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Fly infestation

Does anyone have any tips for eradicating flies from a conservatory?

I have a double glazed conservatory and am plagued by thousands of black flies, smaller than blue bottles, which seem to come from inside the framework of the conservatory. As soon as there is a bit of sunshine and warmth, out they come. Fly spray kills the adults for an hour or two, before more appear. I am looking for a way to kill and stop them from reproducing and laying eggs inside the framework of the building. There is a hollow channel, part of the construction, where they seem to come from. The gap is so narrow I cannot get my fingers inside to clear it out. I did use a teaspout cleaning brush. It was gross! Almost like soil, there were so many corpses etc.
 I soaked some piping cord in fly killer and pushed that into the channel which worked for a while. I think I am going to have to pull it out, resoak it and put it back. I sweep up dead flies on a regular basis, they get caught on cobwebs and plants. One year I used a couple of sulphur candles which helped partially but ruined the chrome handles on the doors of the conservatory. I am reluctant to use them again.
I call the flies, field flies, they are not blue bottles. They are dull black/dark brown and there are thousands of them. Ugh!


  • Ladybird4Ladybird4 Third rock from the sunPosts: 34,460
    Hello Joyce. Flies mostly need rotting organic material to feed on and this is where they will lay their eggs so that the larvae will have something to feed on when they hatch out so it is unlikely they will lay eggs inside the framework of the conservatory. Do you have open doors and windows in your conservatory? This is where flies normally enter a structure like that. There are a few natural scents that put flies off. Basil, cinnamon and mint to name a few. If you can grow these outside your conservatory it may reduce the number of flies you have entering the building.
    Cacoethes: An irresistible urge to do something inadvisable
  • They could possibly be cluster flies.
  • If you search for 'The strange life of the cluster fly' it should bring up an item about them (it's by Rentokil.) They gather in my loft space but tend to come into rooms if there is a tiny gap showing light from below. They also gather in gaps around window frames, even the smallest gap that allows for the window to open without catching the frame.
  • ObelixxObelixx Vendée, Western FrancePosts: 28,045
    Sound like cluster flies to me too. We get invasions in spring and autumn as they fly out of the roller shutters on the outside of the house and come in thru open windows.  We leave the windows open so they can escape again and sweep up the numerous corpses every day.

    No idea why they do it or what they feed on and they don't invade every window and shutter.  I use self sticking circular thingies on the windows affected and fly paper coils to catch them.  Harmless, I think, but a bit of a PITA in such numbers.
    "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
  • In the item I mentioned above it does say that they don't lay eggs on food like other flies might, so that's good news.
    They lay their eggs in the soil and the larvae are parasitic on earthworms. The larvae pupate in the soil and emerge as flies in the spring.

  • a1154a1154 Sunny South Scotland Posts: 996
    We had cluster flies living in the window frames one summer, it was absolutely horrible. After a good clean out we used a pen from rentokil, round the frame, and never had them again. 
    To be honest I don’t think they come every year, so I’m may not be significant, but we felt happy with it. 
  • We used to use fly papers but so ugly.
    We have insectivorous plants...Sarracenias, Darlingtonia and butterworts.
    They do a great job especially the butterworts at catching the very small flies.
    Sundews are another that are brilliant with the small pesky ones.
    Great plants and not ugly.
  • pr1mr0sepr1mr0se Posts: 1,172
    We had infestations of cluster flies for a few years - and the loft was, on one occasion, just like the film The Amityville Horror!  A friend who worked in pest control confirmed that they were cluster flies - and warm lofts/window frames etc are good places for them to congregate over winter, coming out on warmer days.
    If it's any consolation, he confirmed that they don't breed indoors and don't land on dirty stuff.  They mainly eat pollen - and our choisya was smothered in them each year when they came out of their winter clusters.
  • Thank you everyone.
    I am sure they are cluster flies as they do not seem to go into the house. I have also had them coming out of old wooden window frames in the house, which  I have now been waiting 2 years to have replaced, due to Brexit and Covid production and delivery problems.
    I forgot I did try fly papers a couple of years ago, they caught a lot of flies but again, they are so gross to look at and there are so many flies. I want to eradicate the flies permanently. They are in the frame of the conservatory where the roof glass meets the wall glass, there is a lot of debris inside the frame so ideal for egg laying. When I tried clearing out the debris with the teapot spout brushes it involved flicking out the debris a bit at a time because the slot along the frame is so narrow. I assume the debris has built up from thousands of flies not being able to crawl through the slit into the conservatory.
    The conservatory faces South so I keep both roof windows, a single and a double door, plus 5 fan light windows open 24 seven, during most of the summer because it gets so hot inside. Too hot to grow plants, I even have a parasol over the table to give some shade. (My daughter told me I am the only person she knows who has a parasol indoors)
    I think I am resigned to wearing a visor and protective clothing, removing the piping cord, washing and resoaking it with fly killer then stuffing it back into the frame after brushing out the frame as best I can, while balancing on a step ladder.
    I am in my eighties so find the job somewhat daunting to think about, let alone carrying it out.
  • @Joyce Goldenlily From information I have gleaned from the internet, and from what @pr1mr0se was told by the pest control friend, (see posts above) you need not worry about them laying eggs indoors if they are cluster flies.
    Perhaps you could somehow use the pen mentioned by @a1154 ? If you could apply it along the channel edge it might deter the flies from crawling in there. 
    But how to prevent them from flying into the conservatory through open windows and doors -- I can only think some sort of flyscreen would be needed.
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