Written by Joe Ballenger

Flies can be tricky to talk about. There are well over 100,000 described species, and it’s likely that there’s at least one million which haven’t been described yet. Most are small enough to not be noticed, but some do show up in large numbers in people’s houses. These are usually scavenger species associated with people.

So when we get a question like this…

Your Name: Sheree
Your Bug Question: We have an 80 year old barn that we tore down to the bare wood and replaced with siding and insulation and interior wood walls. The whole barn I would say is not tight, some small areas where bugs can come inside. This summer we have an infestation of flies!! Bombed twice in 2 weeks and still have live flies. what else can we do? Pest control comes out and sprays inside and around the base of the building. Worst year for flies!!!

…I tend to think that they’re blue-bottle flies or house flies, which live in trash or manure.

Blue-bottle fly, also known as a blowfly. Likes: long walks on rotting corpses. Dislikes: Earthworms
Image credit: DRSPIEGEL14, via Flikr
License info: CC BY-NC-ND 2.0

If you hear hoofbeats, you’re usually dealing with horses. When you hear about flies around the house, the assumption is that you’re dealing with a common species like house flies or blue-bottle flies. That’s usually what you’re dealing with.

Usually. Not always. Flies are weird, so a thorough description of the environment can really shed light on this situation.

Later in our conversation, Sheree clarified:

Hi Joe,

The barn has been restored to a shop with no livestock in it. New wood inside and out except original hardwood frame which is now covered with new wood. We live beside a farm with cattle. And we have horses on the other side, at least 300 + feet on either side of this barn.

I am a Veterinarian, that is why I am asking you. Nothing dead or dying here, just flies!

Thought you might have some new ideas….

So let’s set the stage for this mystery. We’ve got an old building, a lot of flies, and no rotting matter to attract them. So they’re not house flies, or blue-bottle flies. These hoofbeats belong to zebras, not horses.

I’ve heard of an identical situation.

It’s a major plot point in The Amityville Horror.

The Amityville incident is largely assumed to be a hoax, and I am not suggesting this structure is haunted. However, I do think Sheree is experiencing the same biological phenomena which inspired that scene from this movie. I think that because the scene in the video above is based on a real phenomenon which commonly happens in old buildings.

So what’s going on here? Why do mysterious flies sometimes show up in old structures?

In the fall, old buildings are sometimes invaded by mysterious flies when there’s no rotting matter to attract the standard scavenger species. These flies can even appear in the dead of winter, which really freaks people out. That’s the basis for the scene above…people are so freaked out by these flies that people assume their houses are haunted.

Fall Migrations

Fall is an important time for insects. The days get shorter, and the temperatures get colder…so insects are looking for places to overwinter. We wrote a post about stink bugs migrating indoors, and people have also seen aphids preparing for winter. Elsewhere on the internet, we’ve seen ladybird beetles doing the same thing.

Insects like to come indoors mostly because it’s warmer than it is outside. Houses have a lot of small, tight, enclosed spaces bugs would like to overwinter in. To them, the space between your outer wall and the insulation is just as good as a hollow log or an animal burrow.

So, there are a lot of bugs which like houses at this time of year.

So why flies?

Flies in Old Houses

Cluster flies can be a real…cluster…um, nuisance.
Image credit: Katja Schulz via Flikr
License info: (CC BY 2.0

The flies which inspired the Amityville scene aren’t blue-bottle flies, but they’re distant cousins called Cluster Flies. These flies, in the genus Pollenia aren’t scavengers. Instead of feeding on rotting matter, their larvae attack earthworms.

In the fall, these flies start looking for places to spend the winter. They’re good at squeezing into small spaces, so they can sometimes end up in houses. In this case, they ended up in a stable.

So the flies in Sheree’s stable are earthworm-eating home-invading cluster flies. They’re not attracted to rotting matter, but they’re attracted to the warm house which has lots of cozy living space.

This is a totally normal thing which happens, and it has nothing to do with cleanliness. Or ghosts.

So why do the flies show up in the dead of winter?

Hibernation is at least partly temperature dependent. If the temperature increases, the flies come out of hibernation. When the heater kicks on, flies emerge from their hiding spaces and head towards the warmest place. The outside is cold, but the inside is warm.

The result is a fly infestation in the dead of winter.

What about control?

The good news is this: this is one of the few times when we can talk about control.

The bad news is that it’s because there’s not a whole lot you can do.

You can spray for the flies inside, but they’ll be coming in from the outside. So your best bet is to inspect the outside of your house, and seal up all the cracks as tight as you can to prevent them from getting in.

The Bottom Line

The Flies Equal Evil horror movie trope is something which has interested me for a long time, because it’s totally explainable by an entirely natural phenomena. Most flies people find in their houses are associated with rotting things, but this is a really important exception to that rule.

The flies invading Sheree’s stable are parasitoid flies looking for a place to sleep the next few months. They’re not evil, just sleepy.

Unfortunately, there’s not a whole lot you can do to control them. They live in earthworms as larvae, and earthworms are pretty much everywhere. So if your area has a healthy earthworm population, your area probably has a healthy cluster fly population as well.

But, hey…the stable isn’t haunted. So there’s that.