Cluster flies are sluggish, dark gray flies slightly larger than house flies, with golden hairs on the thorax. They gather on sunny south and west walls in late summer, crawl into walls and attics to spend the winter, and appear at windows on warm winter days. They don’t breed indoors, bite or spread disease. Vacuum the ones you see, seal gaps before late August, and skip indoor sprays.
Table of Contents
How to Identify Cluster Flies
- Size: about 3/8 to 1/2 inch, a little larger than a house fly.
- Color: dull gray or grayish-brown with darker markings, sometimes with light patches on the abdomen.
- Key feature: short golden-yellow hairs on the thorax, easiest to see under a hand lens.
- Wings: at rest, they fold flat over the back with the tips overlapping.
- Behavior: slow, clumsy fliers that are easy to swat, often buzzing in circles at windows.
- Smell: crushed or caught flies give off a sweet smell like buckwheat honey.
Cluster fly vs house fly
| Cluster fly | House fly | |
|---|---|---|
| Size | 3/8 to 1/2 inch | About 1/4 inch |
| Thorax | Golden hairs | Four dark stripes |
| Wings at rest | Overlapping over the back | Held apart |
| Behavior | Sluggish, gathers at windows in winter | Quick, around food and garbage |
| Breeds indoors? | No | Yes, in garbage and decaying matter |
Cluster Fly Life Cycle
Cluster flies belong to the blow fly family but aren’t filth flies. Females lay eggs in cracks in the soil in spring and summer. The larvae find earthworms, burrow inside and feed on them as parasites, then pupate in the soil. Adults feed on flower nectar and water. There are about three or four generations a summer, and wet summers that favor earthworms tend to lead to more flies in fall.
Why Cluster Flies Come Into Your House
From late August through September, adults look for a sheltered place to spend the winter. They gather on the warm, sunny south and west sides of buildings during the day and slip in at dusk through gaps under eaves, around windows, in siding and around pipes and wires. Window screens don’t stop them.
Inside, they collect in attics, wall voids and the upper floors of the house. Homes standing on high ground or in open areas often seem to get the most.
Flies on windows in winter
Cluster flies go dormant for the winter but wake on mild, sunny days or when furnace heat warms the walls. They crawl out of gaps around baseboards, window frames, light fixtures and outlets and head for the light, which is why you find them buzzing at windows in spare rooms and attics. Most leave in spring. They don’t reproduce inside.
Are Cluster Flies Harmful?
No. They don’t bite, aren’t known to carry disease and don’t damage your home. They can leave small dark spots of droppings on windows and walls.
The bigger risk comes from large numbers dying in wall voids and attics. Dead flies feed carpet beetles and larder beetles, which can then move into woolens and stored goods.
How to Get Rid of Cluster Flies
- Vacuum them. The simplest way to deal with flies at windows; empty the vacuum outdoors.
- Seal the house before late August. Caulk gaps around windows, doors, siding, fascia, soffits, chimneys and utility lines, and fix damaged screens. Sealing by Labor Day is the main way to prevent the next winter’s flies.
- Seal inside too. Caulk gaps around baseboards, window and door trim, light fixtures and exhaust fans where flies come out into rooms.
- Use light traps in attics. UV light traps with sticky boards catch flies in attics and unused rooms, though they won’t remove them all.
- Consider a professional exterior treatment. A licensed applicator can treat the outside walls in late summer, just before the flies gather. It breaks down in sunlight within a week or so and only supplements sealing.
What doesn’t work
- Sticky fly paper and fly strips; cluster flies aren’t attracted to them.
- Indoor sprays and foggers, which don’t reach flies in walls.
- Treating the lawn or killing earthworms; the flies travel hundreds of yards.
Cluster flies arrive at the same time as other fall invaders, including Asian lady beetles, stink bugs and boxelder bugs. Sealing the house in summer keeps out all of them.
Cluster fly control supplies
- UV fly light trap for attics and spare rooms
- Handheld vacuum for flies at windows
- Exterior caulk for sealing gaps before fall
- Foam backer rod for wider cracks
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FAQs
Cluster flies are slow, dark gray flies a bit larger than a house fly, with golden hairs on the thorax. Their larvae live as parasites in earthworms outdoors, and the adults shelter in houses over winter.
Cluster flies entered your walls and attic in late summer and fall to spend the winter. Warm, sunny days and furnace heat wake them, and they crawl out toward the light of windows, often in rooms you rarely use.
No. Cluster flies don’t lay eggs or reproduce indoors. Their young develop in earthworms in the soil outdoors.
No. They don’t bite, aren’t known to spread disease and don’t damage the house. They are a nuisance and can leave small dark spots on windows and walls.
Vacuum the flies you see and seal gaps around windows, siding, soffits and utility lines by late August. Light traps help in attics. Sticky fly paper doesn’t attract them, and indoor sprays don’t work.
Cluster flies are slightly larger, sluggish and easy to swat, with golden hairs on the thorax and wings that overlap over the back at rest. House flies are quicker, with gray stripes on the thorax and wings held apart.
Sources
- Penn State Extension: Cluster Flies
- Colorado State University Extension: Cluster Flies and Other Winter Flies
- Purdue Extension: Nuisance Flies, Cluster and Face Flies (E-235)
- Iowa State University Extension: Cluster Fly
- University of Kentucky Entomology: Cluster Flies, Face Flies and Blow Flies (ENTFACT-624)
- University of Wisconsin Extension: Larder Beetles
Facts in this guide are checked against the sources above and reviewed when they change. See how we research in our editorial policy.