Cluster Flies on Windows in Winter: Causes and Fixes

Large, slow flies buzzing at your windows in winter are usually cluster flies. They crawl into attics and wall voids in late summer and early fall to spend the winter, then wake up on warm, sunny days and wander toward the light of your windows. Colorado State University Extension notes that the cluster flies in your house are dormant and cannot reproduce indoors, and Michigan State University adds that they do not bite, spread disease or infest food. Vacuum the ones you see now, and seal exterior gaps before late August so fewer get in next year.

Why cluster flies show up on windows in winter

Cluster flies spend the summer outdoors. As days shorten in late summer, Colorado State explains, the adults stop reproducing and look for dry, sheltered places to wait out winter. From as early as mid-August into early October they gather on sun-warmed south and west walls and slip in through cracks around windows, soffits, vents and siding.

Once inside, they settle into wall voids and attics, often on upper floors, which is why they are also called “attic flies.” They stay mostly dormant through the cold months, but a warm, sunny afternoon or the heat of the house can stir them. The University of Maryland Extension notes they often become active on warm, sunny winter days, and the ones that wander into living spaces head for the brightest spot in the room: the window.

The University of New Hampshire Extension describes the frustrating part. Just when you think you have them under control, more appear the next day, which makes it look like they are breeding in the house. They are not. They are simply coming out of hiding a few at a time.

How to tell cluster flies from house flies

Size and color: Slightly larger than a house fly, about 3/8 inch, and dull dark gray, not metallic, according to UMD Extension.
Golden hairs: Michigan State and Colorado State point to a patch of yellow or golden, crinkly hairs on the thorax, the section behind the head.

Behavior: They are sluggish and clumsy, fly lazily, buzz against glass and are easy to swat. Dead ones often pile up on windowsills.
Season: They show up in fall, on warm winter days and in early spring, rather than in the middle of summer.

Shiny blue or green flies are a different story. Colorado State notes that blue bottle flies and the black blow fly also shelter in buildings, but they develop in dead animals, meat scraps and garbage. If you see metallic flies indoors, look for a hidden source such as a dead rodent. Our guide to lots of flies in the house suddenly walks through that search, and our article on flies in the basement covers the other common indoor flies.

Where cluster flies come from

Cluster flies are not “filth flies.” Colorado State Extension explains that their larvae develop in the soil as parasites of certain earthworms, so they are most common near lawns, pastures and moist ground. Up to four generations can develop outdoors over a summer. That also means there is nothing to clean up indoors, since they are not breeding in your trash or drains.

Colorado State also notes that treating your lawn with insecticide will not prevent them. It has never been shown to kill the developing larvae, and the adults can fly in from several hundred yards away.

Are cluster flies harmful?

No. Michigan State University says cluster flies do not feed or lay eggs during the winter months, do not damage fabrics or infest stored food, do not bite and are not known to transmit diseases. They are a nuisance, though like other flies, large numbers can leave small dark specks on windows and walls. If you are dealing with those marks, see our article on brown spots on walls from flies.

The one indirect issue is dead flies. Colorado State notes that large numbers of dead flies in wall voids or attics can attract scavengers such as dermestid (carpet) beetles and rodents, so vacuum up piles when you find them.

How to get rid of cluster flies on your windows

Vacuum them. They are slow and rarely try to escape. UNH Extension recommends taking the bag or canister outside afterward, or the flies may crawl back out.
Swat stragglers. A fly swatter works well on these sluggish flies, and UMD Extension recommends swatting or vacuuming any that appear indoors.

Use a window or light trap. Colorado State notes cluster flies are not attracted to most food-baited fly traps, but ultraviolet light traps can catch them in attics and other spots where they gather. UNH suggests flypaper strips can help where there is little air movement.
Close interior gaps. Seal gaps around window frames, outlets, light fixtures and ceiling fans so fewer flies can move from wall voids into your rooms.
Skip indoor sprays and bug bombs. Spraying flies on windows does nothing about the hundreds that may still be in the walls. Our article on whether foggers kill flies explains why foggers usually disappoint.

When to call a professional

If flies keep coming out of the walls all winter, a pest control professional can apply a desiccant dust such as diatomaceous earth or silica aerogel into the wall voids where they gather. Colorado State notes this usually needs special equipment and a professional. Any pesticide used should be applied according to its label, which EPA reminds consumers is the law.

How to prevent cluster flies next year

Prevention has to happen before the flies move in. Colorado State says sealing is the fundamental step, and without it, it is not possible to control a cluster fly problem.

Seal by mid-August. Colorado State advises sealing entry points by mid-August and no later than Labor Day, and UNH suggests finishing before about September 10.

Focus on sunny, upper walls. Caulk cracks around windows and siding, especially on the south and west sides and upper stories.
Screen vents. Washington State University recommends screening attic and crawl space vents with mesh small enough to keep adult flies out, and keeping window and door screens tight.
Consider a late-summer exterior treatment. Colorado State notes a professional or homeowner can apply a labeled residual insecticide to exterior cracks and crevices in late summer, but only as a supplement to sealing, not a substitute.

If you have other seasonal pests to deal with, see our guide to bugs in a Christmas tree.

FAQ

Why do I have so many flies on my windows in winter?

They are most likely cluster flies that moved into your walls or attic in late summer or early fall. Warm, sunny days or indoor heat wake them up, and they head toward the light of the windows.

Do cluster flies lay eggs in your house?

No. Colorado State Extension says cluster flies in buildings are dormant and cannot reproduce indoors. Their larvae develop outdoors in earthworms.

How long will cluster flies keep showing up?

Often until spring. Colorado State notes most of them move back outdoors in early spring, and Michigan State notes that some find their way into living areas instead as they try to get out.

What kills cluster flies instantly?

A vacuum or a fly swatter is the quickest and safest option indoors. Insect sprays on windows only kill the few you can see.

Are cluster flies a sign of a dead animal?

Not usually. Cluster flies breed in earthworms outdoors. If the flies are shiny blue or green, though, they may be blow flies, which can point to a dead animal in a wall or attic.