Cluster Flies are a common pest in and on homes, cottages and businesses. Cluster flies derive their common name from their habit of entering a home or building in the fall and gathering together in clusters to overwinter, usually in the walls, attic or upper portions of buildings. Cluster Flies are strongly attracted to light and heat sources such as warm windows or lights. Generally noticed on the first warm days of spring when they congregate on the inside of windows, but can be a problem anytime throughout the year.