Cluster Flies in the House All of a Sudden? Why Big, Slow Flies Show Up in Ohio Homes Every Fall

It is usually an upstairs bedroom or a window on the sunny side of the house. Out of nowhere there are a dozen large, slow flies bumping against the glass, and the next day there are more. The house is clean, the trash is out, and nothing is rotting anywhere. That combination almost always means cluster flies, and the first thing to know is that they did not come from anything in your house.

How to tell a cluster fly from a house fly

To most people a cluster fly looks like an ordinary house fly, which is what makes them confusing. The differences are in behavior. Cluster flies are larger and noticeably sluggish. They fly slowly, bump into things, and are easy to swat. A house fly is quick and erratic. Cluster flies also gather in numbers in the same window or the same room, which is where the name comes from.

The difference that matters is where cluster flies breed: outdoors, in your lawn, as parasites of earthworms. Adult females lay their eggs near the openings of earthworm burrows, the larvae develop inside the worms, then pupate in the soil. There are several generations over a summer. None of that happens indoors, and cluster flies do not reproduce inside a house.

Why they are in your house

The last generation of the summer is the one that heads for buildings. Iowa State University Extension puts the migration in mid to late September, and it is the same story as the stink bugs and Asian lady beetles that arrive around the same time: the flies are looking for a protected place to spend the winter. They land on the warm, sunny side of a house, work their way in under the eaves and siding, and settle between the walls and in the attic, often resting in large clusters in the dark corners.

From there they get into living space through the gaps you never see: openings in window frames, around electrical fixtures, and along trim. That is why they appear in a bedroom that has no obvious way in.

Why you see them again in the middle of winter

Cluster flies go dormant in the cold. On a warm winter day, and especially in early spring, the ones in the wall voids warm up, come out of dormancy, and start moving sluggishly toward light. You find them at a sunny window in January and assume something new is happening. It is the same flies from September, waking up.

Are they harmful?

No. Cluster flies do not bite, do not damage the house or furniture, and do not breed indoors. They are a nuisance, and a dead one crushed on a windowsill has an unpleasant smell, but that is the extent of it.

What actually works

Prevention is a late summer job. The University of New Hampshire's fact sheet gives a specific deadline: seal cracks in the siding and around windows with caulk, and cover attic vents with wire screening, before September 10. That is earlier than most people think about fall pests, because the flies move in before the first cold snap that brings the stink bugs.

• Seal under the eaves first. Iowa State calls out the eaves specifically as the most important place to close gaps.
• Caulk around window frames, trim and where utilities enter the wall.
• Screen attic and gable vents.
• Replace worn door sweeps.

For a house that gets cluster flies every year, an exterior treatment of the sunny walls and eaves in September, before the flies arrive, is the professional step. It is the same timing and the same surfaces as our stink bug and fall invader treatment, so one visit covers the lot.

What does not work

Treating the attic after the flies are in it is largely ineffective. Iowa State notes the flies stay hidden under insulation where a treatment never reaches them. Foggers have the same problem. The University of New Hampshire lists chemical control as the last consideration, after everything else has been tried. Indoors, the honest answer is a fly swatter or a vacuum, with the vacuum bag emptied outside.

The rest of the September crowd

Cluster flies are the early arrivals. A few weeks behind them come brown marmorated stink bugs, multicolored Asian lady beetles and boxelder bugs, all using the same walls and the same gaps. Our guide to Ohio's fall invaders lines them all up, and if the "flies" in your attic turn out to have red stripes, start with the boxelder bug post instead.

Frequently asked questions

Why did cluster flies show up all of a sudden?

Because the last summer generation moves to buildings within a short window in September, and because a warm winter day wakes up flies that were already in your walls. Both look sudden from inside the house.

Do cluster flies mean something died in my wall?

No. Flies drawn to a dead animal are a different problem. Cluster flies come from earthworms in the lawn and are only using the house for the winter.

Will they come back next year?

Very likely, unless the entry points are sealed. The lawn produces a new generation every summer, and a house that worked as a winter shelter once will be found again.