Cluster Flies
While they don’t pose a potential health risk like some other flies, the nuisance of having cluster flies can be unbearable for the homeowner. Windows, walls and light fixtures covered in cluster flies can look like a scene from a bad horror movie. These sluggish flies can appear to be common house flies, but are slower, larger and darker in colour. Their name comes from their habit of “clustering” on the sunny side of structures before entering in the fall.
Behaviour
The behaviour of cluster flies varies greatly from other types of flies. They do not breed, lay eggs or feed indoors. Their eggs are laid in the soil where the larvae become parasites on earthworms. They then pupate in the soil and emerge as adult flies to start the cycle again. The process from eggs to adult can take anywhere from three to five weeks and can be completed as many as four times in a season, depending on conditions. They are a long lived fly, capable of surviving through their winter dormancy in a structure.
Cluster flies become a nuisance when they overwinter in homes, laying dormant in attics and wall voids. They enter the home in the fall through cracks, soffits, vents and small gaps in the building after clustering on the exterior. They will typically emerge in early spring or on warm sunny days in the winter and are strongly attracted to light, often filling light fixtures and windows with dead flies. The dead flies in an attic space can provide food for mice and may also attract secondary pests such as larder beetles.
Control
Sealing of cracks and screening of openings will aid in excluding flies from the structure and chemical treatments need to be properly timed. As cluster flies are most active in the spring and fall, treatment is timed to control the flies during these seasons. Spring treatment is aimed at reducing numbers on the interior, while fall treatment is directed at excluding the flies from entering for the winter. Both treatments involve treatment of the attic space whenever possible. Your PROTEC technician can provide you with treatment options and answer any of your questions regarding cluster flies.