Cluster Fly

Cluster flies are not usually a health hazard. However, cluster flies are a nuisance, and are parasites on our friendly earthworms. The eggs are deposited singly into cracks in the soil of lawns or open areas and hatch in approximately three to seven days. The larvae, or maggots, of clustering flies develop as parasites in the bodies of earthworms, after which they enter the soil where they pupate. The whole life cycle from egg to adult lasts between 25 and 39 days, depending on environmental conditions. There may be up to four generations per year. Adult cluster flies start looking for winter shelter when the days shorten in late summer and early fall. They are often attracted to the warm south and west sides of buildings, particularly those located in open and exposed areas.

After sunning themselves during the day, the flies will look for a warmer place as the sun goes down and will eventually crawl into any small opening they can find in the house siding or under eaves. They may then settle within the walls or attic to hibernate. The problem becomes more evident on days when temperatures rise and the flies become more active, gathering in windows where light attracts them.