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All You Need to Know About Cluster Flies

cluster flies

July 6, 2018 by Chris Chapman

We’ve all dealt with annoying houseflies at one time or another, especially in summer. Irritating as they are, homeowners and businesses can deal with these unhygienic pests pretty easily with a decent fly killer, as you’ll mostly only be dealing with a small number of flies a day. However, cluster flies are an entirely different proposition. As their name suggests, they cluster in large swarms of hundreds or more, meaning they can quickly take over a home, overwhelming traditional fly-killing methods and becoming very difficult to get rid of!

So, have you encountered these prolific pests before, and are you at risk? To find out, read on to learn everything you need to know about cluster flies and how to get rid of them.

What Are Cluster Flies?

Cluster flies

Cluster flies are usually smaller than other species of fly

Cluster flies are a species of fly that – as the name suggests – gather in large clusters of hundreds or even thousands. They’re actually parasitic and will lay eggs in earthworm burrows during the summer. When the eggs hatch, the tiny larvae burrow inside the body of an earthworm, where they’ll stay until they’re ready to pupate and turn into a fully-grown fly.

Identifying Cluster Flies

Cluster flies are smaller than most other species of fly and usually have a slightly yellowish tint to them. There are actually 8 species of cluster fly in Britain alone, so you might find they vary slightly in size and colour. You’ll rarely find one cluster fly alone and will usually be able to identify them by the sheer number of flies in a swarm. If you do find a small number of flies that aren’t part of a larger cluster, check around your home as it’s highly likely you have a larger infestation hiding out somewhere!

Where Are Cluster Flies Found?

earthworm cluster flies

The humble earthworm plays host to cluster fly larvae

As mentioned above, cluster flies are parasitic flies that breed by laying eggs in earthworm burrows. This means they’re much more common in rural areas near fields or woodland. During the summer, they’re quite happy buzzing around outside and generally won’t bother people. However, at the end of summer, as the weather cools, they start to look for somewhere warm and dry to shelter through autumn and winter. This means our cosy weatherproof homes are prime targets for a cluster fly swarm!

The most at-risk part of your home or business is the loft space. This is because the warmth that’s generated up there is incredibly attractive to cluster flies, however, they can be found in other parts of your home too. Cluster flies can crawl through tiny cracks and voids in your wall to explore different rooms of your building, so you might see them swarming around windows as well.

What Problems do Cluster Flies Cause?

Cluster flies aren’t quite as unhygienic as other species of fly as they don’t lay eggs in food, however, they do still pose a hygiene risk, especially in kitchens and restaurants. While they don’t carry disease themselves, they will still transfer bacteria from whatever they land on around your premises, and they’ll hibernate alongside other species of fly which do carry disease while leaving droppings all over the place. It’s not pleasant!

Cluster flies also cause a distinct, sickly odour that can attract other disease-carrying pests, so it’s important you keep an eye on your loft space during the autumn months to make sure you don’t have any unwanted lodgers!

How to Tell if You Have Cluster Flies

cleaning windowsill of flies

Check for signs of flies around your window and windowsill

As cluster flies tend to gather in your attic space, you might not even realise you have them! Many people don’t venture into the attic until it’s time to dig the Christmas decorations out, so if you live in an at-risk rural area, it’s worth regularly checking your attic through the autumn unless you want to get an unwanted early Christmas present!

When you go into a room with a cluster fly infestation, the first sign you’ve got a problem will be the odd sickly-sweet smell that’s left by fly droppings. If you go up there with a torch, you’ll also end up disturbing the flies who’ll be attracted to the light source, so be prepared to get swarmed!

You may also start noticing small numbers of cluster flies around the windows in other parts of your home, along with those sickly-smelling droppings that look like bits of black powder. If you do notice this, that’s a sign you should further investigate your loft space right away!

How to Get Rid of Cluster Flies

cluster fly killer

The Insect-O-Cutor comes with a deep catching tray to hold many more flies than normal electric fly killers

While it’s difficult to prevent cluster flies from entering your home in the first place, you can get rid of them with the right electric fly killer. Normal electric fly killers might not be effective as the sheer number of flies in a normal cluster fly swarm can quickly overwhelm your average fly trap.

Luckily, there are specialised cluster fly killers which have been designed specifically to deal with high numbers of flies. These fly killers offer a high-capacity zapping grid along with an extra-deep catching tray to hold higher numbers of flies. A cluster-fly killer can also wipe out other species of fly too, so it’s a wise investment if you live in a rural area.

An important note about cluster fly killers is that they work by using a UV bulb to attract the flies in using UV light. The bulbs lure the flies onto an electrified zapping grid, where the fly gets fried! You’ll need to replace the UV bulbs in your fly killer every 12 months or so as the element that provides UV light degrades and stops attracting flies. Check out our detailed guide to fly killers for more information on how this works and why it’s important.

You can also combine this with some insect-killing sprays to make sure you catch any flies that the electric fly killer happens to miss. It’s advisable to run a hoover or a brush around your loft afterwards to clear up any remains as leaving dead flies lying around is a hygiene risk in itself.

Wiping out cluster fly infestations as soon as possible is important, as even though the flies will move back outside come the spring, they’re likely to return when autumn rolls around. This is thanks to the faint pheromones and scents left by the original infestation.


Pest control

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3 responses to “All You Need to Know About Cluster Flies”

  1. mr. bean says:

    They also attract other pests. My loft is full of frogs now.

  2. EVEJ says:

    I’m so fed up of them now. First saw fee of them in January by my patio doors, eventually saw them coming through laminate floor gaps. I used dethlac spray and they were gone. Now it’s April and they’re back and they seem to be coming from more than one area. The house is old and full of gaps and cracks so I’ll never seal everywhere. These ones appear one at a time every 10 mins or so, wish there was something that stopped them wanting to come in

    • Chris Chapman says:


      You can stop cluster flies from returning by killing the parent flies. The parent flies leave a pheromone which basically leaves a trail for the swarm to return next year. Killing the parents while they’re in your home means the flies don’t get the chance to leave their pheromone trail, so the swarm won’t be able to find their way into the home again.

      We recommend using a combination of an electric cluster fly killer and an aerosol spray or a smoke bomb to make sure you get every last one. Usually, they’ll gather in the attic, so make sure to check there too. Hope this helps you get rid of the pests!
      – Chris

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