When you see swarms of small flies suddenly springing up around your sink, you are probably having a drain fly infestation. These flies live and breed in organic matter that is trap inside a moist drain, so you do not to worry about getting rid of them, as it can be easy as removing gunk that has built up inside your pipes. Here are the steps you can follow:

1. Identify The Problem.

Examine your drain, which most probably contains standing water, as it is where the problem likely lies. As you can see, drain flies do not stray far from the area that they have chosen as their home, so it is rare for them to spread all over your home. It is important to verify that the problem is coming from your drain itself, as you might actually have another type of fly infestation from food sources, such rotting fruit.
Place a strip of clear tape over your drain, sticking it over the center with the sticky side facing down, but do not completely cover your drain with the tape. This will prevent the flies from flying up, providing you with solid evidence that it is the source. Even if there is not drain flies on the tape after your first check, you should repeat this process for at least four nights to know discrepancies in their breeding cycle.

2. Destroy Their Breeding Ground.

After you are sure that the flies are coming from your drain, remove the filter from drain and clean it out thoroughly. As you can see, an effective way to destroy these pests’ breeding ground is eliminating all the grime and gunk that they may lay their eggs in.
If you are not using your drain regularly, pour 4 to 8 liters of warm water into it to moisten it slightly. However, if you are using your drain on a daily basis, it would be moist enough, so you do not need to take extra measurements to moisten it.
Using a metal brush, clean the drain hole as far down as possible. While at it, twist the brush while slowly moving it up and down in order to remove gunk and grime from the sides of the pipe. You can also use a plumbing snake to fish out other debris that might lie further down the drain.
Pour a drain cleaner into the pipes, applying roughly 125 ml of the solution around the drain’s edge.
Finish things up with a plunger after letting the drain cleaner to sit in the sink for several hours. Then, flush it through with plenty of water.

Preventing Flies to Come Back

To prevent flies from coming back, you should keep your drain clean by cleaning it at least once every month, though a thorough cleaning is not needed. You can simply pour 125 ml of drain cleaner into your drain and let it sit for several minutes to maintain cleanliness. But for a rarely used drain, such as that of your basement or garage, it is recommended to pour a little mineral oil down the drain to prevent further infestations until you run water into it.