Flies in the house, and the two things that actually cause it
A sudden mass of flies indoors is a reading on a source, not a fly problem. Councils put it at two causes: infested food matter, or a dead animal something has laid eggs in. And if it is autumn and they are upstairs, it is probably neither.
By The Pest GuyLast updated
The short answer
Work out which fly it is first. It changes everything that follows.
If the house has suddenly filled with ordinary flies, something is breeding. Lewisham Council reduces it to two causes: either there is infested food matter somewhere nearby, or there is the carcass of a dead animal in which the flies have laid their eggs. The job is to find it, and the spray is the afterthought.
If it is autumn, there are hundreds of them, and they are in an upstairs room or the loft, it is probably a different animal doing something completely different. Cluster flies come in from the fields to hibernate. They are not breeding in your house, their presence is not a sign of poor hygiene, and there is a case for doing nothing at all.
If it is a warm day in high summer and the winged insects have elbowed antennae rather than fly bodies, that is a third thing again, and the shortest-lived of the lot. A flying ant swarm is a mating flight. Any one peak lasts only a few days, but they come in several peaks across a season that runs June to September, so a swarm passing is not the end of them. There is genuinely nothing to buy for it either way.
And if they are very small, they are almost certainly neither. Tiny flies are a fruit bowl or a plumbing problem, and fly spray is the wrong tool for both. If it is the fruit bowl, getting rid of fruit flies starts with whatever they are breeding in.
Telling them apart, which is the part that decides what to do
Three different animals get called flies in the house, and the honest answer to each one is different. This is the whole diagnosis.
| The tell | House and blow flies | Cluster flies | Fruit and drain flies |
|---|---|---|---|
| What you see | One or many, all year, around food and bins | Hundreds at once, autumn or spring, upstairs or in the loft | Very small, near the fruit bowl, a plant pot or a drain |
| Where it came from | Food matter or a carcass, inside or right outside | A field. Larvae are parasitic on earthworms in the soil | Fermenting fruit, or sludge where water evaporates |
| Breeding indoors? | Yes, and that is the whole problem | No, never. They are hibernating, not infesting | Yes, in the fruit or the trap of the drain |
| What actually ends it | Find and remove the source | Time. They leave or die off, and return next autumn | Bin the fruit, or find the water |
Sources: Lewisham Council's flies information sheet, Adur & Worthing Councils, Rochdale Borough Council, and the cluster fly fact sheet published by Cotswold District Council. Cluster flies look like house flies apart from some golden hairs behind the head, and Rochdale notes those hairs rub off easily, so the season and the room are better tells than the insect.
A sudden mass of bluebottles is a search, not a spray
The reason it feels like an invasion is arithmetic, and the reason spraying does not hold is that the source is still there.
Nothing arrived. Something hatched. A female bluebottle lays up to 600 eggs and they can hatch in less than 24 hours; the larvae develop over about a week, and the adult emerges roughly two weeks after that. Houseflies lay in batches of up to 150, hatching in 8 to 48 hours depending on temperature, and can finish larval development in five days. One level tablespoon of moist food debris is enough to support 100 houseflies from egg to adult, which is the figure worth keeping: the source does not have to be dramatic.
So the question is where. Lewisham names the places a carcass hides, and they are the places nobody looks: lofts, for birds and squirrels, behind old fireplaces, under floorboards, and behind bath panels, kitchen appliances and cupboards. Bluebottles breed on meat, meat products and offal, or on excrement, so a bin that has held either will do the same job as a dead animal.
Worth clearing up the flies that did not breed here too, and not for tidiness. Council guidance on cluster flies is explicit that dead flies left on a loft floor "can lead to infestations of undesirable insects, such as carpet beetles", which "feed on fabrics". A loft with a season's worth of them on the floor is a food supply for something that eats wool, so the dustpan is doing two jobs.
In a house the carcass is very often a mouse that took bait and died in a cavity, or a rat under a floor. If anything was baited in the weeks before the flies started, that is the first place to look. It is the same discipline that answers where silverfish come from: find what is feeding them rather than treating what is walking about.
The honest part, which the pages selling a visit tend not to print: a dead animal cannot always be found. If it is in a cavity or under a floor, you may end up waiting out the cycle rather than removing anything. That is a real outcome and it is worth knowing before you pay someone to look.
