Over wintering adults of the cluster fly, Pollenia rudis Fab, are an annual problem in many houses. Cluster flies are slightly larger and darker than house flies, and have a coating of short golden hair on their thorax. These flies normally live outdoors where they occur on flowers and fruits. In late summer and autumn, when the weather turns cool, cluster flies may congregate on sunny walls. They enter the home to over-winter through any available access. In the home, the flies hide, often in a cluster in attics and wall voids, in dark corners, cracks, under clothing in closets, beneath curtains, behind pictures and furniture etc. On warm, sunny days in the winter and spring, the flies often emerge from their hiding place and swarm on windows causing great annoyance by their buzzing and by their presence. Dead flies provide a food source for larder beetle larvae and can contribute to the presence of other nuisance insects in the house.
Biology and behaviour
In April cluster flies lay their eggs singly in cracks in the soil. During the summer, the eggs hatch and the emerging larvae (maggots) penetrate earthworms and develop as a parasite in its host. There are several generations during the summer. The adults stop egg laying in late August and September, and seek hiding places in which to over winter. Most of the flies spend the winter outside in and on fence posts, under stones, and in other protected places. Cluster flies are usually a greater problem in rural and suburban areas due to the large grassy areas that favour high earthworm populations.
Cluster flies are not related to livestock production. They do not develop in manure or manure pits. They are not interested in food, do not develop in garbage, and are not a sign of unsanitary conditions.
Because of the small size of these insects it is almost impossible to prevent the entry of the flies in the autumn. Screens can be fixed to loft windows and roof vents. Points of entry such as cracks, crevices can be sealed with caulking or other crack filler. Flies found on the windows can be killed with a fly swatter and removed with a vacuum cleaner. Insecticides suitable for indoor control of flies can be applied.
Planting tall vegetation on the south and west sides of the house will reduce the attractiveness of these walls to flies seeking warmth and shelter in autumn. A registered pesticide sprayed on the sunny walls of the house may help.
Treatment of grassed areas near houses to control earthworms is not effective since adult flies can travel long distances.
Whenever insecticides are used, read and follow the instructions on the label carefully.