Do Cluster Flies Hibernate in Attics?

Yes, cluster flies commonly hibernate in attics. These insects are well known for seeking shelter inside homes, especially attics, to survive cold winter months. During the fall, when outdoor temperatures start to drop, adult cluster flies look for warm, protected places to spend the winter. Attics, wall voids, and sometimes upper floors or rarely-used rooms are favorite spots because they are relatively undisturbed and insulated from harsh weather.

Cluster flies are different from ordinary houseflies. They tend to be larger, move more slowly, and are not attracted by garbage or food scraps—cluster flies feed on earthworms outside but never lay eggs or feed inside your home. Their main reason for coming inside is to "overwinter," which is a simple term for hibernate during the cold season.

In the fall, you might see cluster flies gathering in large numbers on sunny sides of your house. They're attracted to the warmth and will use any tiny opening to get inside. Common entry points include:

• Gaps around attic windows
• Cracks in siding, soffits, and eaves
• Unsealed vents and cable openings

Once inside attics, cluster flies don't breed or cause damage. They go into a state called diapause. This is like a deep sleep, similar to hibernation, where their bodies slow down to conserve energy until spring. They're mostly inactive during this time, but if there are mild spells of weather or if the attic warms up (for example, from central heating), some flies might "wake up" and start wandering inside the house. That's why homeowners often notice sluggish flies near windows or lights in late winter or early spring.

What makes attics ideal is that they are usually dark, dry, and rarely disturbed by people or pets. Insulation and structure trap warmth, giving the flies a stable place to ride out winter. However, large numbers of dead cluster flies can attract other pests like carpet beetles.

To manage cluster flies in your attic:

• Seal up potential entry points in late summer before flies come in.
• Install fine mesh screens on vents and attic windows.
• Vacuum up any surviving flies in spring to prevent secondary infestations.
• Consider professional pest control for persistent or large-scale infestations.

In summary, attics are a prime location for cluster flies to hibernate. They cluster together in large numbers, stay inactive through winter, and may accidentally appear in living spaces as they try to exit in spring. Proper sealing, home maintenance, and sometimes targeted pest control are the best ways to keep them out.