Question: For the past week, we keep getting hordes of giant houseflies flying out of our laundry room.  We can”t find where they”re coming from or why.  They always appear on the fluorescent light fixtures and everytime we kill them all, a few hours later there are more.  How do we figure out where these are coming from?

ANSWER  What you have seen are probably cluster flies.  They are sometimes mistaken for the common house fly, but they have a completely different life cycle. These flies are very common inside houses, especially  during the early spring.

The life cycle of the cluster fly (Polenia rudis)  begins in the spring when they leave the overwintering sites (attics and wall voids of houses) 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. Each time the female returns to lay eggs in the soil.  However, the flies of the last generation (mid to late August) will spend the winter in a protected location, and start the life cycle over again in the following spring.

Cluster flies begin searching for a place to spend the winter during the last month of summer, usually in late August and early September.  They will gather on the sides of houses that are warmed by the afternoon sun. They collect there by the hundreds or thousands.  Some of the flies find openings and make their way inside the house. They spend the winter inactive until the temperature goes up.

Control of these flies is not easy.  You can try spraying the sides of the house in the fall to kill those there or prevent them from collecting there.  Use an insecticide that is labeled for cluster flies. Follow the label directions for mixing and application. You can try using a fogger in the attic once they seem to be in there. A vacuum cleaner can be used to remove them from windows and walls inside the house.

Inspect the outside of the house. Make sure attic vents are all screened. Repair window screens and check weatherstripping on exterior doors.

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

Question: Do you have a treatment for house flies?

Question: I have house flies caked on the outside of my house. I have a black tin roof and white siding. Is this normal? If not, is there anything that can be done about them. They are swarming, it sounds and looks awful.