Fungus gnats, often mistakenly called black flies or fruit flies, are small flies that live mostly, along with their lookalike cousins, the shore flies, in our gardens, but are most noticeable when they move indoors when they take up residence in our houseplants and seed trays. Tiny as they are, their habit of flitting around the house and flying into the faces of your guests (humans exhale carbon dioxide and this attracts them) can be rather embarrassing.
In actual fact, fungus gnats and shore flies rarely do much damage to plants. The adults don’t feed on plants at all while their larvae, which look like small white worms with a darker head that live in the top layer of potting mix, feed mainly on soil particles, fungus and algae. They can however sometimes damage the fine roots of very young seedlings.
The most obvious fungus gnat control is simply to let the soil dry out a bit more between waterings, to a depth of an inch or so (2,5 à 3 cm): the larvae simply can not survive in a dry environment. You can also try top-dressing the soil with Gnat Nix, a recycled glass material that looks a bit like perlite. It creates a physical barrier that keeps the adults from being able to lay their eggs in the potting mix below.
However, you have to be patient at first. Even if you kill the larvae by letting the soil dry out a little more or prevent the adults from laying new eggs with Gnat Nix, that doesn’t eliminate the adults gnats right away and you may still see them flitting about for a week or so. However, without no younger generation to replace the adults, you will soon be able to invite guests over again… and all that, without having had to use any toxic pesticide!