https://youtu.be/jT9JqfLeLFo
These sluggish flies appear in early spring and are collectively comprised of three flies: the cluster fly (gathering in clusters and give off a sickly, sweetish odour), the face flies (usually found near farm homes or homes located near pastures and finally the blow flies (the loud buzzing pest that is attracted by manure or dead animal carcasses). Cluster flies are not usually a health hazard. They hibernate in wall voids, attics and closets. They are a known to leave stains on walls and curtains. Larder beetles are attracted to these flies when they die, which can cause even more problems. Cluster flies do not mate, eat or lay eggs inside homes, as common flies do. Clustering flies can be controlled with a fly swatter but beware because they release a nasty smell and may stain whatever they are squashed to.
Gogreenpestcontrol.ca Ladner Tsawwassen Delta B.C. Randy Bilesky
https://www.linkedin.com/pulse/clustering-flies-randy-bilesky?published=t
http://www.delta-optimist.com/opinion/blogs/blog-clustering-flies-1.18848602