Cluster flies are common throughout Ontario.  The flies enter houses and other buildings starting in early September looking for a place to spend the winter.  Their name comes from their habit of clustering on outside walls and windows on sunny days.  They congregate in attics.  They are a greater problem in rural and suburban homes.

The cluster fly develops as a parasite in certain species of earthworms.  They do not appear as a result of farm operations, manure piles or garbage.  They are more numerous in rural homes because of the large areas that support a certain species of earthworm.  

The adults leave the lawns, gardens, and fields in September to fly to protected areas such as buildings, fence posts, debris, and stones.  Winter mortality is very high.  In April and May, they return to the field and garden to lay eggs in cracks in the soil.  On hatching, the larvae penetrate earthworms.  They develop through the larval and pupal stage to the adults.  There are several generations in a season.

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