Did you know that there are types of flies that have no interest in filth or foods that we eat? The Cluster fly, also known as the attic fly, is identified as a large and lethargic flyer. Normally we think of flies landing on our food for a quick bite or feeding on waste or even dead animals and bringing that inside. We may think this is the only type of fly in our environment, but cluster flies are different and have no interest in filth or waste of any kind. 

Cluster fly larvae are parasites of earthworms which are their only food source, and they breed outdoors. Cluster flies may have as many as four generations per year with adults feeding on pollen from flowers. The only time the adults enter buildings is when they seek shelter, most generally from the cold but some scholars think the cluster fly may also enter buildings to escape intense heat. This type of fly cannot breed inside the home. 

During late summer through fall, this type of fly is drawn to the south and west walls of a home because those areas receive the greatest amount of sunlight. These attic flies will cluster in groups, thus the name cluster flies. Wall voids, suspended ceilings and attics are favorite places where the fly hangs out. 

While overwintering in the above mentioned places, cluster flies may invade the living area of the home being fooled into thinking that heat from the heating system mimics a “false spring”. Favorite areas of entry are canned ceiling lights, loose fitting trim around doors, windows, ceiling boxes, roof vents, and soffits. If cluster flies die inside walls, they can be food for other insects like carpet beetles or larder beetles making the problem worse. 

With spring comes the need to procreate once again, and the cycle starts all over again.  

Here are your tips to avoid cluster flies in your house! 

  • Start in June and July sealing as many cracks and crevices as possible that would allow entry of the cluster fly. Forgotten areas are around water hydrants and areas where wires enter the structure. Roof lines and overhangs are also favorite areas for the fly. 
  • Next step is a good exterior perimeter treatment that will be in place before nature brings us the combination of warm days and cool nights.  

Just a note!  Once the flies are in the structure, they may emerge at any time making exterior sprays ineffective.