All About the Cluster Fly - a cluster fly on a mango

All About the Cluster Fly

October 2021

All About the Cluster Fly

Autumn brings us many agreeable things. Golden leaves falling from trees, chestnuts making the pavement prickly, Guy Fawkes night, pumpkins and Hallowe’en and the clocks going back. In rather less poetic style, autumn is also the season for cluster flies. Thus this post is all about the cluster fly.

All About the Cluster Fly - a cluster fly on a mango
A cluster fly on a mango

Cluster fly facts!

  1. The cluster fly, aka Pollenia, is common throughout the UK – in the main in rural areas. As I daresay you can guess, its common name derives from its habit of gathering together, in large numbers, in upper rooms or in roof spaces to hibernate.Unlike with the bluebottle, the presence of cluster flies in your home doesn’t indicate poor hygiene.


  1. Britain has over twenty cluster fly species recorded. And most of them you’d need a microscope to differentiate between them.


  1. The most common species is Pollenia rudis. In summer and autumn, you can often find it sunning itself on sheltered fences or tree trunks.


  1. The BPCA A-Z of pests tells us that the cluster fly is dark-greyish in colour and is about 8mm long. It has yellowish hair on its back and its wings overlap.


  1. They can leave tiny spots of dark-coloured excrement on windows or walls. Though not harmful to humans it doesn’t look nice and can be a tough cleaning job.

They become a major nuisance in rural homes and buildings when they overwinter. Or, more to the point, come the spring when they’re trying to escape back outside.

In a building with a heavy infestation there can be several thousand of these flies. It sometimes happens that a few thousand of them form a cluster. Such a thing, if not identified as cluster flies, encourages the assumption of dirty conditions being present. And with that a public health concern. While that’s not the case, it’s clear they’re a severe nuisance, with their sluggish mode of flying around living quarters in large numbers. And besides that, they pong! A cluster fly mass has a characteristic sickly, sweet smell that serves as a good indicator of their presence.

Biology and behaviour

Besides squatting for the winter in upper rooms and roof spaces you can also find them around the edges of window frames. Indeed anywere, where there are gaps that they can gather in.  They’re on record as setting up a winter home in tunnels made by beetles in timber and even in animal burrows.

Before they settle down for their long winter sleep they’ll have filled their abdomens with fat globules. That store of nourishment is what helps them survive the months of inactivity.

Come the return of warmer days, they become active again. Note though, our temperature-controlled homes can trick them into thinking spring has sprung and make them emerge earlier than they should.

Once outside conditions are stable, the adult fly cracks on with reproduction. It does that by laying its eggs in damp soil or rotting vegetation. The larvae hatch about a week later and commences looking for earth worms. Poor old Wiggly Woo!

The cluster fly diet

The humble earthworm forms the major food source for the Pollenis rudis lavae. The main species of earthworm that cluster flies infect are:

  • Aporrectoda caliginosa
  • Aporrectoda chlorotica
  • Eisenia lucens
  • Lumbricus rubellus
  • Lumbricus terrestris

The process is somewhat grisly. The larvae eat their way through the integument section of the earthworm’s epidermis. While they’re feeding they leave the spiracles outside the earthworm. Meanwhile, inside the earthworm, the larvae carry on noshing until they’re ready to pupate. When they are ready, the larvae bores its way out of the earthworm and tunnels its way back to get close to the surface of the soil where it pupates.

Worms aside, the adult cluster fly favours an herbivore diet feeding on many types of organic matter. Plant sap, fruit, flowers and the like.

Getting rid of cluster flies

It’s not impossible to remove a small number of cluster flies with a vacuum cleaner or even an aerosol fly spray. 

But for a heavy infestation it’s really, for your own peace of mind, best to call in pest control experts such as us. If you have cluster fly concerns don’t hesitate to get in touch. Drop us a message on our webform here or call 01793 780 600. 

The social sort?

If you hang out on social media you can see what we’re up to on Facebook, Instagram and LinkedIn