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: What you have seen may be cluster flies, but without a specimen I'm not sure. These flies are sometimes mistaken for house flies because they look similar to the common house fly. Cluster flies have a completely different life cycle. These flies are common inside houses in fall and winter.

The life cycle of the cluster fly begins in the spring when they leave the overwintering sites and lay their eggs in the soil. The larval or maggot stages of this fly attack and eat earthworms. There are several generations of cluster flies during the summer, and each time the female returns to lay eggs in the soil. The flies of the last generation (late August) will spend the winter in a protected location, and start the life cycle over again the following spring.

Cluster flies spend the winter relatively inactive in attics and wall voids. They begin searching for a place to spend the winter during the last month of summer, usually in late August and early September. They seek out the sides of houses that are warmed by the afternoon sun. They collect there by the hundreds or thousands.

Control of these flies is not easy. Call your local Orkin Branch office, and a highly trained Orkin Pest Specialist 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. They are sometimes mistaken for house flies because they look similar to the common house fly, but they have a completely different life cycle. These flies are most common inside houses this time of year.