Cluster Flies

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The house is doing things. Rooms full

of flies at the wrong time of year. . .

It’s the passage to hell!

             –The Amityville Horror (1982)

Indeed, while other demons may infest our worst-case wildlife encounter scenarios, the image of rooms full of buzzing, crawling, window-specking flies just about tops it for sheer disgust. And in the annals of housekeeping horror stories, the cluster fly is king. And queen. And insatiably voracious maggot.

pest-library-img1Unlike the unrelated (and more effectively controlled) house fly they resemble, cluster flies are not attracted to foods in the home and don’t develop in manure or garbage. They belong to the same insect family as blow flies, and are parasites of earthworms. In fact, cluster flies were probably introduced to North America in Colonial times by sailing ships using earthworm-rich soil as ballast, thus making Pollenia rudis one of the first (but sadly, not the last, not even close) cross-continental pest species to invade our shores.

Adult cluster flies lay their eggs in soils near earthworm tunnels. Upon hatching, the fly maggots seek out and burrow into the worms, which they feed upon until they mature as larvae. At this stage they leave the bodies of the worm hosts and pupate in the soil, eventually emerging as adult flies. The flies mate and begin the cycle anew, with three or four generations maturing per season. It is the last generation of adult cluster flies that hibernates in protected nooks as cool weather approaches.

Shoveling cluster flies in New Zealand

Shoveling cluster flies in New Zealand

Alas, those nooks are all too often located inside human dwellings. The flies squeeze in through tiny cracks around windows and doors, air vents, in siding and shingles –anywhere –and congregate in partitions until, on cool days, they come out in droves to enjoy the warmth of previously clean sunny windows, the ceilings of heated rooms, the bright light bulbs next to our heads. In Ashburton, New Zealand, where cluster flies have been steadily encroaching in recent years, masses of them dropped out of the town hall ceiling in the midst of an autumn festival and began crawling in people’s hair and down their shirt fronts, presumably looking to get toasty. Partygoers fled screaming into the streets. It’s enough to send even the housekeeping challenged on a vacuuming rampage.

Unfortunately, trying to stay ahead of seemingly endless cluster fly numbers is about as successful as emptying the ocean with a teacup. It doesn’t help that flies can survive a quick trip into the vac bag and will crawl back out through the tube later to re-infest. Swatting them leaves a greasy stain. Shooing them outside is redundant. Placing clear plastic cups of water mixed with a few drops of dish soap on window sills might work as a home-made trap if one can overlook the not inconsiderable downside of water rings, spillage, and the pathetic buzzing chorus of drowning flies whose dead clumps must then be disposed of somewhere –daily, if not by the hour.

Even after they die, cluster flies torment us. Cleaning them out of the walls and floorboards of unoccupied or seasonal buildings can take weeks of shoveling fly bodies by the bushel basket load. The Public Press writer Stephen Morris reports in his blog that a carpenter friend of his has found walls in old houses so packed with cluster fly carcasses that they provided respectable insulation against the severe cold of New England winters.

“I’d rather freeze,” Morris observes. Who wouldn’t?

If this weren’t bad enough, the presence of dead cluster fly remains can attract other insects, such as the varied carpet beetle, a serious pest that feeds not only on carpets but on a wide range of materials including cereals, furs, woolens and other fabrics, and horn. In museums, where cluster flies tend to be endemic, the carpet beetle is a notorious ravager of tapestries, feathers, taxidermy exhibits, and entire collections of insect specimens– as well as the cluster fly bodies that brought them there in the first place.

And there is no way to completely control cluster flies. Especially vulnerable are isolated buildings located near pastures or large lawns and gardens, golf courses, or other turf areas that support earthworm hosts (houses surrounded by large trees seem to have a lesser problem). Removing earthworms from the soil as a preventative is impractical as well as counterproductive. Worms are important components of the ecology, and anyway cluster flies would simply emerge from wider surrounding areas. Careful caulking of possible fly entryways around window casings, door jambs, soffits and eaves, as well as adding vapor barriers, can help.

A recently patented gadget called the Cluster Buster (available from various online retailers) might be worth a try –the trap is filled with finely ground egg shells in a plastic container that attaches to window frames. The flies crawl inside, the powdered eggshell clogs their respiratory system, the flies die, you throw it all away. No chemicals, no smell, no mess. Allegedly holds up to 1000 flies, hence the name.

Nonetheless, to get to the Cluster Buster, the flies still have to be indoors. As blogger Morris notes, “From what I can tell, the only way to stop them from getting in is to wrap your house in a giant plastic bag.”

About the only good thing that can be said for cluster flies is that they inspired the catchy title tune on the 2000 album “Farmhouse” by the rock band Phish. “Welcome, this is a farmhouse,” the obviously autobiographical lyrics tell us:

We have cluster flies alas

And this time of year is bad.

We are so very sorry,

There’s little we can do

But swat them.


The University of Vermont Extension Service fact sheet, EL 11, “Cluster Flies,” by G,R, Nelson, former Extension Entomologist, Plant and Soil Science Department, University of Vermont.

Virginia Cooperative Extension, “Cluster Fly,” by Eric Day, manager, Insect Identification Laboratory, Pub. 444-255, August 1996.

The Bug Clinic website, “Cluster Flies,”

Cornell Cooperative Extension document

Blog by Stephen Morris, “As the Cluster Flies”

The Lyrics Catalogue,

From “Farmhouse” by Phish: Farmhouse, c.2000

Cluster fly shoveling photo and related New Zealand anecdote copied from