Symptoms
- Many slow-moving flies appearing suddenly indoors, often near windows
- Flies clustering in attics, wall voids, or upper rooms
- Occurs mainly in fall (entering) and spring (emerging)
Likely causes
- CommonCluster flies seeking overwintering shelterThese flies enter homes in fall to survive winter, then become active again and try to exit in spring, both times appearing indoors.
- CommonGaps around windows, siding, or the roofline providing entryThey enter through small exterior gaps, particularly on sun-warmed south and west-facing walls.
What to do
- Vacuum up visible flies rather than swatting them, which avoids stains and is more effective for larger numbers.
- Empty the vacuum bag or canister outside afterward so flies don't work their way back out.
- Seal exterior gaps around windows, siding, soffits, and rooflines to reduce future entry, focusing on sun-exposed walls where they tend to congregate before entering.
- Install or check window screens to prevent additional entry while windows are open.
- Avoid spraying pesticide directly on large numbers of flies indoors, since dead flies in a wall void can attract other pests.
- Expect some recurrence each fall and spring even with good sealing, since cluster flies can be persistent.
- Call a pest control professional for an exterior perimeter treatment in early fall if the problem is severe each year.
Tools and parts
- Vacuum
- Caulk gun
When to call a pro
Call a pest control professional for an exterior treatment if cluster flies are a severe recurring problem each year.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.