They are fuzzy, buzzy, smell like honey (sort of), and I can’t convince my nearly three-year-old son that they are not bees. But even most four-year-olds know that cluster flies are not bees. They look like house flies on steroids, and we tend to treat them that way.
Cluster flies are not house flies. They are a little larger, buzz louder, and are a lot hairier than house flies. When you’ve got the fly swatter ready, one of their best features is that they fly a lot more slowly than house flies, too. More importantly, cluster flies are not classified among the “filth flies,” as are house flies. They are “flesh flies,” which is oh, so much better – at least if it’s not your flesh they are after.
And it is indeed not your flesh they are after, unless you are an earthworm, specifically in the genus Allolobophthora. Unless you keep them for bait or as pets, you shouldn’t have any earthworms in your house, no matter how lax your housekeeping. So what are the cluster flies doing there, and how do you get them to leave?
For those two or three readers who have never had cluster flies in their houses, here’s what happens: in spring and on warm days throughout the winter, these big, hairy cluster flies appear out of nowhere inside your house, congregate on sunny windows in ones and twos or by the hundreds, buzz like crazy, and, if you’ve got a lot of them, make you feel like you are on the set of a horror movie.
Those cluster flies originally arrived by boat. Gordon R. Nielsen, an emeritus extension professor of the University of Vermont, points out that during the last ice age, the glaciers ground all of New England’s earthworms down to nothing. No earthworms, no cluster flies. European settlers repopulated the area with earthworms that stowed away in ship ballast - amidst the root balls of plants and imported topsoil. No one is sure, but chances are that today’s cluster fly maggots are descended from stowaways on the stowaway earthworms.
We focus on the cluster flies in our houses, but on the first warm day of spring, Nielsen says, you’ll find cluster flies all over your lawn looking for earthworm burrows. When they find one, they lay their eggs. When the eggs hatch, the maggots (which are fly larvae) will find and dine on an earthworm. Still underground, they pupate, then emerge as adult cluster flies.
Adult cluster flies sip flower nectar and the juices of ripe fruits through the summer. Come fall, they find a protected spot to spend the winter. That might be a crack in a tree, or it might be in the attic or under the clapboards of your house. They huddle in clusters, unseen inside the walls.
In spring and on warm winter days they appear. You kill them, and then more appear the next day. Many people think this means they are breeding in the house, Nielsen says, but they are not. Cluster flies don’t eat in your house. (That’s why those sticky fly traps don’t work on them.) They don’t lay eggs there, either. They don’t do anything but try to get outside, where the earthworms are.
As the temperature drops, they go back to their hiding spots, only to reappear when the temperatures rise again. If you think you’ve gotten rid of them by vacuuming them up, you had better empty the vacuum outside, Nielsen warns. Most cluster flies merely shake off the dust and march right back out of the vacuum after it’s been turned off.
Houses near pastures have the worst cluster fly problems. If you can’t move, the best defense is tight house construction, says Nielsen, who, as the son of a carpenter, takes these things seriously. Most contractors just don’t get the polyethylene vapor barrier tight enough, he says. And most don’t put a plastic screen behind the soffit and ridge vents, which really helps keep cluster flies out.
At my house, we have our local exterminator spray a chemical repellent outside the house in early autumn, which doesn’t kill anything but simply prevents the insects from getting into the house. Still, a cluster fly shows up occasionally, and when it dies, my son mimics his mom by picking it up with a tissue and throwing the “bee” away.
Nielsen’s well-carpentered house still gets a few cluster flies, too. What does this knowledgeable insect scientist recommend doing about those flies? “Hit them with a fly swatter,” he says.
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