Although blow flies and face flies can present a risk of food contamination because of their breeding and feeding habits, cluster flies are not usually a health hazard. However, cluster flies are a nuisance, and are parasites on our friendly earthworms.
Blow flies or face flies may hibernate in homes that are located near livestock farms, or places where animal wastes or garbage accumulation are not well managed.
Cluster flies hibernate in secluded areas of houses (like in wall voids, attics, closets, and empty rooms). They are a particular nuisance because they leave stains on walls and curtains. Also, if flies die in wall voids, they may attract larder beetles, which will feed on the dead flies and then migrate to other areas of the house.
Adult cluster flies start looking for winter shelter when the days shorten in late summer and early fall. They are often attracted to the warm south and west sides of buildings, particularly those located in open and exposed areas. After sunning themselves during the day, the flies will look for a warmer place as the sun goes down and will eventually crawl into any small opening they can find in the house siding or under eaves. They may then settle within the walls or attic to hibernate.
A fly problem becomes more obvious on days when temperatures rise and the flies become more active, gathering in windows where light attracts them.
Clustering flies can enter your home through the tiniest cracks around door and window frames, but also through any other small unsealed opening. To prevent infestation:
- Make sure that all screens fit properly and that there are no cracks or holes that would allow flies to enter your house.
- Weather strip windows and doors.
- Fill any cracks or crevices with caulking around doors, window frames, and other openings in the building structure.
- Install insect screening over air vents in soffits.
Clustering flies can be controlled by simply killing them with a fly swatter. Be aware that cluster flies will release an unpleasant odour and may stain surfaces when squashed. They can also be released outdoors or vacuumed up and the vacuum bag disposed of.
Devices designed to trap clustering flies are also offered on the market. These traps, placed in windows where flies are a problem, contain a powder that works like quicksand, trapping the insects that fall in.
If the infestation is bad enough, pesticide products can be used. Pesticide products can be bought in hardware or grocery stores and garden centres. They can be sprayed around the outside surfaces of window and door frames to prevent entry. Check the label for product-specific directions.
There are also products that can be sprayed on interior window and door frames, baseboards, localized areas of floors or floor coverings, underneath furniture, and in closets and crevices and other places where insects hide or are seen.
You can also choose to contact a professional pest control operator to solve the problem.
You should not try to control cluster flies by attempting to control the earthworm population, because earthworms are the most natural means of producing fertile soil and the cheapest lawn aerators.