Do cluster flies reproduce inside homes?

It's worth being precise here because the precision is the whole answer. Cluster flies are not generalist flies that happen to also live in earthworm burrows sometimes — the earthworm relationship is obligate, meaning it is the only way the species reproduces at all. A female lays her eggs at the entrance to an earthworm burrow in soil. The larvae that hatch actively seek out and penetrate an earthworm's body, where they develop as internal parasites for roughly 11 to 14 days, according to the University of Connecticut. UNH Extension confirms up to four generations complete this way over a single summer.

Every link in that chain — the burrow, the worm, the soil — is something your house does not contain. A wall void has no earthworms in it. An attic has no soil. There is nothing for a cluster fly to reproduce with indoors, which is a different and much stronger claim than 'they usually don't' or 'it's rare.' They cannot, structurally, complete their life cycle inside a building.

The pattern that convinces people otherwise is real: flies show up at a sunny window, you deal with them, and more show up the next day, and the day after that. UNH Extension names this exact phenomenon and its cause — a cluster of dormant adults tucked into an attic or wall void reactivates in waves as sunlight warms the space, and individuals crawl toward the glass at different times. It looks like reproduction. It's closer to a slow-motion exit, spread over weeks, of a fixed number of flies that all arrived the previous fall.

That fixed-number detail matters. Because nothing is breeding, the population inside that specific harborage is a depleting one from the day the first hard frost sends the last stragglers in. Some die from the stress of dormancy or from wandering into a dry, unsheltered part of the structure. None are replaced. The flies you're vacuuming off the sill in March are the tail end of last October's arrivals, not evidence of a nest you haven't found yet.

If cluster flies bred indoors, the search for a nest or an egg mass would make sense, and killing adults would meaningfully suppress next month's numbers. Because neither is true, that entire approach is wasted effort. There is no nest to find. Killing adults in a void doesn't reduce a growing population, because the population isn't growing — it only adds carcasses, which is its own problem (dead flies in a wall void are a food source for carpet beetles and other dermestids, and enough of them cause odor).

The candid, evidence-backed response is two separate actions on two separate timelines. If they're already inside: vacuum what shows up at the windows and remove the bag outdoors — that's genuinely the right tool, not a consolation prize. If it's not yet fall: seal the exterior — caulk siding cracks and gaps around window and door frames, and screen attic, soffit and gable vents — finished before UNH's September 10th benchmark, well ahead of Manchester's first hard frost. That's the only action that changes how many flies show up to overwinter in the first place, because it's the only point in the cycle where you're acting on the insect outdoors, before it becomes a dormant houseguest with nothing left to interrupt.