Cluster Fly

Cluster flies belong to the genus Pollenia, a group originally native to Europe and now widespread across North America and other temperate regions. They are named for their distinctive habit of gathering in dense clusters on sunlit exterior walls in autumn as they search for sheltered spots to spend the winter.

Unlike many household flies, cluster flies do not breed or feed indoors; buildings serve only as an overwintering shelter. Their larvae instead develop outdoors as parasites of earthworms, an unusual life history among common flies.

Adults are somewhat larger and slower-moving than house flies, with a duller, hairier appearance that becomes more apparent up close, especially the irregular golden hairs scattered across the thorax.

• Dark gray to black body, 8-10 mm long, slightly larger than a house fly
• Fine, irregular golden or olive hairs scattered across the thorax, lacking the metallic sheen of blow flies
• Wings overlap closely over the abdomen at rest
• Slower, more sluggish flight compared to house flies
• Lookalikes: house flies are smaller, faster, and lack the golden hairs; blow flies have a shiny metallic blue or green body that cluster flies lack

Cluster flies are found throughout temperate regions, including much of North America and Europe. They breed outdoors in soil near earthworm populations in lawns, gardens, and fields. In autumn, adults seek out buildings, congregating on sun-warmed exterior walls, especially south- and west-facing surfaces, before entering through small gaps to overwinter in attics, wall voids, and window frames.

Outdoors in warmer months, adult cluster flies feed on nectar and the juices of ripening fruit, while their larvae are parasitoids that develop inside earthworms. As temperatures drop in fall, large numbers of adults gather on sunlit walls and then move indoors to overwinter, remaining largely dormant and sluggish through the cold season. On unseasonably warm winter days, clusters may become active indoors before resuming dormancy.

Females lay eggs in soil near earthworm burrows. Newly hatched larvae seek out and enter an earthworm host, developing internally as parasitoids through several instars before emerging to pupate in the soil. Adults emerge in warmer months and may produce several generations through summer and early fall. As cold weather approaches, the final generation of adults seeks shelter indoors, overwintering as dormant adults rather than continuing to breed.