Why Do Cluster Flies Keep Coming Back?

Monday 14/09/2026

It’s fair to say cluster flies are particularly… gross.

It is annoying enough to have one fly about in a room; it is something quite different to discover hundreds of them gathered together in a loft, in the roof space or around a window.

Unfortunately, after a building has become a favourite place for cluster flies to spend the winter, the flies have the habit of appearing again.

So, what’s going on?

What Are Cluster Flies?

Cluster flies are a bit bigger than the average house fly and are usually darker in colour and move more slowly.

Despite appearing indoors in huge numbers, cluster flies aren’t associated with poor hygiene in the same way as many other fly species.

In fact, most of their lifecycle occurs outside. The larvae grow by parasitising earthworms in the soil, and the adult flies feed on nectar and other plant material.

The problems begin when the weather starts to cool down.

Why Do Cluster Flies Come Indoors?

When the temperature falls in the autumn season, adult cluster flies start to search for a sheltered place where they can spend the winter.

Unfortunately, our buildings provide exactly what they’re looking for.

Lofts, roof voids, wall cavities and other quiet, protected spaces can serve as perfect places for the flies to spend the winter. They can get in through tiny gaps around roof tiles, eaves, windows, vents and other openings.

Once they are in, they usually end up gathering together, or ‘clustering’, in large groups. Hence the name.

On warmer autumn days, overwintering flies can become active again and emerge from their hiding places. This is often when customers first become aware of the scale of the problem, as large numbers suddenly appear around windows and other light sources.

Why Do Cluster Flies Keep Coming Back?

Suitable buildings can experience year-after-year infestations, with flies returning to established overwintering sites each autumn. Cluster flies release pheromones which can help attract other flies. This is one reason the same buildings can become a recurring problem.

It makes it very difficult to achieve complete prevention since cluster flies are able to get in through extremely small gaps, especially in older buildings which have a great number of possible entry points.

Proofing obvious entry points can help reduce access, but it isn’t always possible to make a building completely cluster-fly-proof.

Controlling Cluster Flies

Cluster fly control isn’t necessarily about finding a single entry point and sealing it. With large infestations, the focus may instead be on managing the flies once they have gathered at their overwintering site.

Electric Fly Killers (EFKs) can help control cluster flies, particularly as they are drawn towards light. For heavy cluster fly activity, an EFK with an electric kill grid may be more suitable than a glue board unit, as large numbers of flies can quickly fill the glue boards.

For suitable enclosed spaces such as lofts and roof voids, insecticidal smoke bombs can be an effective way to treat cluster flies. Always check the product label and follow the manufacturer’s instructions before use.

Don’t Wait Until They’re Everywhere

Cluster flies are seasonal; as temperatures start to drop, you should watch out for buildings that have a record of cluster fly activity.

When it comes to cluster flies, an infestation disappearing in spring doesn’t necessarily mean it has disappeared for good. They may just be back next autumn.