Massachusetts is home to several fly pest species, such as greenhead flies, black flies and fruit flies. Another common fly pest species that often makes an appearance within homes and buildings in the state is the Pollenia rudis species, or the cluster fly, as they are more commonly known. These flies are also commonly referred to as “attic flies” due to their habit of sheltering within a home’s attic during the winter months. Adult cluster flies, which are a bit larger than typical house flies, emerge during the late summer and early fall seasons in Massachusetts in order to search for heated homes where they can safely overwinter until the spring season arrives.

Homeowners are most often pestered by nuisance cluster flies during the fall when the flies are attempting to enter a home, and during the spring when cluster flies are attempting to get back outside again. When cluster flies do gain access to a home, they usually fly straight for the attic, where massive amounts of flies can remain for months without a homeowner’s knowledge. Sometimes, however, cluster flies fail to make it into an attic, as large numbers are often spotted in windows between the glass and the screen.

While cluster flies inhabit an attic during the winter, the cold climate keeps them within a largely dormant state where they cluster together into groups, but sunny days in the winter can make these flies active again, at which point they sometimes move to lower levels of a home in large swarms. Although cluster flies are a serious nuisance within a home, the flies are not attracted to food, garbage or any other type of filth, and they are not considered a threat to public health. In order to prevent cluster flies from entering a home, it is recommended that all open cracks and crevices on a home’s exterior be caulked. Homeowners are also urged to swap their screens for storm windows during early September, as cluster flies often gain access to a home through weaknesses and small openings in window screens.

Have you ever spotted a large amount of flies within your attic?