Drain Fly Identification & Control

Quick Identification

• Size: About ⅛ inch long
• Color: Grayish; body and wings densely covered with fine hairs giving a fuzzy, moth-like appearance
• Key Features: Broad, hairy wings held roof-like over body at rest; weak, fluttering flight (moth-like); found near sinks, showers, and floor drains
• Common Names: Moth fly, sewer fly, sewer gnat
• Habitat: Bathrooms, kitchens, basements — anywhere with slow or infrequently used drains
• Active Season: Year-round indoors
• Risk Level: None — nuisance only; indicator of drain biofilm or plumbing issues

What You Need to Know

Drain flies are the small, fuzzy, moth-like flies you find resting on bathroom walls, usually near sinks or showers. They’re weak fliers that tend to stay close to their breeding site, making them easy to locate. Their larvae develop in the gelatinous organic film that accumulates inside drains, particularly in floor drains, shower drains, and sinks that aren’t used frequently. The Illinois Department of Public Health notes that they’re sometimes called sewer flies because they can infest raw sewage.

Drain flies don’t bite and don’t spread disease in residential settings. They’re an indicator pest — their presence tells you that organic buildup has accumulated in your drain lines. The fix is mechanical: clean the drain thoroughly with a stiff brush and enzymatic drain cleaner (not just bleach, which doesn’t penetrate the biofilm). For floor drains that are rarely used, running water periodically prevents the trap from drying out and keeps organic material from accumulating.

If drain flies persist after thorough drain cleaning, there may be a hidden moisture source — a broken drain pipe, a leak under a slab, or sewage seepage. Contact Sanctuary Pest Control at 815-993-3472 for assessment, and consider having a plumber inspect your drain lines.