Finding a swarm of small, gnat-like creatures flying out of your sink overflow hole can be unsettling. Commonly known as drain flies, these pesky creatures lay their eggs in the gunk that builds up in the pipes. Once the eggs hatch, the larvae burrow into the pipe sludge and dine on any available organic matter (grease, hair, toothpaste) until they morph into adults and fly out of the drain. It's not enough to kill the flies you see. To rid yourself of drain flies, you must also kill the eggs and the larvae that are hiding in the pipes.
How to Get Rid of Gnat-Like Bugs Out of Your Overflow Hole
Spray any visible adults with a commercial pesticide formulated specifically for flying insects.
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Cover the overflow hole and any open drains with strips of masking tape. Check the tape daily to determine which drains are infested. If drain flies are present in the pipes below, adults will be stuck to the tape.
Pull any visible debris from any affected drains with a pair of needle-nose pliers or large tweezers. Insert the bristle end of a drain stick into infested overflow holes. Slowly pull the drain stick from the hole while twisting to collect sticky residue.
Combine 1/2 cup of baking soda and 1/2 cup of standard table salt. Insert a funnel into the overflow hole. Spoon the baking soda blend into the funnel, shaking it gently to move the powder into the pipe. Follow this mixture with 1 cup of distilled white vinegar. Wait five minutes, and then add 1 qt. of boiling water. This will kill any eggs or larvae while dissolving greasy build-up and clearing the pipe.
Pour 2 tbsp. of washing soda into 1 qt. of hot water and mix well. Funnel this solution down the overflow hole to remove any stubborn residue. Wait 10 minutes, and then flush the pipe with hot water. Repeat this process to clear any affected drains.