Description: at about 9 mm long, the cluster fly is slightly larger than the common house fly. The thorax is dark olive grey, with fine golden hairs, which can easily be rubbed off. There are no distinct lines or stripes behind the head, and the abdomen has irregular light and dark grey areas.
Biology: the female lays eggs in the soil near the burrows of earthworms. The tiny maggots that hatch from the eggs seek out earthworms to feed upon. As cold weather progresses, adults seek protected places to spend the winter, particularly during September and October (activity can often be noted again in early spring). In many cases, the flies seek shelter within walls, roof voids and attics, usually returning to the same building year after year and often favouring south facing buildings.
Control: complete control of cluster flies is almost impossible since the over wintering adults are concealed in walls, attics, etc. When possible, cover the louver to the attic with screen and use a caulking compound to seal openings to the outside. Adults are sluggish enough in the winter to be picked up with a vacuum cleaner. Smoke and ULV treatments will provide a knock-down in lofts, but as with any loft treatment, always check first for the presence of bats before proceeding.