Mystery of dead Bluebottles

Topher

In 2021 the cottage got a rat problem which was solved by Rentokil, we all guessed that the rats had been accessing the roof space from outside, and descending chimneys to get food from the kitchen (guessed this because of locations of rat droppings). During the rat infestation and after Rentokil dealing with them, hundreds of dead bluebottles were found in the dining room and on the stairs and landing connected to the dining room. The house was uninhabited from August 2022 and cleared of all but carpets, curtains and furniture .... no food has been left in this building. Periodically we go and find another load of dead bluebottles, hoover them up then go for months without finding any more. The last lot of dead bluebottle flies occurred over Christmas 2025, again several hundred, but none before for several months. I started this with coverage of the rat problem as it’s the only thing we thought could have caused the bluebottle invasions, but of course we could be going down a blind alley with that. What is causing them? We never see or hear live bluebottles, just get the sudden lot of dead ones. The area where we find them has a cellar underneath with a small sealed window to outside, there are other areas of the house open to the places we find them, but the bluebottles don’t go into the other areas of the house that they could easily access, just dining room, stairs and landing and window sill of same. 

twopenny

The first lot could have laid eggs in a crevice so the 'explosion' .
Any small area of access will do.
I had a phase of this and found it came from a tiny hole at the inside corner of a window. Not a hole right through but just into the plaster. Could have led to the cavity in the wall.

Went on periodically until I found and filled the hole.

ka7e

The flies must be finding a constant source of food to keep emerging over such a long period of time as the eggs are laid on decaying meat or other organic matter. Do you have a partially blocked fireplace? I had an infestation of bluebottles after a bird died in a chimney and the flies emerged in an upstairs fireplace.

greenbee

Are they definitely bluebottles? Could they be cluster flies? 

If the house is empty, it's probably worth using insect foggers, but also opening and cleaning all the windows and spraying the frames with permethrin (cluster flies tend to hibernate/lay eggs in window frames). 

Lorian

Fortefog are the brand I've used, but also if they are cluster flies they hate the smell of citronella. Sainsbury's used to sell a big gel citronella stick but I've not had to buy any for over a decade.

GrubbyGirl_2

How did rentokill get rid of the rats?  if they used poison there could be a few dead rats for the flies to feed on, in which case you need to find them

Momogeew

Exactly, until a/any food source stops the insect activity will perpetuate.

QrizB

One of the buildings at my workplace used to get regular "dead fly" problems at this time of year.

The building was unheated but had significant glazing. It seems the flies entered the building to hibernate over winter. You'd then get a period of settled, clear winter weather. Daytime sunshine would warm the building up and the flies would awaken. Then the overnight frost would kill them all off.

There would be thousands of dead flies crunching underfoot if you had to visit and couldn't find the broom to sweep them up ...

Topher

thank you for comments, I hadn’t thought of something being in the chimney, the chimney would be the most sensible guess as being their conduit. It also means I can use something in the unused closed stove to kill/deter flies if this is their route. 

Lorian

If they are cluster files they just hibernate in lofts during the winter.

Topher

Thank you to everyone who brought up Cluster flies. I’ve looked up identifying features of cluster flies and bluebottles and I’m 99% confident from the more rounded body form, the black colour with a glint of blue, that they are Bluebottles. I was up close and personal with them yesterday as I vacuumed and flushed and generally disposed of them. They were distributed across nearly all the upstairs rooms, both staircases and most upstairs window sills more on one side of the house than the other. The dining room at the foot of one stair case has a significantly higher number. There are three roof access hatches of very small size, one directly over the staircase I described. Absolutely no flies in the fireplace we previously thought the rats had been using as a conduit to the roof space. If they’re dying as they emerge, they would seem to be coming downwards.