What Are Drain Flies?

Drain flies — also called moth flies, sewer flies, or sewer gnats — are small, fuzzy, moth-like flies that are about 2-5mm long. They have large, leaf-shaped wings covered in tiny hairs that give them a fuzzy or furry appearance. Unlike house flies or fruit flies, drain flies are weak fliers that tend to make short, hopping flights and rest on walls and ceilings near their breeding source.

If you're finding these tiny flies in your bathroom, kitchen, or laundry room, you're not alone. Drain flies are one of the most common household pest complaints we receive from St. Augustine homeowners, and our coastal humidity makes conditions particularly favorable for them. The good news is that drain fly infestations are very treatable once you identify and address the breeding source.

Drain fly resting on a bathroom sink drain
Drain flies are commonly found near bathroom and kitchen sinks where organic buildup inside the drain provides a breeding surface for their larvae.

Where Drain Flies Come From

Drain flies breed in the gelatinous organic film that coats the inside of drains, pipes, and other surfaces where moisture and organic matter accumulate. This film — sometimes called biofilm — is made up of bacteria, fungi, and decomposing organic material. It's the slimy coating you can sometimes feel if you run your finger inside a drain opening.

Common breeding sites in St. Augustine homes include:

Close-up photograph of a drain fly showing its distinctive fuzzy wings
Drain flies are easily identified by their fuzzy, moth-like wings and small size. They're weak fliers that tend to rest on walls and ceilings near their breeding source rather than flying around the room.

Why St. Augustine Homes Are Especially Vulnerable

St. Augustine's climate creates conditions that accelerate drain fly breeding:

How We Eliminate Drain Flies

Spraying adult drain flies is a waste of time and money. New adults emerge from the drain every day, so killing the ones you can see doesn't solve the problem. Effective drain fly elimination requires addressing the breeding source:

  1. Source identification: We inspect all potential breeding sites to determine exactly which drains are producing flies. We use a simple but effective test — covering suspect drains with tape overnight and checking for flies stuck to the underside in the morning.
  2. Biofilm treatment: We treat identified breeding drains with products that break down and remove the organic biofilm where larvae develop. This eliminates the breeding surface and kills existing larvae.
  3. Adult treatment: We treat areas where adult flies are resting to reduce the immediate population while the source treatment takes effect.
  4. Prevention recommendations: We provide specific guidance on maintaining your drains to prevent biofilm from re-accumulating, including cleaning schedules and product recommendations.
Multiple drain flies on a bathroom wall near a shower area
Finding multiple drain flies resting on bathroom walls is a clear sign of an active breeding source nearby — usually in the shower drain, sink drain, or overflow hole.
Quick identification test: Not sure if you have drain flies? Place a strip of clear tape over a suspect drain before bed, sticky side down, leaving gaps around the edges for airflow. Check the tape in the morning — if drain flies are breeding in that drain, you'll find adults stuck to the tape. This helps pinpoint exactly which drain needs treatment.

Preventing Drain Fly Recurrence

Once we've eliminated your drain fly infestation, these steps will help prevent them from returning:

Areas We Serve

We provide professional drain fly control throughout Northeast Florida:

St. Augustine
St. Augustine Beach
Vilano Beach
Hastings
Elkton
St. Augustine Shores
Palencia
World Golf Village
Nocatee
Ponte Vedra Beach