The 4 fly species you see inside
I get asked a few times every day in summer: “Johan, which flies do I actually have?” Because there are different species, and each one requires a different approach. Let me explain which flies you see most often indoors and how to recognise them.
The Common Housefly: Everyone Knows It
This is the fly everyone knows. The common housefly (Musca domestica) is 6-7 millimetres in size, grey-brown in colour with four dark stripes on its back, and has red eyes. It buzzes noisily and lands on your food, your walls, everywhere basically.
What does she do? She eats liquid food and vomit. Yes, really. They spit on solid food to digest it and then suck it up. That is why they are so filthy - they spread bacteria from one place to another.
Recognition: If you see a medium-sized, noisily buzzing fly that lands anywhere and is hard to catch, it is probably a housefly. They are the fastest and most agile of all.
Fruit flies: Small But Massive
You probably know this one. Fruit flies (Drosophila) are tiny - 2-3 millimetres, light brown to yellowish, and have red eyes. And they never come alone. If you see one, there are ten. If you see ten, there are a hundred.
What do they descend on? Ripe and rotting fruit, wine, beer, vinegar, vegetables that are spoiling. They can smell where sugar is fermenting, and they go for that. A banana lying too long? Within a day there are fruit flies around it.
The annoying thing about fruit flies? They multiply at lightning speed. A female lays up to 500 eggs in her short life of 8-10 days. Those eggs hatch after only 24 hours. You get the picture: within a week, you can go from zero to hundreds.
Blowflies: Large And Noisy Visitors
Humming flies are larger than houseflies - 10-14 millimetres, solidly built, and they make a deep humming sound (hence the name). They are blue-metallic, green-metallic, or black-glossy. Beautiful colours actually, if they weren't so filthy.
Blowflies lay their eggs on cadavers, meat, fish, and other organic rubbish. That's why you often see them near rubbish bins, dead animals, or when meat is rotting somewhere. If you suddenly have humbugs in your house, something is dead somewhere - a mouse in the wall, a bird in the chimney, or a rat under the floor.
Last month, I had a client who suddenly had 10 humbugs in the living room. I searched and found a dead mouse behind the fridge. As soon as we got rid of it, the flies were gone within a day.
Drain flies: The Bathroom Guests
Drain flies (also called sewer flies) are small, black flies measuring 2-5 millimetres with long antennae and hairy wings. They look like tiny moths. You see them mostly in the bathroom, kitchen, toilet - anywhere there are drains.
Where do they come from? They breed in organic material in your drains - hair scraps, soap scum, food scraps, anything that sticks there. The larvae eat that stuff, and after 1-2 weeks, adult flies emerge from your drains.
Recognition: If you see little black flies sitting on the wall near your sink or shower, and they fly a little awkwardly, they are drain flies. They are poor fliers and often sit still.
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Where do flies breed? (Find the source!)
OK, you know which flies you have. But most importantly: where do they come from? Because if you don't remove the source, they will keep coming. Let me explain to you where flies breed and how to find the source.
Rubbish And Waste: The Number 1 Breeding Place
This is by far the most common source. Flies lay their eggs on organic waste - food scraps, fruit and vegetable peelings, food waste packaging, dirty dishes.
Houseflies and bluebottles love this. They can smell your dustbin from miles away. Literally. A fly can smell the smell of rotting food up to 7 kilometres away. That's why your dustbin should be tightly sealed.
My advice: empty your bin daily when the weather is warm. Use a bin with a lid. Rinse out the bin regularly. And if you collect organic waste? Do that outside, not inside.
Fruits And Vegetables: Fruit Fly Paradise
A fruit bowl on the table seems harmless. But as soon as that fruit gets ripe and starts to spoil, the fruit flies come. They lay their eggs in the peel, and those larvae eat their way inside.
What should you look out for? Fruit in the fruit bowl (especially bananas, peaches, tomatoes), potatoes and onions that are sprouting or rotting, open wine or beer bottles (even a small leftover attracts them), leaky containers in the pantry.
Tip from me: store ripe fruit in the fridge. Throw away spoilt fruit immediately (outside!). Rinse out empty wine bottles before putting them in the bottle bank.
Drains And Sewers: Where Drain Flies Live
In your drains, there is a layer of organic material - hair, soap scum, food scraps, grease. This is perfect for drain flies to breed in. They literally live in your drains.
