a few hundred metres away has almost none.

Why they appear in rooms on sunny days

Cluster flies do not breed inside your home, and they are not looking for food. During winter they are largely dormant. However, a warm, sunny day, or central heating warming the roof space, can wake them. Confused by the warmth, they move towards light, which often brings them down into bedrooms, landings and bathrooms, where they buzz against windows and light fittings. This can happen repeatedly through autumn, on mild winter days, and again in early spring when they try to leave.

Are cluster flies harmful?

Cluster flies do not bite, they do not sting, and because they do not breed in food or waste they are not associated with the spread of disease in the way house flies can be. The problems they cause are mainly practical:

  • Large numbers of dead and dying flies in lofts, windowsills and light fittings.
  • Staining around windows and on paintwork where they gather.
  • A sickly, sweet odour when large numbers are present in an enclosed space.
  • Dead flies attracting other insects, such as carpet beetle larvae, which feed on them and may then move on to natural fibres in the home.

For holiday lets, bed and breakfast accommodation and wedding venues in converted rural buildings, the appearance of flies in guest rooms can also be a real reputational issue.

What you can do yourself

  • Vacuum them up. Using a vacuum cleaner with a nozzle is the simplest way to deal with flies in living areas. Empty the bag or canister outside promptly.
  • Proof the obvious gaps. Sealing gaps around window frames, under fascias and where pipes pass through walls reduces the number of flies that get in. Be realistic, though: a traditional roof has many entry points, and complete proofing is rarely possible.
  • Keep lofts tidy. Clearing dead flies from the loft each spring reduces odours and removes food for carpet beetles.
  • Avoid DIY sprays in living areas. Aerosol fly sprays are not designed for use in loft voids and will not reach flies hidden in gaps in the roof structure.

How professional cluster fly treatment works

An effective treatment starts with confirming that the flies really are cluster flies, because the approach is quite different from a house fly or blowfly problem caused by something decomposing in a void. We then locate where the flies are resting in the building, which is usually in the loft or roof space.

Treatment is timed for when the flies are gathered, typically in autumn. In a loft, a professional insecticide treatment such as a fog or smoke can reach the areas where flies are clustered. People and pets need to stay out of the treated area for a period, and we explain exactly what preparation is needed beforehand. Because new flies arrive each year, many rural homeowners choose an annual autumn treatment as part of their routine maintenance.

A note on similar pests

Not every mass of insects in an autumn loft is cluster flies. Harlequin ladybirds, some other overwintering flies and occasionally wasps from an old nest can cause similar concern. If flies are appearing in large numbers in one room and seem to be breeding, particularly in summer, something may have died in a wall or under a floor. That needs a different response. If you are unsure, send us a photograph for free identification.

When to call us

Call us if flies are appearing in living areas every sunny day, if the loft is carpeted with dead flies, or if you run holiday accommodation in a converted rural building and guests are noticing. We regularly work in older properties across Newark, Southwell and the surrounding villages. Read about our cluster fly control service or request a personal quote and tell us about your property.