If you’re like many homeowners, you’ve dealt with annoying flies ruining your summer barbecues and outdoor dinner parties. You may have even become accustomed to whipping out the flypaper and heavy-duty bug zappers the minute you hear the familiar buzz of a fly. These annoying pests are likely house flies, which can pose significant health risks to you and your family. But have you ever seen large, sluggish flies loitering inside your home in the autumn and winter? They may be cluster flies.

cluster flies

What Are Cluster Flies?

Though they are both a part of the Order Diptera, cluster flies and house flies are in two different families and are different species of fly. House flies often grow no larger than ¼ of an inch long, while cluster flies grow to be twice as large, measuring ½ of an inch long. House flies are gray in color, while most cluster flies appear black. Additionally, cluster flies have golden hairs growing on their body whereas house flies do not.

While house flies can zip quickly through a room, cluster flies are more sluggish in their speed, and easier to swat. They prefer to stay away from busy areas of a home and will seek out quiet spaces away from humans and animals.

What Causes Cluster Flies in your home?

As larvae, cluster flies exist as parasites inside of earthworms. As they mature, they enter the pupal stage , and after 1-2 weeks, they exit from the pupal case. This process typically occurs during the spring and summer, and as fall arrives, these pests head indoors in search of shelter. They’re often only seen inside homes in the winter, while they wait out the cold weather. Cluster flies may group together when they settle down in the lesser-used areas of your house.

Though these pests are larger than the average house fly, their size does not stop them from sneaking into your home through tiny cracks and holes in your home’s facade. Once inside, they tend to stay in unused areas, such as in the attic, in order to hibernate on cold days. However, on warmer days, they become more active and may be seen in other areas around your home.

Are Cluster Flies Dangerous?

Because many homeowners often associate house flies with spreading harmful bacteria and disease agents, as outlined by the World Health Organization. Some people may believe that cluster flies can be as dangerous as house flies. However, unlike some of their other fly relatives, cluster flies do not carry dangerous diseases and they do not bite humans or animals. There’s also no risk of these pests laying eggs in your home and causing an infestation. Cluster flies hibernate through the winter and wait until the late spring and summer, when they’re back outdoors, to lay their eggs.

You may see cluster flies in your home in the early spring, as these pests are attracted to warmth and light. It’s not uncommon for groups of cluster flies to gather near windows on bright, sunny days to try to escape. You may also begin to find cluster fly excrement on windows or walls around your home, which appears as small, dark-colored stains. These pests are little more than a nuisance to homeowners, but still, the presence of these large, sluggish flies can be unappealing.

Preventing Cluster Flies in Your House

It’s unlikely you’ll be able to fully prevent cluster flies from living inside your home, as these pests can sneak in through tiny cracks in your attic or through windows and doors. Still, you can limit your chances of dealing with a cluster fly population by filling in all visible cracks and holes in your home’s exterior with caulk, and replacing any torn window screens and vents before the cooler months arrive.