Councils do recommend using an insecticide as well, following the instructions on the packaging, and a spray is the reasonable answer to the flies already in the room. It is the order that matters. With the source still in place you are treating the output of something that is still producing.
Cluster flies are not an infestation, and your loft is not dirty
The autumn case, and the one where the honest advice is closest to doing nothing.
Cluster flies spend the summer in fields, where their larvae are parasitic on earthworms, and emerge as adults at the end of summer. In autumn they gather on the outside of buildings, particularly on sunny, south-east facing, light-coloured walls, and as the afternoon cools they crawl into crevices for shelter, mostly under the eaves and fascia boards and into the roof space. The name is literal: they form clusters to hibernate.
Two things follow from that, and both cut against what gets sold. Their presence is not a sign of poor hygiene, which the Cotswold fact sheet states outright. And fitting fly screens is of little use against this species specifically, because it squeezes through small cracks around tiles and window frames rather than coming in at a window.
Autumn is also when house spiders start turning up in the same rooms, and the two are related: a room with more insects in it feeds more spiders. That is a seasonal peak in sightings rather than an arrival, in both cases.
They will very likely be back. The flies return to the same property year after year, similar buildings nearby may be unaffected, and the fact sheet is refreshingly plain that it is not known why one building is preferred: possibly the lighter colour, the sunnier spot, or pheromone markers left at an old roost. Rochdale tells you to expect a new lot the following year for the same reason.
One consequence is worth acting on even if you leave the flies alone. Rochdale notes that dead cluster flies on a loft floor can lead to infestations of carpet beetles, which feed on fabrics. So the clearing up is not cosmetic, and it matters more than the spraying does.
Before anyone sprays a loft, the question about bats
This is the part that is not a preference or a piece of good practice. It is the law, and the duty is yours as well as the contractor's.
Cluster flies gather in roof spaces. So do bats. The fact sheet Cotswold District Council publishes puts the position in one sentence: it is illegal to use insecticide if there is any possibility that it might come into contact with bats, and the occupier and the chosen contractor both have a legal duty to ensure the bats come to no harm. Rochdale says the same thing in plainer words, that if there is evidence of bats you must not use insecticides.
Read the wording carefully, because it is stronger than it first looks. The test is not whether you have seen a bat. It is whether there is any possibility of contact. And the duty does not transfer to whoever you hire. If you engage someone to fog a loft, you are not handing the responsibility over with the invoice.
This site does not give legal advice and cannot tell you what is in your roof. What it can tell you is that this is the question to settle before anything is sprayed, and that an organisation that works with bats is the right place to ask, not a contractor quoting for the treatment.
What a treatment does not do
Worth reading before you agree to one, and it comes from the contractor's own terms rather than from anyone's opinion.
The Cotswold fact sheet carries the schedule of terms for its own cluster fly treatment, and two lines in it are more useful than anything in the advice above them. The service is to inspect and treat the active habitat with insecticide with the intention of controlling the problem, and then, verbatim: this may not eradicate the cluster flies. The price includes one treatment only.
The same sheet is equally straight about the DIY version. An ordinary aerosol fly spray in the loft will kill all the cluster flies it hits, but will not penetrate the cracks where flies are hibernating. A vacuum cleaner and fly papers deal only with the ones you can see. None of that makes any of them useless. It makes them a way of reducing the flies in front of you, which is a different purchase from ending the problem.
That distinction is the one to carry into the conversation. If you are quoted for a treatment, the questions worth asking are what it is expected to achieve, whether it is one visit or a programme, and what happens next autumn, because the sourced answer to the last one is that they will probably be back.
If the source is outside, one thing does help
Only for the outdoor case, and it is worth being clear about that. If flies are coming off bins or a refuse area, catching them out there is working on the source. If something is breeding under your floor, this does nothing at all and finding the carcass is still the entire job.
What I’d use
Outdoor baited fly trap
Zero In Ultra Power Fly Trap
A reusable outdoor trap, ready-baited with an 8g sachet that is activated with water and lasts up to four weeks.
It is bait rather than spray, which is what makes it usable where a spray is not: around bins, a stable or a refuse area, outdoors, with nothing to land on surfaces people touch. The listing puts its attraction radius at 10 metres.
