Cluster fly larvae develop inside earthworms living in the ground outside of homes.
Flies in attic room.
Cluster or attic flies are the genus pollenia in the blowfly family calliphoridae.
Individual flies found around the house can be sprayed with household insecticides.
These are not the same as the blue bottles that buzz hard against the windows in the warmer weather or the house flies and smaller flies that are active in summer too.
Remember attic flies want to cluster indoors when it starts to get a lot cooler outdoors.
Do this in the summer or early fall at the latest.
Spray as best you can around your roof chimney eves ventilation holes etc.
Once the flies are dead it is important to clean the area thoroughly so that other bugs such as roaches do not move in to eat the dead flies.
Flies that gather in roofs around windows and on walls inside in the autumn and winter are called cluster flies also known as attic flies.
Apply insecticide to the outside of your attic to prevent flies from coming in.