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Cluster Flies (Pollenia rudis)
Cluster flies are parasites of earthworms during their larval stage but become a household nuisance in autumn when they gather in large numbers on the warm, sunny sides of buildings and enter through gaps to overwinter in attics and wall voids. Indoors, they cluster together in large masses and emerge on warm winter days, appearing sluggish and disoriented. While they pose no health risk, their sheer numbers and buzzing presence are highly unpleasant. Preventive exterior treatments in late summer and sealing entry points are the most effective control strategies.