While most overwintering insects burrow within the soil, amongst leaf litter or in various nooks and crannies around our gardens, some will have chosen to spend the bleak winter months inside our home, where they remain dormant in cool, dry, frost-free spaces, such as lofts.

It is not surprising when the Christmas decorations emerge from the loft that a lacewing, ladybird, or even a dormant queen wasp appears hidden amongst them. However, there is one overwintering insect, if found, is likely to be quite visible and considered to be an infestation -the cluster fly. 

Where are cluster flies found?

Cluster flies are most prevalent around grassland areas, where the larval stage of their lifecycle lives underground and predates on earthworms. They usually have 2 or 3 generations a year, with the final adults emerging from the ground at the end of summer to find and feed on late season nectar.

Throughout autumn days, as their name suggests, they’ll gather in groups often on the south facing walls of the houses that they’ve selected to be their winter refuge, where the flies will remain until the evening when the light begins to fade, crawling up walls and under eaves and into roof spaces, where they remain dormant for the winter months, sometimes in thousands.

Although cluster flies are similar to houseflies, their abdomens have light and dark grey patches, and their thorax have short golden hairs; they pose no health risk to us, although they can become a nuisance in homes that remain warm throughout the winter.

How to manage cluster fly infestations

Eradicating hibernating cluster flies from a home loft during winter is not usually necessary since they won’t damage the house since spring usually brings them out of hibernation, and they leave the house naturally.

However, if cluster flies become a problem, an industrial vacuum cleaner can remove them or, as a last resort, a professional pest controller can fumigate them, as long as there’s adequate ventilation and no bats in the loft.