Field plate · Pollenia spp.
Cluster fly
Slow, dull-grey flies that crowd attic windows in autumn and blunder out on mild winter days.
Plate 7 of 28. Drawn in profile to the identification marks named below; the scale rule is a drawing convention — sizes are given in the text.
Identification marks
Slightly larger than a house fly, dull grey-brown, with crinkled golden hairs on the thorax that wear off with age. Sluggish in flight, and it does not hover around food.
Look-alikes it is confused with
House flies and flesh flies. House flies are quicker, breed in waste indoors, and stay near kitchens; cluster flies ignore food and make for the brightest window.
Where it hides in a house
Upper floors and attics, clustered in wall voids, behind curtains, and around window frames — often in houses on high, sunny ground. The larvae develop outdoors in earthworms, not in anything inside the house.
Health & property risk, stated plainly
Pure nuisance. They carry nothing of note into the house, do not infest food, and do not breed indoors. The harm is numbers, mess, and dead flies feeding carpet beetles and other scavengers.
What actually works
Seal soffits, ridge vents with mesh faults, window casings, and siding joints in summer, before the autumn entry. Vacuum live flies; fit fine screens on attic windows; a light trap in the attic catches winter wanderers.
What doesn't
Treating bins, drains, or the kitchen — that is house-fly logic aimed at the wrong insect. Insecticide sprayed into a void after entry kills some flies and leaves the entry points open for next autumn.
When to call a licensed pro
Call a pro for repeated heavy invasions of a tall house, where the work is exterior sealing and targeted void treatment done from ladders and lifts.