Why do Cluster Flies Cluster?

Somewhere back around 1980 I read a book, and subsequently watched the inevitable movie version of it, called "The Amityville Horror". Now, when the book came out it was touted as a true story, although I suppose I really should have been more skeptical of that claim, and a few years later it was revealed that the whole thing was dreamed up one night by a couple of people scheming at a beer party. However, it was more tantalizing to think that this scary story was true, and that a house actually WAS inhabited by ghosts and demons that seemed to be associated with the mysterious blood-red room deep in the basement below. Shoot, at that time my wife and I had just bought an old house in California, and in its basement the previous owners had chosen to paint the little bathroom blood-red - walls, floor, sink, toilet, everything. Needless to say, we never used that bathroom, but it was nasty stuff other than demons that kept us out.

Anyhow, in the movie version of this fable one scene depicts an eerie moment at a second floor window, when hundreds of large flies suddenly appear, as if out of nowhere, and are buzzing around at the window. Obviously, the storyline suggests, these flies were placed there by some supernatural force in the house.

Well, only a few years into my long, enjoyable career with the professional pest management industries, this mystery took on a much more logical, pragmatic reason, and that reason is……Cluster Flies. A very common and unfortunate experience that many people throughout North America have each year is the Invasion of the Cluster Flies…...hmmmmm, I see a new storyline evolving.

Cluster Flies are scientifically known by the name Pollenia rudis, and their nasty little habit is to spend the winter as an adult fly, cozied-up inside the homes or other structures that we provide for them, pretending they have found a cave or a hollow tree. They tend to gather there, hidden away inside the wall voids, attics, or other protected, hollow spaces, in very large numbers, perhaps in the tens-of-thousands, and possibly to use the same house year after year if we let them. In all likelihood there are odors left by the previous generation of adult flies that hibernated there that attract the next generation to the building, an advertisement that this is a great place to hide and survive the cold months of winter.

What do they look like?

If they were some pretty, shiny, golden red colored fly, I suppose we could at least enjoy the display of colors, but unfortunately Cluster Flies are dull. They are pretty big for a fly, being bigger than our standard old House Fly, but smaller than the Horse Fly that fed on your leg last summer while you were swimming at the lake. Cluster Flies are dull black in color, but if you take a really close look - I mean a REALLY close look, with a magnifying glass - at the sides of their body just behind the head, you will see that there is a covering of short, curly, golden-yellow hairs, and this makes them a Cluster Fly.

Another key might be an annoying habit they have, of flying into your face, or plopping down on your arm as you are reading a book. These are pretty sluggish fliers, particularly when they are first coming out of the hiding places in the spring or on a warm winter day, and don't dodge things in front of them very well. They sort of plod along and then just drop onto a surface to rest.

Finally, one reference book states that when the flies are crushed they have "an odor like buckwheat honey". Now, I haven't tried this myself, and may not even recognize "buckwheat" honey's odor, but I suppose it's worth a shot.

But my house is clean, why do I have flies?

Unfortunately, the Cluster Fly is not particularly discriminating in its choice of winter abodes, other than choosing a place where it can hide. Whereas the "filth" flies such as the House Fly or the shiny Blow Flies are looking for dead animals or rotting garbage, the Cluster Fly does not seek such a habitat for its own larvae. As it turns out, the larva of the Cluster Fly (the "maggot" stage) eats earthworms. It is a parasite of our little garden friends that are living in the soil. The adult fly lays her eggs in cracks in the soil, the eggs hatch, and the larva wriggles down into the soil until it stumbles onto an unlucky earthworm, digging into the worm and feeding on it from the inside for a few weeks.

Because they are not breeding in some unsanitary condition that can be cleaned up, the control of Cluster Flies cannot take place at the larval level, as most other kinds of fly control will stress. In fact, we generally would prefer to have lots of earthworms in our garden, since they are so beneficial to the soil environment, and no control measures are available to eliminate Cluster Flies at their early stage.

