Cluster Fly

Pollenia rudis — the common cluster fly — is one of the most extraordinary household pest insects and a major autumn nuisance pest across temperate North America and Europe. Adults are 7-10 mm long with dark gray-brown bodies, golden-yellow short hairs across the thorax, and translucent wings. The species' parasitic life cycle is unique among major household pest flies: adult females lay eggs in moist soil in late spring through early autumn; eggs hatch in 1-2 weeks into mobile maggots that actively SEEK OUT EARTHWORMS as host. Once a maggot finds an earthworm (typically Lumbricus and Aporrectodea species), it BURROWS INTO the worm through the body wall and develops as an internal parasite, consuming the host worm's tissues over 2-3 weeks. The maggot then leaves the now-dead worm, pupates in soil for 1-2 weeks, and emerges as an adult fly. The earthworm-parasitism is a continuing topic of soil ecology research — cluster fly populations contribute to local earthworm population dynamics across temperate agricultural and garden soils. The species' household pest status comes entirely from the AUTUMN AGGREGATION BEHAVIOR. Adult cluster flies emerging in late summer and early autumn seek sheltered overwintering sites and are strongly attracted to large warm vertical structures (especially south-facing walls of houses). They enter buildings through cracks, gaps around windows, vents, and other small openings, and aggregate in attics, wall voids, behind insulation, and in unused upper rooms by the THOUSANDS in single buildings. The flies do not feed, breed, or cause direct damage indoors but emerge into living spaces on warm winter days, accumulate on south-facing windows, and produce a sweet honey-like odor when crushed (a defensive secretion) plus dead-fly accumulations that some homeowners find disturbing. The species is a continuing topic of residential pest management. Despite the dramatic numbers and the parasitic life cycle, cluster flies do not bite, sting, or transmit any human disease.