Cluster Flies
Pollenia species
NuisanceWhat it is
Slow-moving flies that enter buildings in late summer or fall to overwinter in attics and wall voids. Their larvae develop as parasites of earthworms outdoors; indoor adults do not breed in garbage or drains.
How to recognize it
- Slightly larger than a house fly
- Dark gray with fine golden-yellow hairs on the chest
- Wings overlap over the back when at rest
- Slow and sluggish, often buzzing at windows on warm days
- Crushing them can leave greasy smears
Where you will find it
Attics, wall voids, and upper floors, then around windows in winter and early spring. Larvae develop outdoors as parasites of earthworms.
Found across much of the US.
Why it matters
They do not breed indoors or bite, but large groups are a nuisance and leave stains.
Look-alikes
- House fly: Faster and more active, with a dull stripe pattern on the chest, and it breeds in garbage and waste.
- Blow fly: Shiny metallic blue or green, and drawn to rotting material.
What to do
- Vacuum up the ones that get in.
- If the attic has a big cluster, call a professional rather than spraying.
- Contact a professional in late summer or early fall, before they move in.
When to call a professional
- You get large numbers every winter.
- You want exterior treatment before they move in.
- You see many in the attic or in a room that is difficult to reach.
Questions
Where do cluster flies come from?
They develop outdoors, and the adults move indoors in fall to overwinter.
Are they a sign of dirt?
No. They aren't attracted to garbage or filth.
Do they bite or spread disease?
They don't bite, and they're mainly a nuisance.
Related reading
Related pests
Occasional Invaders
A broad category for pests that enter buildings incidentally but usually do not reproduce indoors. Examples can include boxelder bugs, lady beetles, millipedes, ground beetles, and other seasonal invaders.
Read more →Boxelder Bugs
Black-and-red true bugs associated with boxelder and some maple trees. They often gather on sunny exterior walls and enter buildings in large numbers to overwinter.
Read more →Clover Mites
Tiny reddish mites that feed on grasses and ornamental plants. They often invade buildings in large numbers around windows and foundations but do not bite people or damage household items.
Read more →Lady Beetles
Predatory beetles, many of which are beneficial because they feed on aphids and other plant pests. Some species, especially multicolored Asian lady beetles, enter buildings in large numbers to overwinter.
Read more →This page is general information, not a diagnosis. Pests vary by region and situation, and an on-site inspection is the reliable way to know what you have.