Which drains? Kitchen sink (especially if you throw food waste in there), shower drain (full of hair and soap), bathroom sink drain, floor drains in basement or garage.
How do you test if they are in your drain? At night, stick transparent tape over the drain. If there are flies on the tape in the morning, you have found your source. Then clean that drain thoroughly with special drain cleaner or boiling water.
Cadavers: The Source Of Hummingbirds
If you suddenly have humbugs inside, something has died somewhere. Often a mouse or rat that has died in the wall, under the floor, or in the attic. It could also be a dead bird in the chimney or attic.
How do you find it? Follow the humbugs - they come from the direction of the carcass. Pay attention to smells (you can smell rotting meat). Look in places you don't normally go: behind the fridge, under the floor, in crawl spaces.
Last week, I had a client with humbugs in the living room. I finally found a dead pigeon in the kitchen drain pipe. As soon as we removed it, the problem was solved within a day.
Manure And Compost: Garden Problem
Do you have a compost bin, animals in the garden, or use manure as fertiliser? If so, you attract flies. Especially in summer, this can be a huge problem.
What works. Cover compost bin well, no meat or dairy products on the compost, undercut manure immediately in the ground (do not leave it on the surface), clean chicken run or rabbit hutch regularly.
Why do flies come to your house?
Good thing you asked! Because flies don't just come. They are attracted to specific things. And if you know what they find attractive, you can ward them off. Let me explain.
Smell Of Food: The Main Reason
Flies have an incredible sense of smell. They can smell food from miles away. What attracts them?
Sugars and fermentables: Ripe fruit, soft drinks, juice, wine, beer. Fruit flies are the first to descend on these.
Proteins and fats: Meat, fish, cheese, eggs, milk. Houseflies and bluebottles love these.
Rotting organic matter: Rubbish, compost, manure. This mainly attracts bluebottles.
I often see people not realising how many odours their homes emit. A dustbin without a lid, a fruit bowl with overripe bananas, dirty dishes in the sink - these are all invitations for flies.
Heat: Flies Love Warm Weather
Flies are cold-blooded. They are most active at temperatures between 25-35 degrees. That's why you suffer the most in summer, and hardly any in winter.
What does this mean? In warm weather, flies multiply at lightning speed. A housefly can go from egg to adult fly in 10 days in summer. In winter, it takes months (or doesn't happen at all).
Flies also enter your house because it is cooler or warmer there than outside. On a hot summer day, they seek shade and water. On a crisp spring day, they seek warmth.
Season: Peak Periods For Flies
When do you suffer most from flies? That depends on the season:
Spring (March-May): Flies awaken from their winter hibernation. You will especially see houseflies that have hibernated in your home.
Summer (June-August): Peak season! All fly species are super active. This is when I get the most calls.
Autumn (September-November): Flies seek shelter for the winter. You can see them in the house trying to hibernate.
Winter (December-February): Few to no flies. Most die, some overwinter in sheltered areas.
Open Windows And Doors: The Access Route
This sounds obvious, but it is the main way flies come in. You open a window for fresh air, and within an hour there are flies.
What can you do? Use screens for windows and doors. Keep doors closed as much as possible. If you have to have a door open for a long time (for example, during a move or party), put a fan in the doorway - flies can't fly against it.
Flies also get in through cracks and holes. Check your window frames, wall cracks, vents. A fly will fit through an opening of 3-4 millimetres. That's tiny!
Light at night: especially for bluebottles
Some fly species (especially bluebottles) are attracted to light. If you sit with the light on and the window open at night, they will come inside.
My tip: at night, close the curtains or blinds before turning on the light. Or use yellow light - it attracts fewer insects than white light.
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Health risks of flying
You may be thinking, “It's only flies, how bad can it be?” Well, let me be honest: flies are one of the nastiest insects there is. They spread diseases and bacteria, and they are a huge problem especially in the hospitality and food industry.
Spreading Bacteria And Germs
Flies are walking bacteria factories. Why? Because they land in the filthiest places - poop, cadavers, rubbish, sewage - and then they land on your food. And they take all those bacteria with them.
What diseases can flies spread? Salmonella (food poisoning), E. coli (diarrhoea and abdominal pain), Shigella (dysentery), Typhus, Cholera (especially in tropical countries). They also spread worm eggs and other parasites.
How does that work? Flies have hairs on their legs and body that bacteria stick to. They also spit on your food to digest it, and that vomit is full of bacteria. When they defecate on your food (which they often do), bacteria are also released there.
