• The cluster fly averages between 1/4 to 3/8 inch long, dark gray in colouring. Cluster flies closely resemble house flies although they are usually larger and their thorax displays a yellowish sheen. 

The cluster fly is a parasite of earthworms. They breed outdoors in lawns and fields during the spring and summer.

• Female Cluster Flies lay their eggs in cracks in the soil, which hatch in three days. The larvae use earthworms as a food source.
• The larvae feed for about 22 days then followed by entering the pupae stage. This stage lasts 11-14 days before emerging as adults. Adult cluster flies feed on flowers. Four generations hatched per summer.
• Cluster flies begin to enter structures in large numbers as winter approaches. The cluster fly is seeking warm sites with protective cracks for shelter, crawling back as far as they can get. It is important to consider treatment before this happens.
• Cluster flies can squeeze around the edges of windows that are weather-proofed. Increased large clusters of flies can huddle inside wall voids, attics, and false ceilings. Attics of homes is a common place for infestations to occur. 
• Unseasonably warmer weather may cause the cluster fly to emerge thinking it is spring, going for the warmer air outside. Cluster flies fly very slowly when they just wake up and are strongly attracted to light. 

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