Cluster Fly
Overview:
This species is a common pest throughout Europe, Canada and the U.S. (except in those states that border the Gulf of Mexico). It gets its name from the habit of forming tightly compacted “clusters” of hibernating individuals, usually found in wall voids and attics. They can be very frustrating for the homeowner who not only will notice cluster fly activity during the spring and fall, but also during warm, sunny days in the winter.
Identification:
Often confused with houseflies, but are larger, more robust and about 3/8” (8mm) in length.
Sluggish in their movements and fly slower than the housefly.
Dark gray in color, non-metallic.
Thorax will lack any distinctive stripes, but will have numerous short golden-yellow hairs along the sides and at the base of the wings.
The dark gray abdomen will have irregular, lighter patches.
Unlike the housefly, the Cluster Fly’s wings overlap at the tips when at rest.
The wing’s 4th longitudinal vein will have a characteristic forward bend near the tip, almost meeting the 3rd vein at the wing margin.
When crushed they will have a distinct Buckwheat Honey odor.
Biology:
Adults over winter or hibernate in sheltered areas.
Emerge in the spring to mate.
Eggs are laid in cracks in the soil and will hatch in about 3 days.
Larvae are parasitoid upon certain earthworms.
Egg to adult developmental time varies between 27-39 days. They can have up to 4 generations per year.
Habits:
As the days shorten, during early fall, Cluster Flies begin to look for a place to over winter, sometimes traveling up to a mile to do so.
Are attracted to the “warm” sides of structures, usually the South, West and sometimes East walls.
Will enter void areas such as attics, walls, window and door casings etc. through any number of small cracks, gaps, ill-fitting screens, vents and other openings, which make it very difficult to “pest proof” the structure.
They are a problem in the autumn, winter and spring; autumn, when seeking to over winter; warm, sunny days during the winter; and in the spring when emerging.
May be stimulated to activity from any heat source, such as a furnace, but usually require the warmth of the bright sun heating up the walls.
Once stimulated, they are attracted to the light from the windows and doors.
During warm, sunny days they can be found on windows, crawling up the panes or frames, around lamps and light fixtures.
There is NO reproduction in the walls or during their hibernation.
Treatment:
Long-term control and exclusionary methods are preferred as this can be a continual problem, year after year. It is not recommended to kill these flies while they are in the wall voids as the bodies of these insects attract dermestid beetles. Exclusion methods should be implemented during June and July. Exclusion methods would include:
Screening (#16 mesh) all vents (roof, soffit, weephole etc.).
Repairing cracks in foundation, stucco or brick.
Replacing deteriorated caulking or weatherproof sealants around windows, doors, brick mold, cable TV lines or other such openings.
Make sure all doors, windows, screens etc. are in good repair. Windows and doors should fit and close tightly.
Residual treatments can be performed as a barrier. These treatments usually can be performed between August 15th and 30th, which is about the time of the last seasonal emergence of the adult flies. Microencapsulated materials should be used.
RELATED ISSUE
More and more of these and other pests which try to overwinter in your home like box elder bugs and lady bugs are dying in peoples walls and attics, only to become the food source for other insects. For more information, see our Larder Beetle page.
FAQ's:
“I can’t wait until summer… What can be done right now?”
Temporary but immediate relief can be obtained by using a vacuum cleaner and space sprays in individual rooms. To prevent reentry seal all possible entry points, use caulk, foam sealant or even tape around window frames and pulleys, doorframes, baseboards etc. Remove outlet covers, switch plates seal then replace. Light fixtures, ceiling fans etc. can be removed to the base plate, seal the wall and replace the fixtures.
In suspended ceilings you can use No Pest resin strips, sticky traps or fly paper. A 60-watt light can be placed in the suspended ceiling. The flies are attracted to it, which causes them to exhaust their energy and die right around the light (vacuum them up!).
