How do I get rid of cluster flies in the loft?

This is one of the few household insect problems where the standard advice — find the breeding site — does not apply. Cluster flies breed in soil outdoors and simply choose the warm, sheltered, south or west-facing side of a building to spend the winter. That means the response is proofing and tolerance rather than treatment, and it explains why houses near pasture get it every year.

Safety

What you'll need

• A vacuum with a long hose
• Boards to stand on, and good light
• Fine mesh, mastic and expanding foam for the roofline gaps
• Dust mask
• A window fly trap or a UV trap, for the rooms below (optional)

Step by step

1. Confirm they are cluster flies.Slightly larger than a house fly, sluggish, dark grey with golden hairs on the thorax, and overlapping their wings at rest. They gather in large numbers on a warm loft gable or a window, and they are notably slow rather than the fast darting of house flies.
2. Understand where they came from.The larvae parasitise earthworms in grassland and woodland. Nothing in your house produced them, so there is no bin, drain or dead animal to find. This is the key difference from every other fly problem.
3. Note when they appear.Large numbers arrive in autumn as the temperature falls, and they reappear on warm winter days when a bit of sun warms the loft and wakes them. A sudden appearance in February is the same population, not a new one.
4. Vacuum rather than spray.A vacuum with a long hose clears both live and dead flies quickly. Dead flies attract carpet beetles and other scavengers, so removing them is worth doing for its own sake.
5. Seal the roofline gaps.The point where the soffit meets the wall, gaps at the verge and the ridge, missing mortar at a gable, gaps around a soil vent pipe, and the joint at a dormer. This is the actual treatment.
6. Keep the ventilation.A loft has to breathe. Use fine mesh over vents rather than blocking them, and never seal soffit vents solid — condensation and rot are a far bigger problem than flies.
7. Deal with the room below.Flies reaching bedrooms come through the loft hatch and around ceiling light fittings. Draught-seal the hatch, and check that downlighter holes and pipe boxing are sealed.
8. Use a trap in the loft rather than a spray.A UV or sticky trap in the loft space handles the residual population quietly. Insecticidal smoke or fogging is sometimes used by professionals, but it is a repeat annual expense rather than a fix.
9. Accept partial success.A house next to permanent pasture will get some cluster flies most autumns. Proofing reduces the numbers substantially; it does not usually eliminate them, and anyone promising elimination is overselling.
10. Do the proofing work in late summer.Before the autumn arrival, and after any nesting birds have finished — active nests are legally protected and it is an offence to disturb them.

Tips

• Concentrate the proofing on the south and west elevations. That is where the sun warms the wall and where they choose to enter.
• A loft that reaches a decent temperature on winter afternoons will produce flies in the bedroom below. Sealing the hatch is often the change the household actually notices.
• Combine this with loft insulation and ventilation work — the access, the boards and the mask are the same, so do it once.

Common mistakes

• Searching for a dead animal or a breeding site — Cluster flies do not breed indoors. Time spent lifting insulation looking for a source is wasted, and the actual answer is at the roofline.
• Sealing the loft airtight — It causes condensation, damp insulation and eventually rot in the roof timbers. Mesh over openings, not blocking them, is the correct approach.

If it doesn't work

Questions people ask