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Cluster Flies - Not Your Ordinary Buzzer!
Cluster Flies—Not Your Ordinary Buzzer!

Question. Mike: We have flies all summer long; and even in the middle of winter if the sun comes out for a few hours. We have been told that these are cluster flies, and that they come from the ground every three weeks with a new hatch. The flies leave droppings all over our vinyl siding that has to be cleaned several times a summer with a bleach type cleaner. We have been told the flies come from earthworm castings, and we do have thousands of worms that surface every time it rains. We have tried several different products in liquid form and do get some temporary relief, but only for a couple weeks, and then its right back to the heavy concentrations. We live in the country with farms all around—including lots of cows a quarter mile away; could that be part of the problem?
    ---Dennis in Gaylord, Michigan
Answer. No, those cows could be sending a lot of other kinds of flies your way—face flies, filth flies and the like—but not these weird little puppies!

Cluster flies—so named because they appear indoors in huge clusters—are not attracted to any kind of manure, not even those earthworm castings you mentioned. They're after the poor earthworms themselves! Here's the deal: Female flies lay their eggs in earthworm-rich soil in the Spring; the eggs hatch and the larvae (a.k.a. maggots) go through that soil looking for earthworms to parasitize. After a few weeks, these nasty fiends move out of the deceased worms and find a nice place in the soil to pupate—that is, transform into their adult shape. Adult flies then emerge to mate and continue this cycle throughout the warm season—not quite every three weeks, but quickly enough that there can be four generations a year, even in a short-season region like yours.

But they do not swarm on houses during this time, which means that your siding is being stained by something else, most likely the legendary 'artillery fungus' that breeds in wood mulches. On a hunch, I emailed Dennis to ask if there was wood mulch around his home, and he answered "yes". Get rid of that mulch and you'll get rid of the stains. (You'll find lots more details about bad mulches at THIS PREVIOUS QUESTION OF THE WEEK.)

Back to the flies: The finalgeneration of the season does cluster on homes. They're looking for a way to get inside so they can spend the winter hibernating indoors. Then they'll wake up in the Spring and try and leave through the same cracks and crevices they used to gain entry to their warm and cozy winter home—much like the notorious multi-colored Asian ladybug, stink bug, box elder bug and other home invading insects we've discussed in depth many times on the show. (You'll find one such discussion at THIS PREVIOUS QUESTION OF THE WEEK.)

So our initial advice is the same as with those other pests—do everything you can to deny entry to the creatures. Make sure window screens fit tight, and seal and caulk up any little holes you can find on the South-facing side of your house, which is where that last generation of the season clusters at the end of summer, looking for a way in.

Unlike those other home-invaders, cluster flies like to enter up high on that warm, South side, leading to their other common name of "attic flies". So pay special attention up there, where vents and poor-fitting windows often offer easy access.

Interestingly, Gale Ridge, an entomologist with the Connecticut Agricultural Experiment Station at New Haven, notes that these creatures seem to prefer homes in open settings with wide expanses of lawn. "Structures surrounded by large trees rarely have infestations," she writes. So maybe plant some trees—especially on that oh-so-attractive South side.