House fly or cluster fly? How to tell the difference

Not every fly in the house is a house fly. Flies buzzing round the kitchen in July and flies crawling up the bedroom window in January are usually two different insects, there for different reasons. Telling them apart tells you whether you have a hygiene problem to fix or a seasonal visitor to manage.

01How to tell them apart

• Size. House flies are about 6 to 7mm long. Cluster flies are slightly larger and stockier, around 8 to 10mm.
• Colour. House flies are grey with four dark stripes running lengthways down the thorax. Cluster flies are a darker grey, with a chequered pattern on the abdomen and fine golden hairs on the thorax, which you can see close up.
• Wings at rest. A house fly's wings sit apart, in a V. A cluster fly folds its wings flat, one overlapping the other over its back.
• Flight. House flies are quick and hard to catch. Cluster flies are slow and clumsy, and easy to pick off a window.
• Season. House flies are a summer problem. Cluster flies arrive in late summer and autumn, and reappear on sunny days through winter and in early spring.
• Where you find them. House flies hang around kitchens, bins and food. Cluster flies gather in lofts, roof spaces, unused upstairs rooms and around windows.

02Why they come into your home

House flies come in looking for food and somewhere to lay eggs. They breed in rotting food, rubbish and animal waste, so a steady supply of house flies means a breeding site close by.

Cluster flies are not interested in food. Their larvae live in soil as parasites of earthworms, so the adults breed in grassland and fields. In autumn they look for a sheltered, dry place to spend the winter and crawl in through gaps at the eaves, under tiles and around windows. They tend to return to the same buildings year after year.

03The risk each one carries

House flies are the hygiene concern. They feed by softening food with saliva and digestive fluid, they leave droppings wherever they land, and they carry bacteria from waste onto worktops and food.

Cluster flies carry much less risk, because they do not breed in waste. Their nuisance is numbers. A heavily affected loft can hold thousands, they leave small dark spots on walls and window frames, and dead flies build up in heaps that can smell and feed larder beetles and carpet beetles.

04Other flies that gather in houses

Two more are worth knowing. The yellow swarming fly is tiny, about 2 to 3mm, yellow with dark markings, and sometimes gathers in large numbers in upstairs rooms and lofts in autumn, much like cluster flies. Small, furry, moth-like flies around sinks and plugholes are drain flies, which breed in the film inside drains. Our guide to drain flies covers those.

05What to do about each

For house flies, find and remove the breeding site, keep bins and food covered, and fit fly screens. Our guide to flies in the house goes through it.

For cluster flies, cleaning will not help, because they are not there for food. Vacuum up the ones you can reach, seal gaps where you can without blocking ventilation, and think about timing, because treatment works best as they arrive in late summer and autumn. Our guide to cluster flies in the loft covers the full approach.

06A word about bats

Lofts that suit cluster flies can also suit bats. All UK bat species and their roosts are protected by law. If there is any chance bats use your loft, do not use smoke or fogging products there. If you find bats or signs of them, contact the Bat Conservation Trust or Natural England for advice before anything else is done. A professional will check for signs of bats before treating any loft.