The strange life of cluster flies

While having the odd housefly visit in the summer is not often a great cause for concern, cluster flies can be more problematic in autumn as they will typically gather in large numbers. Often they will cluster (as their name suggests!) around windows as they are attracted to light during the day, but they will then move and hibernate in roof spaces at night.

This means that even if you only spot a few, the chances are there will be more and you could have an infestation somewhere in your home. In the autumn, cluster flies will often gather in large numbers on sun-warmed, south-east facing walls during the day. As the cooler evenings arrive they will crawl into crevices or under the eaves and fascias into roof spaces and may start to gather in lofts or wall voids – quiet, undisturbed areas where they can eventually hibernate safely in the warm over winter.

The common cluster fly (Polleniarudis) is large and black and they are very similar to house flies so can easily be mistaken for them. The difference is their yellowish golden hairs on the thorax and a prominent dark and light coloured chequered pattern on their abdomen. At rest, both wings overlap across the abdomen and they tend to be sluggish in flight. There are a few other cluster fly species that vary slightly in size and colour, but the common cluster fly is the one you’ll be mostly likely to find in your loft space.

Historically cluster flies would hibernate in hollow trees, under lose bark or other dry sheltered areas. However, our warm, weatherproof and protected homes are particularly inviting to them.

Once the weather warms in the spring, at about 12°C cluster flies start to become active again. Although, because of our temperature controlled homes, it’s not uncommon for them to emerge earlier having been tricked into thinking spring has arrived. The adult flies will eventually leave in late summer to lay their eggs in soil. The cluster fly larvae are parasitic on earthworms and will pupate in the soil, emerging as adults ready to find shelter for the winter.

Why are cluster flies a problem?

Although these insects don’t bite, they’re considered a nuisance because of the volumes in which they migrate into a property to hibernate over winter. Naturally, you don’t want to share your property with swarms of flies and for a business, it’s not a great way to attract customers either.

Cluster flies can sometimes leave tiny dark-coloured spots of excrement on walls or windows which can look unsightly and be difficult to clean. A secondary issue with cluster flies is that if they die in areas such as wall voids or false ceilings, they may attract other pests such as larder beetles, which are known as stored product insects (SPIs).

Getting rid of cluster flies

Cluster flies can enter your home through the smallest of cracks around door and window frames or through other tiny unsealed openings. You can try and limit access to your property by filling any cracks and crevices in walls, window or door frames with caulking. However, you’re unlikely to stop all of them getting into your loft. DIY fly sprays will have an affect on direct contact with the flies, but not on those already hibernating in cracks and crevices.

If you have a large infestation you are likely to need the help of a professional pest controller, who can use treatments such as residual sprays to target all active flies and others as they emerge from hibernation. Caution needs to be taken however in older buildings particularly, there may be other creatures such as bats that could be affected. All bats in the UK are protected by the Wildlife and Countryside Act 1981 and it is illegal to use any insecticides if it might come into contact with bats.

If you think that you may have a cluster fly infestation, please feel free to contact Rentokil and we will be more than happy to help and advise on a cluster fly or any other fly problems you may have.