Cluster flies

What are cluster flies?

Cluster flies get their name from their habit of gathering in large groups to overwinter inside buildings. As autumn arrives and the weather cools, they look for shelter in the nooks and crannies of houses, sheds and other structures.

As the temperature drops further, they collect in huge numbers, sometimes many thousands of flies, tucked into roof spaces, cavity walls and lofts where they wait out the winter.

Unlike a bluebottle or a house fly, cluster flies are not drawn to food or waste and they do not breed indoors. They come inside purely to hibernate, which is why an infestation appears suddenly in autumn, seems to vanish in the depths of winter, and returns to the same building year after year.

Cluster flies: development & life cycle

Understanding the life cycle explains why cluster flies behave so differently from other household flies, and why they keep coming back.

The female lays her eggs in damp soil and leaf litter, not in your home. After about a week the larvae hatch and seek out earthworms, which are their host: the larva bores into the earthworm and develops inside it, before emerging to pupate in the soil. Because of this, cluster flies are a sign of nearby grassland and fields rather than of any hygiene problem in the house.

The number of generations depends on the weather. Two generations a year is normal, but a warm summer can produce up to four. The adult flies live outdoors through summer, feeding on flower nectar, and it is only as temperatures fall, from late August into November, that they move indoors in large numbers to hibernate.

Professional treatment

By the time large numbers have gathered, the most effective treatment is a space treatment. A trained technician uses a ULV (ultra-low volume) fog to penetrate the harbourage areas and knock down the cluster where it rests.

Where the resting sites can be reached directly, such as roof joists in a loft or the voids behind metal cladding, these can also be treated with a residual insecticide that continues to work over time. After treatment, dead flies should be vacuumed away.

For ongoing relief, UV fly units or discreet sticky fly papers help to reduce numbers in a room, and an external treatment around entry points and cladding can lower the population trying to get in.

Depending on the situation, a follow-up visit may be advised. Where the first treatment is carried out in autumn, a spring visit is often worthwhile, because that is when any survivors leave the harbourage as the sun warms it and re-infestation tends to show.

How to keep cluster flies out - proofing advice

Because cluster flies get in through tiny gaps in the outside of a building, prevention comes down to finding and sealing those entry points, ideally before the autumn influx begins.

• Inspect exterior walls carefully and seal cracks and gaps, particularly on south and west facing sides
• Seal the gap between window frames and the wall with a suitable sealant
• Fit fine mesh insect screens to windows and vents, and cover air bricks and ventilation openings with fine grilles
• Pay attention to the roofline, eaves and around cladding, which are common entry routes

Proofing is rarely one hundred per cent effective on its own, because the gaps cluster flies use are so small, but combined with treatment it makes a real difference to how many get in and whether they return.

Can I keep cluster flies at bay with DIY-remedies like scents or sprays?

A quick word on the popular question of what smell cluster flies dislike: some people report that scents such as citronella, eucalyptus or certain essential oils offer a little deterrence, but there is no strong evidence that home remedies stop an infestation. Sealing entry points and professional treatment are far more reliable.