While neither a public health pest nor a filth fly, cluster flies (Pollenia spp.) are a significant nuisance in a large part of the U.S. They generally live outdoors, but will sometimes overwinter in cracks and crevices on the outside of structures, especially the sunny side. The problem occurs when they emerge on a warm winter or spring day, and emerge inside the structure rather than outside. Once they have emerged indoors, they can be trapped with fly lights placed temporarily and/or window traps, as they are attracted to light.
Prevention is the best cure, though. In early fall, treat around windows, eaves and other exterior cracks and crevices, particularly on the sunny side of the structure. A botanical wettable dust, applied either in dust form or as a wettable powder, is a great choice. The strong botanical repellency will keep flies from overwintering on the structure, and the lack of impervious surface label restrictions means you can treat all the necessary areas on the side of the structure without worry. The wettable dust form of the product will provide longer residual. Begin promoting this service to your customers as an add-on in late summer.