Having overwintered in little-used rooms and attics of old buildings, cluster flies are usually first detected on a warm, sunny day when the flies become active and emerge. There are clusters of them all around the sunnier spots in my garden today.
Cluster flies are so-named owing to their tendency to cluster together. They are harmless and are frequently observed in rural or semi-rural areas. Once a cluster of flies has found a suitable place to settle they emit a pheromone which attracts others of its species.
Though annoying, cluster flies pose no risk to humans as they do not lay eggs in human food. They are classed as a pest because of the large numbers that can 'cluster' together. They are strictly parasitic, the females lay their eggs near earthworm burrows and the larvae then feed on the worms.
Cluster flies will return to the same area year on year, often in even greater numbers.