Cluster flies are a group of fly species belonging to the genus Pollenia, primarily including Pollenia rudis. They are known for their habit of congregating in large numbers, especially during the fall and winter seasons. Cluster flies are commonly found in homes, buildings, and other structures seeking shelter from cold weather. While they may be a nuisance due to their presence, they are not known to pose significant health risks or cause structural damage.
Taxonomy and Classification
Cluster flies belong to the order Diptera and the family Calliphoridae. They are classified under the genus Pollenia, which comprises several species. The most notable species is Pollenia rudis, which is often referred to as the “common cluster fly” or simply “cluster fly.”
Physical Description
Cluster flies are medium-sized flies, measuring about 0.3 to 0.4 inches (8 to 10 mm) in length. They have a robust and hairy thorax, with a non-metallic, dull gray or olive-gray coloration. The abdomen of cluster flies is dark gray with irregular light and dark patterns. When at rest, their wings slightly overlap along the abdomen.
Unlike other common fly species, cluster flies have a distinct behavior of clustering together in large numbers, typically on walls, windows, and other surfaces.
Distribution and Habitat
Cluster flies are found in various parts of the world, including Europe, North America, and Asia. They are most prevalent in temperate regions. In their natural habitat, cluster flies are commonly associated with fields, meadows, and woodlands, where their larvae develop as parasitoids of earthworms. However, during the fall and winter, they seek shelter in buildings and other structures to escape the cold.
Behavior and Life Cycle
Cluster flies are known for their habit of forming large aggregations. As temperatures cool down in the late summer and early fall, they gather in protected areas such as attics, crawl spaces, and wall voids. These sites offer them a suitable environment to overwinter.
The life cycle of cluster flies begins with eggs being laid in soil crevices or areas near earthworm populations. The hatched larvae then parasitize earthworms, feeding on their tissues until they mature. After pupation, adult cluster flies emerge and seek shelter for overwintering.
Interaction with Humans
Cluster flies can be considered a nuisance when they invade homes and buildings in large numbers. They typically enter through small cracks and gaps in windows, doors, and walls. While their presence can be bothersome, they do not bite, sting, or cause structural damage. Cluster flies do not reproduce indoors and are not known to transmit diseases.
Efforts to prevent or control cluster fly infestations focus on sealing entry points, such as gaps and cracks, to prevent their entry into buildings. Insecticides are generally not recommended for cluster fly control unless the infestation is severe.
Conservation Status
Cluster flies are not a species of conservation concern as they are abundant and widespread. They play a role in natural ecosystems as parasitoids of earthworms, contributing to nutrient recycling processes. Their presence in homes and buildings is primarily seen as a nuisance rather than a conservation issue.