When You Need a Plumber for Drain Flies A plumber for drain flies can help when small, fuzzy, moth-like flies keep appearing near a sink, shower, floor drain, or toilet despite regular cleaning. These insects breed in the moist organic film inside drains and pipes, not on the visible surface of the fixture. Quick answer: Drain flies usually stop only after the slimy buildup where they lay eggs is removed. A plumber is the right call when flies return after thorough cleaning,