Drain Flies in NYC and Brooklyn Apartments: What They Are and How to Eliminate Them

# Drain Flies in NYC and Brooklyn Apartments: What They Are and How to Eliminate Them

You've noticed small, fuzzy, moth-like flies — maybe a few, maybe a dozen — hovering near your bathroom sink, shower, or kitchen drain. They appear in the evening, they don't fly very fast, and they tend to rest on walls near the drain. They're drain flies (also called moth flies or filter flies), and they're extremely common in Brooklyn, Queens, and Manhattan apartments.

The good news: they don't bite, don't transmit disease, and they're fixable. The frustrating part: most people treat them wrong, which is why they keep coming back.

What Are Drain Flies?

Drain flies (*Psychodidae*) are tiny (1/8 inch), fuzzy-winged flies with wings held tent-like over their body when at rest. That fuzzy, moth-like appearance is distinctive — they look nothing like a fruit fly or fungus gnat up close. They're most active in the evening and tend to hover slowly near their breeding source.

They are not coming from outside. Drain flies breed almost exclusively inside drain pipes — specifically in the gelatinous organic biofilm that coats the interior walls of P-traps, floor drains, and older cast iron or clay drain pipe sections. They are an indoor problem with an indoor source.

Why NYC and Brooklyn Apartments Are Especially Prone

New York City apartment drain systems have several factors that accelerate drain fly breeding:

Aging infrastructure: Pre-war Brooklyn brownstones, Queens attached houses, and Manhattan buildings built before the 1950s have cast iron and clay drain pipe sections that accumulate dense biofilm much faster than modern PVC systems. The irregular interior surface of old cast iron pipe holds biofilm in a way that smooth PVC doesn't.

Building-wide drain stacks: In multi-unit buildings throughout Brooklyn and Queens, drain stacks are shared. If biofilm is accumulating in the shared stack (not just your unit's pipes), drain flies can emerge into multiple apartments through floor drains or gaps around pipe penetrations.

Overflow drains: The small hole under your bathroom faucet is an overflow drain. It has its own tiny drainage channel behind the sink basin that almost never gets cleaned — and it's a prime drain fly breeding site in NYC apartments. Most people don't even know it exists.

Infrequently used floor drains: Apartments with a basement laundry room, utility sink, or floor drain that doesn't get used often have dried-out P-traps. When the P-trap dries, it opens a direct connection to the sewer — and drain flies can travel up from the building's main drain stack.

The Tape Test: Confirming the Source

Before treating every drain in your apartment, confirm exactly which drain is the breeding source:

1. Cover each suspect drain with clear tape (sticky side down) overnight

2. In the morning, check the tape — drain flies stuck to the underside confirm that drain as the breeding source

3. If no flies are on the tape after 2 nights, it's not the primary source — move to the next suspect drain

This saves you from treating the wrong drain and wondering why they're still there.

What Actually Works (and What Doesn't)

What does NOT work:

• Pouring boiling water down the drain: Kills surface bacteria but doesn't reach the biofilm layer on pipe walls where breeding occurs

• Bleach: Too dilute by the time it reaches the biofilm; kills some surface bacteria but doesn't break down the gelatinous organic matter

• Standard liquid drain cleaners: Not designed to remove biofilm, focused on clearing clogs

What DOES work:

1. Physical drain brushing: A drain brush used to scrub the inside pipe walls — the mechanical removal of biofilm is what breaks the breeding cycle. For the P-trap under the sink, you may need to remove the trap and clean it manually.

2. Enzymatic drain gel: Products containing live bacterial cultures (not bleach-based) that digest organic matter over repeated applications. Apply several evenings in a row to establish the bacterial culture in the pipe.

3. Cleaning the overflow drain: Remove the overflow cover plate (usually one screw) and scrub the channel behind it with a small brush or pipe cleaner. This overlooked location is responsible for a surprising number of persistent drain fly problems in NYC bathroom sinks.

4. Pour mineral oil into unused floor drains: This slows evaporation of the water seal and prevents the P-trap from drying out — closing the connection to the building sewer stack.

When to Call a Professional

A professional drain fly treatment is warranted when:

- You've thoroughly cleaned the identified drain and they're still appearing after 2–3 weeks

- Flies are coming from multiple locations throughout the apartment (suggests a building-level issue in the shared drain stack)

- You see phorid flies (small flies with a humped back and erratic movement) rather than typical drain flies — phorid flies can indicate a sewage leak or decaying organic matter inside a wall void, which requires investigation

For Brooklyn and Queens buildings with older plumbing, building-level drain stack issues require coordination with the building super or management — not just treatment within your unit. Jet Pest Control can inspect your apartment's drain system and coordinate with building management when a shared stack is the source.

Call (718) 710-0330 (Brooklyn, Queens, Manhattan) or (516) 774-5051 (Nassau, Suffolk) for a drain fly inspection. Unmarked vehicles; discreet professional service throughout New York.