The Best Way to Get Rid of Cluster Flies

 

The most effective way to get rid of cluster flies combines prevention, physical removal, and targeted chemical treatments. Since cluster flies are different from common house flies—they’re larger, slower, and breed outdoors but seek shelter indoors as the weather gets colder—you need a special approach that closes their entry points, removes existing flies, and keeps them from coming back.

 

Understanding Cluster Flies

 

Cluster flies are about 7-10 mm long, blackish-grey, and move sluggishly, especially when it’s cold. Unlike house flies, they don’t come in because of filth or to breed inside your home. Instead, they develop in the soil outside in late summer, then enter buildings in the fall to hibernate in attics, wall voids, and unused rooms. In spring or during warm spells, you’ll see them clustered around windows as they try to get back outside.

 

Seal All Entry Points

 

The most important step is prevention—stop cluster flies from entering in the first place. During late summer and early fall (before they invade), carefully check for even the tiniest gaps and cracks on the outside of your home, especially around windows, doors, attic vents, siding, eaves, soffits, and where utility wires enter the house. Use these methods:

• Seal cracks and crevices with high-quality silicone or acrylic caulk.
• Install tight-fitting door sweeps on exterior doors.
• Repair or replace damaged window screens and weatherstripping.
• Cover attic and soffit vents with fine screen mesh.

Cluster flies can squeeze through surprisingly small holes. Pay special attention to the south and west sides of your house, which are warmer and more attractive to them.

 

Remove Flies That Have Entered

 

If cluster flies manage to get inside, you’ll often see them clustering on windows, in attics, or hidden in wall voids. To remove or kill flies already indoors:

• Use a vacuum cleaner with a hose attachment to suck up slow-moving flies. Empty the vacuum bag or canister promptly to prevent bad smells.
• For heavy infestations in hidden spaces (like attics or wall voids), use a pyrethrin-based aerosol spray labeled for indoor use. Follow all safety instructions and ventilate the area afterward.
• Sticky fly traps or fly paper can catch wandering flies in areas you can’t easily vacuum.

Avoid crushing cluster flies—when smashed, they release an unpleasant odor and can stain surfaces.

 

Use Targeted Insecticide Treatments

 

Cluster flies hide inside wall voids, attics, and crawl spaces, so direct spraying is usually not useful for prevention. Instead, apply an insecticide dust or residual spray around the exterior of your home, focusing on areas where they might enter:

• Spray or puff insecticide dust into cracks, crevices, and gaps identified during your inspection. Use products labeled for cluster fly control (often containing permethrin, cyfluthrin, or deltamethrin).
• Apply a residual barrier spray around eaves, windows, siding, and attic vents, following label instructions.
• Best timing: Do this in late summer or early fall, just before cluster flies start seeking shelter for winter.

For large or ongoing infestations, or if you have difficulty reaching high or hidden areas, consider hiring a professional pest control service experienced with cluster fly treatments. They have stronger products and specialized equipment for safe, effective application.

 

Why You Shouldn't Ignore Cluster Flies

 

While cluster flies do not bite, spread diseases, or lay eggs inside, they are a nuisance and can stain walls or carpets, create persistent odors, and indicate entry points that might let in more damaging pests. Regular exclusion and control keep your home comfortable and pest-free.

 

Long-Term Prevention Tips

 

• Repeat inspection and sealing work every year, especially in late summer.
• Maintain caulking and weatherproofing around windows, doors, and utility entry points—they degrade over time.
• Use dehumidifiers in attics or crawl spaces to make conditions less attractive for cluster flies and other insects.
• Keep lawn well-maintained. While you can’t remove cluster fly larvae from all the soil around your home, aerating and not overwatering can reduce their numbers.

Following these steps, you can dramatically reduce or even eliminate cluster fly problems and prevent them from returning each year. Consistent prevention is far more effective than emergency removal once they’re inside. If you ever feel overwhelmed or the problem keeps coming back, don’t hesitate to contact professional pest control for help.