Cluster flies are a type of fly that are part of the genus Pollenia in the family Polleniidae. They are also known as grass flies or attic flies. Unlike other types of flies, such as bluebottle flies, cluster flies do not lay eggs in human food. Instead, they parasitize earthworms, laying their eggs near earthworm burrows, and the larvae then feed on the worms. However, the biology of this group is relatively poorly known and a few have been recorded from other hosts including caterpillars and bees.

Cluster flies seek refuge in cold weather and find their way into attic spaces and similar areas indoors. They often emerge on warm days and cluster at windows attempting to exit, hence the name. Cluster flies are typically slow-moving and are about 7 mm long. They have a widespread distribution, with eight species found in Britain and 31 in Europe. Pollenia species are also numerous in Australia and New Zealand, with over 30 species, and they are common in North America.

Cluster flies are not harmful to humans and do not reproduce inside invaded homes, but they can be a significant nuisance within a home. They prefer to hibernate throughout the winter and will often enter homes through a door or window crack. They’re attracted to warmth from sunlight and tend to cluster together near windows. Cluster flies can be controlled by using a vacuum cleaner, aerosol fly killer, or by calling local professionals who will be able to use smoke bombs or foggers for more effective results. Mechanical exclusion is the best method to keep cluster flies from entering homes and buildings.