How to Get Rid of Cluster Flies in Your Home (and Why You Should Never Swat Them)

It might seem harmless, but a cluster fly infestation can be quite the nuisance—especially if left undeterred. Cluster flies may look like houseflies at first glance, but there are a few easy ways to tell them apart. “Cluster flies are a little bigger, and their bodies are covered in golden, fuzzy hairs,” explains Trent Frazer, lead entomologist at Aptive Pest Control. “Their wings overlap when they’re at rest, and they move much more slowly than regular houseflies.” 

If you see flies grouped together in a sunny spot on a window or wall—especially on upper floors during fall or winter—Frazer says there’s a good chance you’re dealing with cluster flies. Here, learn how to safely get rid of an infestation and how to keep them from coming back the next time the temperature starts to drop.

What Attracts Cluster Flies?

Unlike a regular housefly, cluster flies aren’t attracted to trash or food. “Instead, they’re attracted to structure—particularly to warm and sun-exposed walls and gaps or cracks in old construction,” says Allan Bossel, operations expert and licensed exterminator for Bed Bug Exterminator. “Homes with lawns and gardens, or those near soil-heavy areas, have a tendency to see them more.” 

When cluster flies are inside your home, they’re hiding, not feeding, which means they treat your house as shelter from winter. In the late summer and early fall, these insects start to gather near homes. They sneak in through cracks in siding, gaps around windows, or vents to get into attics, wall voids, and the upper parts of your home. “Once inside, they settle in for the cold months and go into a sort of sleep mode called diapause," says Frazer.

Safe Elimination Methods

This is one of the cases where non-toxic approaches tend to work as a first resort, says Bossel. “These flies are slow-moving and very predictable,” he says. “My first recommendation is simple vacuum removal.” Use a vacuum with a hose attachment and gently remove the flies where they tend to gather—in the windowsills, wall corners, or near your attic’s access points. Immediately empty the canister or remove the bag outside your home.

Why You Shouldn't Swat Them: Vacuum removal is a much better option than swatting, which can backfire. Bossel warns against swatting "since crushed cluster flies release an odor that can end up attracting even more flies to your home.” 

Another preventative method is light disruption. “Close the blinds and curtains in the room where the flies tend to gather. Then, open a window and place a bright lamp near it,” Bossel says. “Cluster flies instinctively move towards light and warmth, so give it at least an hour, and most of them will find their way out on their own.” 

As a last resort, you might also try spraying a residual insecticide if you can reach the cluster flies. "Choose one labeled for indoor use and focus on the cracks and entry points around windows, doors, and baseboards," says Frazer. "Always follow the label instructions and make sure pets and kids are kept away from the treated area until it’s safe."

How to Prevent Cluster Flies

To keep cluster flies out in the first place, denying them access is key. “Check around windows, doors, vents, and siding for gaps or cracks,” says Frazer. “Use caulk or weatherstripping to close up those openings. It’s also a good idea to check screens and vents for any tears or loose edges. Fall is the best time to do this—before the flies start trying to move in for the winter."

You also want to limit exterior lights. “Cluster flies, like most other flying insects, are attracted to light,” says pest control expert Brett Anderson. “Keep exterior lights off or on a timer. You can also change lighting over to warm colored bulbs, which will attract fewer insects.”

When to Call a Pest-Control Professional

Sometimes, despite your best efforts, you might still see large numbers of cluster flies year after year. If that’s the case, it’s probably time to call in a professional.

“Pest control experts have tools, training, and treatments that can target the places these flies like to hide—especially inside walls or attics where store-bought sprays can’t reach,” says Frazer. “A professional can also help identify how they’re getting in and make a plan to stop them for good.”