Dirty drains can harbor drain flies.
Common drain flies live and breed in drains and thrive on the sludge found there. The issue is rarely kitchen and tub drains, since these drains are flushed out regularly so sludge doesn't accumulate. 
Normally, they come up from the floor drains where they live, looking for a food and water source.
Instructions
1. Locate all your floor drains in the lower level of the house, include sump pump openings, and remove the drain covers.
2. Look along the upper edges and on the inside walls of the drain pipe with a flashlight. Probe with a long screwdriver to scratch in the area, looking for sludge buildup on the inside. Drain flies live and breed just above the water level, in the sludge buildup.
3. Work hot water into a scrub brush to break up and destroy the sludge. Scrub the sludge within the drain as far down as you can using the scrub brush. Clean the upper lip of the drain as well. 
Clean the drain covers, especially the underside facing the drain.
4. Flush the drain by pouring hot water into the drain.
5. Pour bleach into an empty spray bottle. Spray into the drain, focusing on sides of the pipe and along the top lip. Pour a quart of the bleach into the drain to kill flies and to contaminate any remaining food source. Avoid using the treated drain for four to six hours.
6. Look under sinks, behind the refrigerator, around the water heater and under the dishwasher for stagnant water. Blot it up and spray bleach over the area where the water had settled. Stagnant water can become a breeding ground for drain flies and other insects as well.
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