Getting Rid of Cluster Flies on Windows in Late Summer

If you’ve noticed a large number of slow, clumsy flies congregating on the sunny windows of your home in late summer and early fall, you’re dealing with cluster flies. This is a common and predictable seasonal event that can be a major nuisance for homeowners.

It’s crucial to understand that this is not a sanitation issue. Cluster flies do not breed in filth or garbage; they are parasites of earthworms in your lawn. The flies you see on your windows are newly hatched adults that are instinctively seeking a sheltered place to hibernate for the winter. The only long-term solution is to prevent them from entering your home in the first place.

The “why”: understanding the cluster fly life cycle

The cluster fly life cycle is unique and the reason they appear so suddenly.

1. Summer: Female cluster flies lay their eggs in the soil of lawns and pastures.
2. Hatching: The eggs hatch, and the larvae burrow into and feed on earthworms.
3. Emergence: In late summer, the new generation of adult flies emerges from the soil.
4. The invasion: Triggered by shortening days and cooler nights, these newly emerged adults seek out hibernation sites. They are powerfully attracted to the warmth radiating from the sunny, south- and west-facing walls of homes. They crawl into tiny cracks and voids around windows and siding to spend the winter.

The flies you see on the inside of your windows are the ones that have made a mistake. They have emerged from the wall void on a warm day, and thinking it’s spring, they fly towards the daylight of your window, where they become trapped.

The field-tested management plan

The strategy for cluster flies is two-fold: manage the ones already inside and prevent the next generation from entering.

Phase 1: Dealing with the flies inside

• The vacuum is your best tool: This is the most effective and cleanest way to remove the sluggish flies from your windowsills and walls. Do not swat them; they can leave a greasy spot. A crevice tool attachment makes quick work of them along window tracks and sills where a standard nozzle can’t get close enough.
• Use a light trap: A simple plug-in UV light trap can be very effective when placed in the room where the flies are most active. A trap like Flowtron’s indoor plug-in unit covers up to 375 sq ft, so one placed near the worst-affected window handles most single rooms without needing to be moved around.

Phase 2: The exterior lockdown (the permanent solution)

This is the only way to stop the annual invasion. These steps must be completed in mid-summer, before the flies begin to seek shelter.

1. Seal everything: This is the most critical step. Using a high-quality exterior caulk like GE’s all-purpose silicone caulk, meticulously seal all cracks and gaps on the upper levels of your home. It’s rated for same-day rain exposure and holds up through freeze-thaw cycles, which matters since these seals need to survive a full winter before you can check them again. Pay special attention to:
   • The trim around all windows and doors.
   • The area where siding meets the eaves and soffits.
   • Gaps around utility lines.
2. Repair screens: Ensure all window and door screens are in perfect condition, with no holes or tears.
3. Check vents: Make sure attic and soffit vents are properly screened with a fine mesh.

FAQ

Q: Are cluster flies the same as house flies? Are they unsanitary?
A: No. Cluster flies are slightly larger and slower than common house flies and have fine, golden hairs on their thorax. More importantly, they are not associated with filth or garbage. They do not breed indoors and are not a significant health concern. They are strictly a nuisance pest.

Q: I keep killing them, but more appear every day. Why?
A: You are only seeing a tiny fraction of the total population. For every fly you see at your window, there could be hundreds or even thousands hibernating safely inside your wall voids and attic. The ones you see are just the “overflow.” This is why sealing the exterior is the only real solution.

Q: Will a professional pest control treatment work for cluster flies?
A: Yes, a professional exterior treatment in late summer is highly effective. A licensed technician can apply a long-lasting residual insecticide to the specific areas where cluster flies congregate, particularly the upper south- and west-facing walls. This kills the flies as they land on the surface, drastically reducing the number that can find a way inside. This is a powerful supplement to a thorough exclusion effort.