We all love to see and feel the sun shining on a winter day and in early spring. It’s a pleasant reminder that warmer weather and summer are close by!

On those nice unusually warm winter and spring days, some homes and businesses are surprised with an unwelcome, noisy guest – the cluster fly (Pollenia rudis Fab.). But it’s not just one cluster fly you discover sluggishly bumping into ceilings and walls, and loudly buzzing around your windowsills. You have a LOT of flies, enough to wonder “why do I have so many flies in the house?”

Cluster flies on a window ledge on a home

Even more confusing is where they came from. Your windows and doors have been firmly shut all winter, and the spring weather is still too cold to dare open them. These flies seemed to have magically appeared. And just when you think you have swatted the last one, more suddenly appear, followed by even more. They just don't go away.

Plus, flies are summer bugs, right?

The cluster fly is one of many overwintering pests that hibernate in homes and businesses from late fall to early spring. They are an unwelcome surprise when discovered. Cluster flies are slightly larger and darker than house flies, with a short coating of golden hair on their thorax. These flies normally live outdoors year-round. In the winter they often take shelter to hibernate in and on fence posts, under stones, and in other protected places.

Cluster flies will also overwinter in homes and buildings by gaining entry through any accessible crack, crevice or torn screen. Between late summer and early fall, cluster flies love to bask in the warm sunshine on walls of homes and businesses. In some cases, they congregate in enormous masses. When evening approaches with cooler temperature, they seek refuge in nearby holes, cracks and crevices. Depending on temperatures, they may come out again the next day to enjoy the sun, and sneak back inside at night. They will repeat this until they are ready to hibernate, in which they will remain indoors until spring.

Once indoors, cluster flies commonly hide in secluded areas like attics and wall voids. They favour dark corners, cracks, beneath curtains, under clothing in closets and behind pictures and furniture. On warm sunny winter days these insects can awaken early in a confused, drunk-like, state thinking it’s spring. Once active in winter or spring, they are a nuisance day and night, with their constant buzzing, disoriented flying and congregating at windows and lamps trying to get back outside.

So, this spring, if you’re asking yourself Why do I have so many flies in my house?, you likely have a cluster fly infestation.

Other surprising and unwelcoming overwintering pests include boxelder bugs, Asian lady beetles, pine seed bugs, and wasps and hornets.

Check out our resourceful pest library to learn more about overwintering pests, how to tell if A cluster fly Environmental Pest Control you have an infestation, and to get helpful tips for dealing with these common nuisance spring pests:

If your home or business has been infested with cluster flies or been invaded by other overwintering pests, and you’d like professional pest control help, give us a call at 1-800-263-5055 or request a quote online.

We're also booking for spring, so if you want to get ahead of future pest issues, we're happy to help!

WHY DO I HAVE SO MANY FLIES IN MY HOUSE?