Why do I suddenly have large flies in my house?

"I’ve started seeing really large black flies in my house all of a sudden. Why are they here, and what can I do?"

Finding large flies in your house can be surprising, especially if they seem to show up overnight. These huge flies are often cluster flies or blow flies. They may come inside when the weather cools, squeezing in through gaps around windows, doors, or vents. Once indoors, they tend to gather in attics, spare rooms, or around sunny windows.

Decaying pests as the source of large flies in the house

If you notice big flies in your house often, they might not just be flying in through open windows or gaps in your home. There could be a hidden food source that has drawn them in, and they could be feeding on the food source, laying eggs, and hatching. This is why it can seem like they’re coming out of nowhere. The food source could be open trash, pet waste, or even something decaying in walls or crawl spaces, like dead rodents.

Flies appearing suddenly in colder weather

Another possibility, especially if large numbers of flies appear during the winter months, is cluster flies. They often show up unexpectedly because they enter homes in the fall to find warm, sheltered spots - such as walls, attics, or other hidden spaces - where they can overwinter. They remain dormant through the colder months, but when indoor heating or sunny winter days warm them up, they become active and gather near windows or light sources. 

In general, removing food and waste quickly, sealing entry points, and keeping window and door screens in good repair can help cut down on the risk of flies entering your home. For flies that might be hatching inside of your home, keep windows open and they should fly out in time.

If you can’t find the root cause of the problem (for example, where a dead mouse or rat might be decaying), one of our technicians can inspect your home and help locate and remove it. As part of our fly control services, Ehrlich can also help seal gaps and repair screens to help prevent these flies in the future.