Why are these large, black, slow-moving flies in my home?

Question: Hello, I have noticed large, black, slow-moving flies, around my home. One day I found 20 within a matter of 30 minutes. I am curious as to why there are so many, and why they are hanging around my home? They tend to hang inside and outside of my four-season, fully windowed porch. Please advise. Thank you!

Answer: If you've noticed big, black, slow-moving flies gathering inside your home, particularly in areas like a sunroom or enclosed porch, you may be dealing with a common seasonal pest known as the cluster fly.

How to Identify Cluster Flies in Your Home

Cluster flies are often mistaken for house flies due to their similar appearance, but there are key differences:

• Size & Color: Cluster flies are larger and darker than common house flies, with a checkered light and dark gray abdomen and short golden hairs on their thorax.

• Behavior: They move sluggishly and are frequently found near windows or in attics and wall voids, especially during cooler months.

• Life Cycle: Unlike house flies, cluster flies do not breed indoors. Their larvae develop in the soil, feeding on earthworms. Adults seek shelter inside homes to overwinter, becoming active again in the spring.

These characteristics help distinguish cluster flies from other types of flies you might encounter indoors.

Why are cluster flies in my home?

Cluster flies typically enter homes in late summer or early fall as they search for warmth and shelter to survive the winter. They’re drawn to sunlit areas on the outside of buildings and often slip indoors through small cracks, gaps around windows, doors, and vents. Once inside, they stay mostly inactive during colder months, gathering in wall voids, attics, or unused rooms. As temperatures rise in the spring, they become active again, often appearing in noticeable numbers near windows or sunny spots inside the home.

How to Get Rid of Cluster Flies

Managing a cluster fly infestation can be challenging. Here are some steps you can take:

1. Seal Entry Points: Inspect your home for cracks and gaps, especially around windows, doors, and vents, and seal them to prevent flies from entering.

2. Reduce Attractants: Keep your home clean, and remove potential food sources that might attract flies.

3. Professional Pest Control: For significant infestations, it's advisable to seek professional assistance from an exterminator.

If cluster flies keep showing up year after year, professional help may be the most effective solution. Orkin's fly control services are designed to address infestations at the source and help prevent future issues through targeted treatment and exclusion methods. Call your local Orkin Branch office, and a highly trained Orkin Pro will come to your home, conduct a thorough inspection, and develop a customized, scientifically proven treatment.

Related Questions

Orkin used the information above to also answer the following questions submitted by Orkin.com users:

Question: We have a big fly problem in our house. Every day I kill many flies and some days hundreds. Our house is clean, and we've never had this problem until this winter. What do we need to do?

Answer: What you have seen are probably cluster flies. While they resemble house flies, they have a different life cycle and are especially common indoors during the winter as they seek warmth and shelter. To reduce their presence, seal cracks and gaps around windows, doors, and siding, and consider contacting an exterminator for help if the problem continues.