Drain flies, also known as moth flies or sewer flies, are small, fuzzy insects that frequently appear around sink and shower drains. Their presence during cold weather often causes confusion about their ability to survive winter conditions. These flies breed in moisture and decaying organic matter. An infestation can persist because of the biological defenses they possess and the unique insulating environment found within a home’s plumbing system.
Drain Fly Survival Mechanisms in Cold Temperatures
The survival of drain flies in cold weather depends entirely on their location, as they are not biologically equipped to withstand true winter conditions outdoors. If exposed to freezing temperatures below 0°C (32°F), adult drain flies and all their life stages will perish quickly. The entire life cycle requires a minimum temperature threshold; for instance, pupation cannot occur if the temperature falls below approximately 7.7°C (46°F). Furthermore, the completion of the entire life cycle ceases when temperatures drop below about 6°C (43°F).
In outdoor environments, such as sewage beds or soil, these flies may enter a state of dormancy known as diapause, often in the larval stage, if they can find a cool, non-freezing shelter. This metabolic slowdown allows them to wait out adverse conditions until warmer temperatures return. However, this dormancy is not a defense against deep, sustained freezes. The appearance of these pests inside a warm home during the winter is a strong indicator that the infestation is originating indoors.
Why Drain Flies Persist Indoors During Winter
Drain flies thrive indoors because the plumbing environment provides an insulated microclimate that mimics tropical conditions. The organic film, often called sludge or biofilm, accumulates on drainpipe walls and serves as a perpetual food source and protected habitat. This sludge consists of decomposing matter like hair, grease, and food particles, which is the sole diet of the larvae. Home temperatures usually remain above the optimal 21°C (70°F) range, allowing the flies to complete their full life cycle in as little as 21 to 27 days. The pipe’s U-shaped trap holds water, which further insulates the breeding mass, meaning the flies are living in a protected environment where winter never truly arrives.
Focused Eradication Strategies for Cold Weather
Successful eradication requires targeting the breeding source within the drain, not just the adult flies. The primary step is the physical removal of the organic sludge where the eggs and larvae reside. This is accomplished using a stiff, long-handled brush or a plumbing snake to mechanically scrub the inside walls of the pipe and the P-trap.
Once the bulk of the biofilm is dislodged, another element is a specialized treatment using an enzymatic or bacterial drain cleaner. These bio-cleaners use natural enzymes or beneficial bacteria to digest the remaining organic matter, effectively removing the larvae’s food source and habitat. Standard chemical drain cleaners or bleach are largely ineffective because they cannot penetrate the thick mass to kill all life stages and may also damage the plumbing.
Adult Fly Trapping
Placing a small bowl of apple cider vinegar mixed with a few drops of dish soap near the drain can trap and eliminate adult flies. This prevents them from laying new eggs while the source is being cleaned.