If you've ever opened a spare bedroom window in late February and found dozens — sometimes hundreds — of slow, drowsy flies on the sill, the chances are you have cluster flies overwintering in the loft. They're not a hygiene problem and they didn't come from your bins; they're a countryside pest that uses houses as a substitute for the hollow trees they evolved to shelter in.
The cluster fly life cycle
Cluster fly larvae are parasites of earthworms. The adults emerge in late summer, mate, and then in October look for somewhere sheltered to spend winter together in large groups — hence the name. South and west facing roof slopes in rural and semi-rural Hertfordshire are ideal: warm in autumn sun, dark, undisturbed and full of small gaps around ridge tiles, soffit boards and eaves vents.
Why they appear suddenly in spring
On the first warm, sunny days of February and March, loft temperatures climb enough to wake them from torpor. They head for light — which means the loft hatch, the spare bedroom window, the bathroom rooflight. A loft holding 2,000 overwintering flies can produce a flush of 50–100 active flies a day in the house below until the weather warms enough for them to leave for good.
What to do
- Vacuum up live and dead flies as they appear — don't squash them, the smell attracts more next year.
- Seal the loft hatch with draught stripping so emerging flies can't drop into the house.
- Avoid blocking external eaves vents — you'll cause condensation problems and the flies will find another way in.
- For repeat annual infestations, a professional autumn loft treatment with a residual insecticide and ULV fog kills incoming flies before they cluster, breaking the cycle.
Cluster flies are a particular problem in older rural Hertfordshire houses near pasture, arable fields and parkland. If you've had them three springs in a row, the same generation's offspring are returning to the same roof void — a single autumn treatment usually solves what years of spring vacuuming won't.