First decide whether the flies are entering or emerging. That distinction changes where you should look.
If they cluster near doors and windows
Check screens, weatherstripping, and doors that are left open. Adult flies follow light, food odors, and moving air.
If many appear while the house stays closed
Look for trash, pet waste, spoiled produce, organic debris in a garage, or even a dead animal in an inaccessible void. Not every fly problem comes from a drain.
Trash needs a lid and regular removal
Clean spills inside bins, remove bags before they overflow, and keep outdoor containers away from entry doors when practical.
Fly type can reveal the source
Large house flies, tiny fruit flies, and fuzzy drain flies develop in different materials. A sharp photo can keep you from cleaning the wrong area for days.
Do not rely on aerosol to solve a source problem
Removing adults reduces annoyance, but more will appear if a breeding site remains. Sanitation, exclusion, and removal of the developing material are the lasting controls.
A sudden cluster of many large flies in one room despite no exposed food can justify professional inspection for a concealed source.
Sources
- UC IPM — Flies — House-fly sources, sanitation, exclusion, and nonchemical control.