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Cluster flies (Pollenia rudis) are a common and deeply frustrating nuisance pest across Montana, especially in rural and semi-rural homes. Unlike house flies, cluster flies are not born indoors, they spend the warmer months outdoors, parasitizing earthworms in your lawn. As temperatures drop in late summer and fall, they begin searching for warm, sheltered spots to overwinter. Unfortunately, your attic, wall voids, and window frames are exactly what they're looking for.
Once inside, cluster flies huddle together in enormous groups — hence the name. On warm winter or early spring days, they become active again and emerge sluggishly into your living spaces, often by the hundreds. They are drawn to light and will congregate on sunny windows, leaving behind greasy smear marks and a distinctive, slightly sweet odor.
In Montana's climate, cluster fly season typically runs from late August through October for entry, with emergence happening in March through May. Homes on hilltops or with sunny south-facing walls are particularly vulnerable.
Size
½ inch (slightly larger than a house fly)
Color
Dark gray with golden-yellow hairs on thorax
Active Season
Late summer entry; spring emergence
Risk Level
Low (nuisance only)
Habitat
Attics, wall voids, window frames
Lifespan
Up to 2 years
SIGNS OF INFESTATION
How to Tell If You Have Cluster Flies
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Flies on windows in spring — Sluggish, slow-moving flies clustering on south-facing windows are the number 1 sign.
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Buzzing in walls — A faint buzzing or humming from inside your walls during warm winter days indicates an active colony.
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Greasy smear marks — Dark smear spots on walls, window sills, or curtains from fly secretions.
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Sweet, musty odor — A faint, sickly-sweet smell in attic spaces or spare rooms.
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Large numbers appearing suddenly — Unlike a stray house fly, cluster flies appear in waves of dozens or hundreds.
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Flies around light fixtures — Dead or dying flies accumulating around ceiling lights in upstairs rooms.
