Ask an Entomologist: Cluster Flies

In late fall (usually September and October) I always hear from friends that suddenly have tons of flies buzzing sluggishly around their house. They didn’t smell anything obviously dead, and are frustrated by the constant (it seems) procession of flies collecting on their window sills.

What gives?Cluster Fly

Cluster Flies get their name from their habit of clustering in attics or near windows. Although they are blowfly relatives, these flies are not carrion flies, but a type of fly parasitic on earthworms. These flies are harmless, and do not carry any human diseases. Unless you have an earthworm farm inside your house, you don’t have to worry about the flies laying eggs on anything, or reproducing.

These flies are just looking for a warm place to spend the winter. You’ll see them clustering on the outside of a home during the fall, usually in a spot warmed by the sun. They’ll find shelter from the winter by crawling into the eaves or under the siding–and you can’t really stop them, since completely sealing your house is almost impossible. (You should try to seal all the big cracks for heating efficiency anyway, and this will help reduce the number of flies.)

If the flies happen to crawl close to the inside of your house, they’ll warm up when you turn on the heat and find a way in–via electrical outlets, baseboards, or an attic door. You can usually vacuum them up, since they aren’t that lively.

You’ll have a second sudden influx of flies in the spring, as the outside of the house begins to warm, and flies that were hiding under the siding, or in the attic, begin to wake up and wander around. Who needs the Swallows of Capistrano–we have the cluster flies to tell us it’s spring!

These flies don’t cause any home damage, although you will see “fly specks,” or small brown spots, where the flies poop. These come off with soap and water.

Mostly these flies are just free cat entertainment. (If your cat eats these flies, they won’t hurt her. protein!)

[Image via Cliffie ]

12 thoughts on “Ask an Entomologist: Cluster Flies

  1. If your cat eats these flies, they won’t hurt her.

    Well, maybe not flies so much, but if your cat has a delicate stomach then bits of insect leg and such may irritate it. I used to have one cat that insisted on catching cicadas in the season and then hacking up all over the house. I don’t think it causes any long-term effects to the cat, but there is the inconvenience of cleaning up cat sick.

  2. Yeah, well, if you have a cat, you’re cleaning up cat yak all the time anyway :)

    Cicadas are a lot bigger and harder than flies, so they would definitely be an issue for a kitty stomach.

  3. Haven’t had a problem with cluster flies that I recall. The big issue now that I have right now is pantry moths. My wife inherited her grandmother’s china, so we think that they hitched a ride up from New York.

    We’ve got cedar blocks in the cupboards, but all they really seem to do is drive them away from the center of the cupboards. Every time we clean out the cupboards and throw out anything that they might be feeding on or laying eggs in, they go away for a while, but they eventually come back.

    The cat likes to chase them too, but the odd time that she manages to catch one, they also don’t agree with her stomach.

  4. You know that they’re cluster flies because they’re hanging out with the Asian painted ladybirds. I was struck in you post with how much the fly and ladybug behaviors resemble each other. Other than the earthworm stuff, of course.

  5. There is actually a company who’ve come up with a fairly ingenious cure for the problem of cluster flies in a house.

    What a cluster fly in a house is looking for when flying about inside a house is a small dark nook to hide in until spring. The company make small boxes which stick onto windows, which are open at the top and have a small, narrow entrance. Inside is a powder made, they claim, of “exploded eggshells”; it is a very light, fluffy powder which sticks to the flies and prevents them climbing out of the device again. Instead they buzz,and end up at the bottom of the boxes.

    So, without insecticides these devices catch and hold cluster flies and reduce the population in a room from hundreds to a very few remarkably quickly.

    The website is http://www.theinsectivore.com/

  6. I’d think the flies’ attraction to light and light bulbs would overrule their desire to find a place to hide. Once they’ve warmed up, they aren’t hibernating. However, sticking this on the window might work.
    It would make more sense to have the opening at the bottom, since these insects naturally walk up surfaces, not down.

    I’d build it as an inverted U–that way they’d walk in at the bottom, keep going up, and then get trapped.

  7. great post and comments for me, who lives in cluster fly country (and those not-native lady bugs too). Toys for cats? Sorry. They have seen and eaten so many that they just ignore them now.

    thanks for more info.

  8. I would really like to know how to effectivly get rid of them. They are very bold and fly right into your face at times. I have a sick daughter and although they say there is no real health issues, her immune system is low. If they are leaving behind those little brown spots (poop) and she touches it without washing her hands she can get sick and even end up at the hospital because of it. I am not trying to be cruel to the flys, but my daughter comes first, so please tell me the best way to make the flys go, without using chemicals. Thank you

  9. I’m actually not concerned with fly cruelty. I swat them all the time.

    I don’t think your fears about their poop are correct; these are not like house flies. I would be quite surprised if there is much to worry about in their poop.

    They are insanely annoying, and almost impossible to control, since that would mean sealing your house up. Dr. Dan posted a link above to a type of cluster fly control; I have no idea if it works or not.

    Vacuum cleaner is the control I use.

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