Cluster Flies are dark grey in colour, approx 6mm in length with yellowish hairs on their backs and reddish compound eyes. Female Cluster fly eggs are laid loosely on damp soil or leaf litter; the larvae hatch after one week and begin to seek out their food source, earthworms. After burrowing into the body of the earthworm the larvae feed for approx three weeks before pupating. The adult Cluster Fly emerges from its pupa stage, which is 12 to 14 days.

The adult flies, of which there can be 2 to 4 generations per year.They tend to congregate during the autumn in large numbers, mainly in roof voids or attic spaces ready to hibernate. A gathering of Cluster Flies has a very characteristic and unusual smell. They are extremely sluggish in flight and can be a real nuisance around the house. Cluster Flies can be difficult to control once they have found their way into a property, due to their liking for inaccessible and closed spaces.

This type of pest infestation can be removed by applying a residual insecticide fogging treatment to the roof void or attic space.