Life cycle of a Cluster Fly
Cluster Flies are a common household pest that are usually found during autumn and winter months. They seek harbourage from the winter, cold and adverse weather conditions which leads to infestations arising, frequently in loft spaces. If you do have an infestation, Cluster flies will regularly be found clustering around window frames and south facing walls as these are the warmest places within buildings. Infestations do not arise due to poor hygiene, flies enter properties solely to escape the cold temperatures outside in preparation for hibernation. The life cycle of a cluster fly is very much dependant on the weather conditions and in the United Kingdom four generations are possible during a calendar year, however, in normal weather conditions two generation is the norm.
The cluster fly life cycle begins when the female flies lay their eggs into damp soil in late summer or early autumn. The fertilised eggs will hatch within a few days and the larvae will instantly pursue earthworms. Earthworms play host in this vital part of the cluster fly life-cycle. Once the larvae find an earthworm they will enter the body cavities of the worm where they will stay for several days. Throughout this period they will feed on the earthworm and when they are of a suitable size they emerge from the earthworm at the pupal stage and will fully pupate after eleven to fourteen days. Commonly the adult flies will emerge in the autumn when the temperature starts to decrease leading to adult cluster flies to search out harbourages from the adverse weather conditions. In the wild this will be inside tree bark and the hollows of dead trees, however, with human habitations increased encroachment upon their natural habitats, Cluster Flies have a new and abundant sources of warmth and shelter for winter hibernation. The adult flies will seek refuge, usually in loft spaces of houses as it is the warmest part of the building. They will hibernate in the cold winter months and will emerge in the spring when the temperature increases. The fertilised females will then lay their eggs into damp soil and the life cycle starts all over again.
Cluster Flies are ‘field’ flies during the summer months and they are of no consequence to us. However, when the temperature decreases in the autumn months the numbers of cluster flies located in human dwellings dramatically increases. Cluster flies are known to return to a single house or building year on year and there are a few theories currently being debated. The dominant theory states cluster flies leave pheromones which attract future generations to the same building. Other factors are colour of the building and temperature, it has been suggested cluster flies are attracted to lighter coloured and warmer buildings. Unfortunately all of these theories are contested and the reason for yearly return of cluster flies to the same location is still up for debate.