Drain Flies

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Drain flies, also known as moth flies, sewer flies or filter flies, get these common names because of their appearance and breeding habits. Areas with a continuous build-up of microbial film will eventually create a constant layer of undesirable polymer slime, where drain fly larvae thrive.

HOW DO I KNOW IF I HAVE DRAIN FLIES?

  • Adult drain flies are not strong fliers and typically can be seen resting on the walls in bathrooms or near the drain that is their breeding area.
  • Drain flies become a nuisance when there is a large infestation.

WHAT DO DRAIN FLIES, OR MOTH FLIES, LOOK LIKE?

  • Adult drain flies are small; about 1/16” to ¼” in length.

  • Drain flies are yellow to brown to black.

  • Mature drain flies have six legs, a pair of wings, and prominent antennae.

  • The key identifying characteristics of the drain fly are the unique pattern of veins, as well as their hairy appearance.

  • Drain flies hold their wings over their bodies like a roof when resting.

  • If crushed, drain flies leave a powdery smudge.

  • People rarely see drain fly larvae since they live in the slime on the inside of drains.

  • Larvae are tiny about 1/8”to 3/8” in length.

  • Drain fly larvae do not have eyes or legs.

  • Drain fly larvae have a dark breathing tube at one end which they can extend out of the slime in order to breathe.

ARE DRAIN FLIES DANGEROUS?

  • No, drain flies do not bite.

  • Although drain flies are associated with filthy habitats, such as drains, there is little evidence that they are disease vectors.

  • However, in areas heavily infested with drain flies, people may be vulnerable to develop bronchial asthma from breathing in dust and body parts from dead drain flies.

WHY ARE DRAIN FLIES IN MY HOUSE?

  • Adult drain flies are often found adhering to the walls of bathrooms, kitchens, or basements.

  • Drain flies rest during daylight hours on walls near drains, near plumbing fixtures, and on the sides of showers and tubs.

  • Drain flies are attracted to light and become more active at night and hovering over drains, sinks and other breeding areas.

  • Drain flies also can breed in protected locations with constantly moist and decaying organic materials, such as under dripping pipes, leaky roofs, and refrigeration equipment.

  • Drain fly larvae may also breed in large numbers at sewage filter plants.  

WHAT DO DRAIN FLIES EAT?

  • Drain fly larvae develop usually in and feed on the gelatinous film (slime) that can build up inside drains, thus the common name of "drain flies".

  • Adult drain flies feed on flower nectar and polluted water.

WHAT IS THE LIFECYCLE OF DRAIN FLIES?

  • Drain flies go through a complete metamorphosis, which has four stages: egg, larva, pupa and adult.

  • Each gravid female lays a mass of about 30 to 100 eggs in the moist, decomposing organic materials along the inside of drains.

  • Eggs hatch within 32 to 48 hours at 70°F.

  • Larvae mature in 9 to 15 days feeding on the decaying organic matter, microorganisms, and algae.

  • Pupae occur in or on the surface of the breeding media and, after 20 to 40 hours, new adults emerge.

  • The life cycle can be completed in one to three weeks.

  • Adults live about two weeks.