Ask The Entomologist, Community

Why are there flies in my house? There’s snow outside!

If your home is anything like ours, it has had an abundance of flies lately. You may ask yourselves “why now?” – after all, it’s snowy out, and there can’t be much food for them. Shouldn’t they die with the cold like everything else?

You’d be entirely right in thinking so. Regardless of the type of fly, most of the good food sources are rather scarce this time of year, and the cold kills many. However, insects are just as uninterested in freezing to death as we are. Most of these gathered indoor flies are trying to avoid that particular fate.

My Mother-in-Law’s elegant, clear sticky traps, thoroughly filled with Cluster Flies and Fungus Gnats.

While we see many house flies, flesh flies, and blow flies in the summer, they aren’t the primary species to invade our homes in the winter (though I’ve met a few of each overwintering inside my home).

The vast majority of our new fly guests are a different group entirely – the Cluster Flies. Until quite recently, the Cluster Flies were grouped as a special sort of blowfly, but just over a year ago, they were given their own scientific family name, Polleniidae.

Cluster Flies get their name from their habit of gathering in large groups, often in windows. Some try to stay warm outdoors, nestled deep inside rotting tree trunks. More sensible ones move inside with us to stay warm and survive the winter. Cluster Flies don’t want anything to do with your food, plants, or pets – they’re just here to avoid freezing.

Eternal optimists, Cluster Flies often come out on warmer days, and try to find their way outside.
Thus the accumulation of dead ones on your windowsills when weather turns cold again.

It’s this habit of moving inside for the winter that is thought to have introduced Cluster Flies to the New World. You see, the Cluster Flies we have now aren’t North American natives. No. They’re thought to have sailed over from the Old World with some of the first European colonists.

Back when European countries were busy sending ships to build colonies in the New World, they brought many unintentional stowaways with them. Many of these scalawags are with us today, for better or worse. In this story, we care about two such emigrants. Earthworms (especially the Green Worm) and Cluster Flies.

It’s thought that earthworms were transported across the Atlantic in ships’ ballast. Meanwhile, some Cluster Flies took refuge from bad weather inside the ships themselves, just like they’re doing in our homes this winter, and so found their way to America as well.

Earthworms flourished when ships made landfall. Many of the formerly-glaciated parts of North America no longer had earthworms, and were ripe for Lumbricid conquest. The spread of earthworms, in turn, opened up North America to the spread of Cluster Flies. Both spread like wildfire, and now can be found all over our continent.

Why are earthworms important to Cluster Flies? You see, Cluster Flies are specialist parasites. Their maggots feed almost exclusively on earthworms. Mother Cluster Flies sensibly lay their little ones near earthworm burrows, and the newly-hatched maggots then see about journeying into earthworm burrows. There they lurk in ambush, waiting for a worm to pass by.

Thinking of tiny maggots catching hold of vast earthworms with their mouth-hooks as the worm passes by very much brings scenes of Frank Herbert’s “Dune” to my mind. I like to imagine the tiny Cluster Fly maggots singing this song as they hunt their earthworm prey, and carve their way inside.

Our friendly, local Cluster Fly – Pollenia pediculata.
As you can see, they’re a bit larger and hairier than our common houseflies, as well as easier to catch.

Because of their worm-based diet, Cluster Flies are actually remarkably clean compared to other flies with carrion- or dung-based diets. While you may not appreciate Cluster Flies’ presence, at least they will not spread disease.

Additionally, both my dog and I can attest to their superb flavor.

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