How to control cluster flies

10 November 2017

Cluster flies can suddenly appear in their thousands in the autumn. They get their name from their habit of clustering together and while they are harmless and do not carry any diseases that affect humans, they are unpleasant. The adult flies start to cluster together from early September onwards to over winter. There are several species found in the UK and they range from 6-8mm long.  

During the summer the flies don’t pose a problem. It’s when they start searching for cover for winter hibernation, that they gather together mainly on south and west facing elevations of people’s homes and offices. They then can venture up into roof spaces, lofts or in and around window frames in the evening where they stay overnight. As winter comes, the cluster flies stay within the roof. Only venturing out in early spring. Cluster Flies can also be a problem behind wall cladding as these flies can enter through the very tiniest of holes. They can also cluster between the roofing felt and the tiles, both of which makes treatment a little more complicated.

Controlling cluster flies can be difficult. To stop them entering a building would be costly and extremely difficult. Sealing window frames and sky lights for example will help along with any other potential entry and exit points. 

There is a wide range of products available for DIY use; including window cluster fly traps, sprays and smokes

To deal with flies behind wall cladding or under roof felt, the best method is to blow an insecticidal dust up behind the cladding. 

In the loft or attic, there are different treatment options. You can install an electric fly killer specifically designed for cluster fly use. These electric ultra violet light fly killers are designed to collect several thousand flies, without impeding the operation of the machine.  We would not recommend using standard electric fly killer in your roof as they are a serious fire hazard. If there is enough room to move about then you can apply liquid sprays via a hand-held sprayer. Applying the spray to all surface areas including insulation and finally directly on to the flies.

Insecticidal foggers and smoke bombs can also be effective. However, you may need to use them more than once during the season.  Flies will enter the roof over many weeks, so your first treatment may eradicate all the flies in the roof but within days, more cluster flies may arrive. 

It is also important to clean up after any treatment has been completed. Dead cluster flies can lead to infestations of other undesirable insects such as carpet beetle. These may invade the property and feed on fabrics. 

Once you’ve had an infestation of cluster flies you can expect a new batch of flies the following year. This is because it is thought the flies leave a pheromone in the roof which is attractive to next year’s clutch of cluster flies. 


Logo 3
Logo 4
Logo 7
Logo 13
Logo 15
Ms1 598x217
Salsa main image
Bpca member logo rgb on white
Abate 14001
Abate 9001
Abate 18001
Nctg logo web
Purple rgb accredited
Rsz seal colour safecontractor sticker
Basis prompt
Think wildlife trademark 0
Cepa official logo
Winner 2019 bpma
Future50 final logo