What is the best way to get rid of cluster flies?

Almost every article answering this question gives one universal method, as if a cluster fly problem is the same thing in August and January. It isn't. Before flies enter your house, the best method is prevention — caulk and screen, and it works close to perfectly when done on time. After flies are already clustered in your attic or wall void for the winter, prevention is off the table, and the best method becomes physical removal: a vacuum. Conflating the two is why so much advice about cluster flies is unsatisfying — it recommends August solutions to a November problem, or resigns itself to a spray that can't reach a dormant insect either way.

For flies already inside, UNH Cooperative Extension's guidance is straightforward: vacuum them. They're sluggish once indoors and don't evade a hose the way an active fly would. The one detail worth doing right is emptying the bag or canister outdoors immediately afterward — UNH notes that flies left inside a vacuum bag can crawl back out into the room. A UV or light trap near a problem window can supplement this by drawing in flies before they scatter, and flypaper is a reasonable mop-up tool in a still-air space like an attic landing. None of these fix the underlying population in the void; they clean up what's visible.

Essential oils, vinegar, cinnamon and similar folk remedies have no controlled evidence behind them for cluster flies specifically — the flies aren't foraging, so there's nothing for a scent to interrupt. The more consequential mistake is reaching for an indoor insecticide or fogger. Penn State Extension is direct that treating flies once they've reached the wall voids or attic isn't advisable, in part because insecticidal dust 'may kill thousands of flies,' but the dead insects left behind become food for carpet beetles, which then go on to attack woolens and stored dry goods elsewhere in the house. You'd be trading a fly nuisance for a fabric-pest problem, and it doesn't even clear the void faster than a vacuum would.

If the straightforward goal is not having this conversation again next winter, the answer isn't a better vacuum — it's exclusion, done before the flies arrive. UNH's guidance is to seal siding cracks and gaps around window and door frames, and cover attic, soffit and gable vents with wire screening, finished before September 10th. Concentrate on the south- and west-facing walls, where autumn sun keeps the surface warm enough to draw flies staging for entry. This is a caulk-gun-and-screen job most homeowners can do themselves in an afternoon, and it's the only intervention on this page with real evidence of stopping the problem before it starts.