In the Netherlands, serious illnesses caused by flies are rare, but food poisoning does occur. Especially in vulnerable groups - babies, the elderly, people with weak immune systems - flies can be really dangerous.
Contaminating food: What to watch out for
If a fly lands on your food, you should actually throw it away. Seriously. Because in the seconds a fly is on your sandwich, it has already left bacteria behind.
Which foods are most at risk? Moist and high-protein foods (meat, fish, dairy, eggs), prepared food eaten cold (salads, buffets), fruit and vegetables, food exposed for a long time (at a buffet or barbecue).
My advice: always cover food. Use covers at a buffet. Don't eat food that has had flies on it. Throw away doubtful cases - better that than getting sick.
Hygiene In Hospitality And Food Industry
In the catering and food industry, flies are a nightmare. The Dutch Food and Consumer Product Safety Authority (NVWA) can shut down a business if there are too many flies. And rightly so, as it is a huge hygiene risk.
I often work with restaurants and catering companies. What I see? If you have one fly in the kitchen, more come. And before you know it, you have an infestation. That's why you have to act immediately.
For catering establishments: strict prevention (screens, covers, no open food), daily cleaning, professional fly trough with UV light, regular checks.
Nuisance And Annoyance Factor
Health risks aside, flies are just hugely annoying. They buzz around your head while eating, land on you, wake you up in the morning with their buzzing. If you have a lot of flies, you can't sit quietly.
It is also embarrassing when you have visitors and there are flies in the house. People then think it is dirty (which is not necessarily true - flies come into the cleanest houses too).
That's why I always say: tackle flies directly. Find the source, remove it, and prevent new flies from coming. Have you tried it yourself but are not succeeding? Call me. I often visit you the same day to see what's going on. And I tell you honestly what you can do yourself and when you need professional help.
Frequently asked questions about recognising flies
These are the questions I get most often about flies in the house. Is your question not among them? Feel free to call me for personal advice.
Each fly species has its own characteristics. Houseflies are 6-7mm in size, grey-brown with four stripes on the back and red eyes. Fruit flies are tiny (2-3mm), light brown with red eyes, and are found en masse around fruit. Blowflies are large (10-14mm), metallic blue or green, and make a deep humming sound. Drain flies are small black flies (2-5mm) with hairy wings that you see mostly at drains. Pay attention to size, colour, behaviour and where you see them - that will tell you which species it is.
Fruit flies come from eggs laid on or in ripe and rotting fruit. A fruit fly smells fermenting sugars from afar and lays its eggs there. Within 24 hours, the larvae hatch and develop in the fruit. After 8-10 days, they are adult flies. So you get them through fruit you buy (the eggs are often already on it), or they come flying in when you leave fruit outside. They are also attracted to open wine, beer, vinegar, and spoilt vegetables. One banana lying too long can produce hundreds of fruit flies within a week.
Yes, flies can be dangerous because they spread bacteria and germs. They land on poop, cadavers, and rubbish, taking all those bacteria with them on their legs and bodies. Then, when they land on your food, they contaminate it. Flies can transmit salmonella, E. coli, shigella, and other pathogens. In the Netherlands, serious illnesses caused by flies are rare, but food poisoning certainly occurs. Especially for vulnerable groups - babies, the elderly, people with weak immune systems - flies are a serious risk. Always cover your food and throw away food that flies have sat on.
Prevention starts at the source. Empty your dustbin daily in hot weather and use a lid. Store ripe fruit in the fridge. Always cover food. Do the dishes immediately or rinse dirty dishes. Clean your drains regularly with boiling water or drain cleaner. Use mosquito nets for windows and doors. Keep doors closed as much as possible. Clean up fallen fruit in the garden. Close your compost bin tightly. Check for dead animals in or near your house. If you do all these, you are already 90% less likely to have flies. The rest are flying visitors that you can catch with a fly swatter.
Flies lay their eggs on organic material where the larvae can eat. Houseflies lay eggs on manure, compost, rubbish, and decaying organic matter. Fruit flies lay eggs on ripe and rotting fruits, vegetables, and fermenting substances. Blowflies lay eggs on cadavers, meat, fish, and animal wastes. Drain flies lay eggs in the organic material in your drains (hair, soap, grease). Each fly species has its own preference, but the common thread is: they lay eggs where their larvae find food. If you remove the nesting site, the flies stop coming.
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