Cluster Flies

The cluster flies are in the blowfly family. Unlike more familiar blow flies, such as the “bluebottle” fly, they do not present a health hazard because they do not lay eggs in human food. The typical cluster fly is about 7 mm long and can be recognized by distinct lines or stripes behind the head, short golden-colored hairs on the thorax, and irregular light and dark gray areas on the abdomen.

Adult females that have survived the winter, deposit eggs in cracks and crevices in the soil. The larvae, upon hatching, parasitize small Earthworms. Control of earthworms to control the cluster fly is not recommended or effective, since flies may have originated from up to a mile away from the site of infestation. The total developmental period of the fly, from egg to adult, varies from 27 to 39 days, depending upon temperature and other environmental factors.
When temperatures drop, the behavior of cluster flies is characteristic as follows – in the afternoon they settle on the upper parts of walls and on roofs, facing south or southwest, sunning themselves. As the sun sets, they crawl into any crevice on the exterior of buildings, but usually near the roof. For a few days, they come out during the day and return to the warmth of the building at night. Eventually they continue to move into the interior of the building and remain there to spend the winter.

Cluster flies are typically slow-moving. They can be a pest in homes, schools, and commercial buildings and prefer uninhabited areas. Cluster flies derive their common name from their habit of entering a home or building in the fall and gathering together in clusters, usually in the attic or upper regions of buildings. The behavior of the flies is a good identifying feature. The flies are strongly attracted to light and will blunder about lights and windows, colliding with any object in their path. They often drop to the floor on their backs and spin noisily until exhausted. The annoyance of cluster flies usually begins mid-August and may continue until April.