Well, thank goodness no other flies annoy us like this, right?

Sorry, but here's the bad news. A second species of fly, called the Face Fly, often does the very same thing, although its larvae live in animal and livestock droppings, so at least there is some sort of Sanitation step that can be taken to minimize the populations of adults. The Face Fly is a close relative of the House Fly, and is recognized by its shiny appearance, striped top of the thorax, and yellowish sides of its abdomen. I once witnessed an invasion - actually the exodus of the flies in the spring - at a house way up in the mountains in the California Sierra Nevada, north of Lake Tahoe. There were, it seemed, hundreds of thousands of the flies, inside and outside this old ranch home, coating cars, tables, clothing on the line, walls, and everywhere else you looked.

There also are many other kinds of insects that choose homes as their over-winter resting sites, and if you will look at another of our BugInfo articles - "Sharing Your House for the Winter" - you can learn about them as well. Fortunately, most of these other bugs will be present in far lower numbers, but not always.

So, how do I keep this from happening?

It's hard to say why Cluster Flies pick a home as the perfect one to inhabit, but once they do they seem to draw all their friends to the party. Again, it likely is odors they give off that cause one to draw the next, and pretty soon their name of "cluster" fly becomes appropriate. Your house may be a color attractive to them, may have just the right light exposure, may have garden plants that attracted the nectar-feeding adults - who knows? You are just the lucky one, and there really is no great way to keep adult flies from flying into your yard.

However, you CAN keep them out of your house, but it can take some work. The ideal solution for keeping any unwanted living organism from invading your home is to close off those places that they enter. This pertains as well to rats, bats, and mice as it does to bugs, but it can be a difficult process, and it will take someone with a very good eye for detail to discover all those holes, cracks, or other openings the flies can squeeze through. Likely places could be where window frames are not sealed well, attic or crawl space vents with broken screens or missing screens, or other poorly fitted joints around the walls, doors, and windows. You also need to check where electric or telephone wires come into the house through the exterior walls, around the electric circuit breaker panel outside, by dryer vent openings, on the roof around vent pipes, chimneys, etc. to make sure flashing and other possible openings don't exist.

Once you find these kinds of portals they should be filled in, and in most cases the opening serves absolutely no purpose, but is essentially a flaw in the construction that should be repaired anyhow. I recently had to reseal around some front windows, with a silicone caulking, because rainwater was leaking in and ending up on the floor and carpet next to that outside wall. These cracks have appeared over time as the house matures, and would have been great doorways for bugs as well as the rainwater.

I've already got the flies, now what do I do?

Once the flies have found your home as their home, for the winter months, you might consider calling a licensed pest management company. Since the flies may be present in large numbers and are likely inhabiting hidden areas that you cannot easily get into, some professional help could be called for. Companies that specialize in pest elimination have a variety of handy tools to work with, from vacuums that can suck large numbers of bugs out of their hiding places to portable devices that can inject a fine fog of insecticide deep into the wall voids to kill the flies that are present.

There also are highly effective - and at low risk to you - dusts that can be injected with the proper equipment, and the dust not only kills these flies but can remain in the voids for several years to (hopefully) kill flies that may enter for awhile into the future. Either way you go, however, the physical exclusion program around the exterior of the home should take place. The benefit to going to all this trouble, to close off the access points the flies are seeking, is that it seals the home to all the other kinds of invading pests as well - other insects, cockroaches, mice, bats, etc. If the cracks are around windows, due to weathering as the structure ages, sealing them up is a necessary fix to keep out rainwater too, another potential problem you'd prefer to avoid.

Professionals also may be able to help in the fall, as the flies begin to gather on the outside walls of your home. At this early stage you can benefit from applications of certain insecticides onto the walls the flies are resting on, killing them before they have the chance to squeeze inside. Hopefully this will break up the cycle and provide relief for you, and avoid enduring the problem these unwanted guests will cause when they all decide to exit in the spring.