- Bait
- One 8g sachet, activated with water, up to 4 weeks
- Attraction radius
- 10 metres, per the listing
- Stated capacity
- Up to 40,000 flies per fill
- Size
- 260mm x 85mm x 85mm
- Use
- Outdoor only
Watch out for
- NO PAGE ON THIS SITE COVERS FLIES YET, so nothing renders this card. See CLAUDE.md.
- Outdoor use only. It draws flies towards it, so siting it matters.
- The bait lasts up to four weeks, then it needs another sachet.
- Screwfix showed a single review when this was written.
That link earns me nothing. It is not an affiliate link, no brand pays for a mention here, and I get the same whether you buy this or ignore it. It is on the page because it is what the job takes.
No brand pays for a mention here, and nothing on this page earns me a commission. The card above links out to a shop, and I get the same whether you buy or ignore it. The link is there because finding the thing is the boring half of acting on advice, which is also why the watch-outs sit on the same card as the reasons to buy.
About this resource
The weight on this page is carried by public-body sources rather than by pest-control marketing: Lewisham Council's flies information sheet, Adur & Worthing Councils, Rochdale Borough Council's cluster fly page, and the cluster fly fact sheet published by Cotswold District Council, which is also where the contractor's own treatment terms come from.
The honest limit is identification. This page can tell you what each case usually means and where to look; it cannot tell you which fly is in your house, and it would be doing you no favours by pretending otherwise from a description. No brand pays for a mention here and nothing on this page earns me a commission.
Common questions
Why do I suddenly have 20 flies in my house?
A sudden large number almost always means something is breeding, and Lewisham Council puts it at two causes: either there is infested food matter nearby, or there is the carcass of a dead animal that the flies have laid eggs in. The speed is what makes it feel like an arrival. Bluebottle eggs can hatch in under 24 hours, and one level tablespoon of moist food debris is enough to support 100 houseflies from egg to adult. Nothing came in. Something hatched.
What does it mean when you have a lot of flies in your house?
It means you are looking at the output of a source rather than a fly problem in its own right. That is why killing the ones in the room changes nothing for long: the next batch is already developing. The exception is autumn, when large numbers appearing in an upstairs room or a loft usually means cluster flies, which are not breeding in the building at all.
Where do I look for a dead animal?
Lewisham names the places worth checking: lofts, for birds and squirrels, behind old fireplaces, under floorboards, and behind bath panels, kitchen appliances and cupboards. It also says plainly that a dead animal cannot always be found, which is the honest part most pages leave out.
How do I get rid of lots of flies in my house?
Find and remove the cause first, because that is the only step that stops it happening again. Councils do also recommend using an insecticide alongside that, following the instructions on the packaging, for the flies already in the room. The order matters: a spray with the source still in place is a subscription, not a fix.
What are the tiny flies in my house?
Usually one of two things, and neither is a small housefly. Fruit flies come from fermenting fruit or old vegetables. Drain flies come from water: they lay eggs in the sludge left as water evaporates, and Lewisham warns that a pool is not even needed, because a trickle from a pipe joint when the toilet is flushed is enough. So the answer is in the fruit bowl or the plumbing, not in the air.
How long will a fly infestation last?
As long as the source does, which is why no honest page gives you a number of days. What is knowable is the speed of the cycle behind it: housefly eggs hatch in 8 to 48 hours depending on temperature and the larvae can finish developing in five days; a bluebottle takes about a week as a larva and roughly two weeks to emerge as an adult. Remove the source and you are waiting out one cycle. Leave it and you are not waiting for anything.
Do I need pest control for cluster flies?
Usually not. They do not breed indoors, their presence is not a sign of poor hygiene, and they are hibernating rather than infesting. It is also worth knowing what a treatment does and does not buy: the fact sheet Cotswold District Council publishes carries its own contractor's terms, and those say the service "may not eradicate the cluster flies" and that "the price includes one treatment only".
Why do they come back to the same house every year?
Nobody is certain. The Cotswold fact sheet is unusually honest about it: the flies return to the same property year after year, nearby buildings may be unaffected, and "it is not known why one building is preferred". The candidates are lighter-coloured walls, sunnier aspects, or pheromone markers left at an old roost, which is also why Rochdale tells you to expect a new lot the following year.