Gnats are in sink drains because there is some type of wet food or debris that is in the drain. According to Orkin, gnats thrive on wet soil but will enjoy any type of wet or soggy environment.
Gnats are a small insect. They have two wings and belong to the fly family. Due to their size, many people think they are baby flies, but even adult gnats are very small. They are not generally harmful but can be uncomfortable and annoying, especially in the home. Gnats generally travel in groups and will form swarms that can be seen from a distance. These swarms are comprised of millions of the tiny creatures.
The easiest way to reduce the amount of gnats in a sink drain is to reduce the amount of food that is in or around the sink drain. Fruit and other food should not be left in the sink, and fruit should never be left open and available on the counter. Reducing the amount of food gnats have to survive on will keep the eggs from surviving and can reduce the amount of gnats in a home. Owners that have homes and kitchens with serious gnat infestations can call a professional instead of trying to handle the problem on their own.