February 9th, 2021 by Sarah Miles

Having any kind of pest infestation in your home is an unpleasant thought, but it’s surprisingly common – and, in most cases, it does not mean that your home is dirty. Many pests establish themselves in dwellings due to the availability of food and shelter, and cluster flies are one such pest.

While there are a few different species of cluster fly, the most common is slightly larger than a housefly with a dirty yellowish look. They are also slower and more sluggish than house flies. However, trying to identify individual cluster flies usually isn’t needed – as the name suggests, they travel around in huge swarms which are hard to miss!

During the summer, these swarms live outdoors and generally don’t bother anyone. However, once summer ends, they’ll seek shelter from the cold weather of autumn and winter in homes, which is when you’re most likely to spot signs you have them.

The signs of a cluster fly infestation

Like many pests, cluster flies prefer to stay hidden away – you would certainly know about it if they didn’t as they live in swarms of thousands of flies. While it’s certainly possible you may spot the odd one – sometimes they’ll move around your house in void walls -  but there are other, more common, signs that you need to be vigilant of.

The sickly, sweet smell

One of the key characteristics of cluster fly behaviour is the sweet, unpleasant smell that they leave. This is produced by their saliva that they deposit on the walls of the space that they occupy. This smell is much more than a by-product of their occupancy; it has an important function. The reason for the smell is the pheromone that the saliva contains. This is specific to the flies in each particular swarm and operates as a marker, to guide them back to your home after they’ve spent the summer outdoors, mating.

As they like spaces that are warm, dark, and quiet, they’re most likely to be found in lofts and attics (they are sometimes known as ‘attic flies’ because of this). If you detect this kind of smell upstairs, or while putting away the Christmas decorations, it may well be a cluster fly infestation.

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Yellow marks

The saliva that contains the pheromone is also yellowish in colour, so if you notice stains on your walls, this is another common sign of cluster flies.

Flies around light sources

Cluster flies are attracted to light so, while they like to stay hidden in your loft, if you shine a torchlight into your loft, many will be tempted out and swarm the light. This isn’t the most pleasant thing in the world, but they won’t harm you and it’s a dead giveaway. If you do see the odd stray one in other areas of the house, it will usually be around light fittings, lamps, or even windows.

What to do if you suspect you have a cluster fly infestation

As with any pest, even though cluster flies are harmless to humans and pets, you need to take measures to eradicate them because you don’t want them living in your home. Timing is as important as treatment for this particular pest due to the pheromone they leave behind.

Left alone, the swarm will leave of their own accord in the summer so you may think you don’t really need to take action, especially if they’re not really bothering you. Unfortunately, that’s not the end of them as the pheromone they left behind will guide them back in the autumn when they seek shelter from the colder weather again. If you don’t want to keep repeating this cycle, it is important to exterminate them before they leave in the spring/summer.

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How to get rid of cluster flies

Due to their numbers, you need to employ a ‘shock and awe’ strategy to ensure you have eradicated the whole swarm. This needs to be a combination of treatments, including automatic aerosol dispensers, smoke bombs, and electric cluster fly killers, and they should be repeated a few times until you can realistically assume you’ve killed them all.