Cluster Flies In The Loft Or House

What's happening?

Cluster flies live in fields in summer, where their larvae are parasites of earthworms. In autumn they look for dry, sheltered places and gather, often in large numbers, on sunny, south-east facing walls before crawling into crevices, mainly under the eaves and fascia into the roof space. A Cotswold District Council fact sheet says their presence is not a sign of poor hygiene, that fly screens are of little use because they squeeze into small cracks, and that they tend to return to the same property year after year. They wake on mild winter days and drift down into the rooms below, buzzing at windows, and leave in spring.

What usually causes it?

• A house close to fields or open land, with warm, sunny, light-coloured walls
• Gaps under eaves and fascias, around tiles and window frames that let them into the loft
• A roof void that is dry and undisturbed, which suits hibernation
• Old roost sites that attract the flies back, year after year
• Warm days in winter that wake them and draw them down into the house

Is it dangerous?

No. They do not breed in the house and they are not a sign of dirt or poor hygiene. They are a nuisance in large numbers, buzzing at windows and leaving dead flies on sills and in the loft.

Can I fix it myself?

Vacuum up the flies you can see and empty the cleaner outside; the Cotswold fact sheet says this only deals with the ones you see and not those hidden and hibernating. An ordinary fly spray in the loft will kill the ones it hits but will not reach the cracks where flies hibernate. Sealing obvious gaps around window frames gives limited help. Before any spray, fog or other insecticide goes into a loft, ring the National Bat Helpline on 0345 1300 228 if bats are possible: bats and their roosts are protected, and disturbing a roost can be an offence.

When should you call a pest controller?

Call a pest controller when there are hundreds in the loft, when they return every autumn, or when you want them treated before they wake in spring. A technician has more effective insecticides and equipment to reach the cracks. Ask them to check for bats first.

What will the pest controller actually do?

• Confirm cluster flies and check the loft for bats and other pests before treating
• Treat the loft and the cracks where flies hibernate
• Advise on sealing entry points under eaves, fascias and around window frames
• Return to repeat the treatment in later years if the flies come back

Who fixes this

• Cluster fly treatment

Questions people ask

Sources & References

1. Cotswold District Council: cluster flies fact sheet
2. Bat Conservation Trust: bats and the law