A table spoon of oil into the drain will prevent them from breading in the water. Do it at night before you go to bed or before you leave for work, so the oil stays as long as possible.

oil?? Do you mean baby oil, olive oil or what? Surely not motor oil?!?

Mineral oil works fine (unlike food oils it does not go rancid) and should only need to be replenished if the drain is used.

Any type of oil works but obviously you don't want to put motor oil down the drain. Cooking oil(mazola etc) or mineral oil are typically what is used. Baby oil would probably work. The idea is to seal the surface of the water so the flies can't use it to breed.

Even though I'm a bit late to the conversation, t wanted to add my experience. We have a stand shower in our bath that used to have a door. I kept noticing little worms and drain flies. The worms are the larva and they were coming from a very weird spot in the bottom of the shower door. I'm a meticulous cleaner and get places that a lot of folks don't think about so I was stumped. After further investigation of the door, I found an area that was impossible to clean where they were coming from. I removed the door, put up a curtain and have not seen a drain fly since. Just wanted to mention that these flies don't always come from the drain.

You can also get something like Invade Bio Foam. You can find it at www.bugfreezone.com or other internet sites that sell pest control products.

At my wife's urging, I purchased BioFoam thru Amazon. It did not work for me. I have also tried various combinations of bleach, drain cleaner, boiling water, vinegar and baking soda.

We struggled with these guys in our bathroom for months. Tried BioFoam, oil, to no avail. I did some research, and tried ammonia. It worked! I poured about 1/2 cup in each drain, and left it for several hours. I repeated once or twice, and then was fly free for 8 months. I repeated the treatment, and they are gone again.

KathyinJax