You may not have heard of them but there is every chance you've seen them. Think tiny little flies floating around your plants. They're as annoying as all hell, and can wreak havoc on you plant fam if not dealt with.
Fungus Gnats are often confused with mosquitoes (although they don't buzz and are much smaller). The adult flies are a risk because they feed on fungal growth, and can therefore become spreaders of diseases. The even bigger threat though, is their larvae, which, if left unchecked, will nibble onto your plants roots and starve them of nutrients. An adult gnat can lay up to 300 eggs in its 3 week lifespan, so it's easy to see how they can get out of control.
The sooner you can deal with them, the better.
Where they live
Moist, rich potting media. If you're prone to overwatering, don't be surprised if you have to deal with them on the regular. You may have actually seen an increase in gnats around your home as they tend to move inside of the cooler months where the temperature is warmer. Coupled with the fact that alot of us forget to reduce our watering now as our plant growth starts to slow, the conditions make for a gnat-paradise.
What to do
The best method for keeping Fungus gnats at bay? Get your watering right. If there is no excess moisture in your potting media, the gnats will be less likely to lay their larvae there. If you need to, consider using a moisture meter to help you understand your plants water requirements.
Because they live on decaying organic matter and fungal growth, any sick plants can be more susceptible to becoming a host. A bit of root rot will be a tasty treat for a freshly hatched gnat larvae, so keeping your plants healthy by feeding them on a regular basis and removing any dead foliage will help prevention.
Neem is often used to counter Fungus Gnat larvae. Depending on how bad your problem is, you can spray neem directly onto the surface or do a 'drench' by pouring pre-mixed neem evenly into your potting mix. (In Australia, while commonly used to deter indoor plant pests, Neem is not registered as a pesticide with the APVMA)
Always use a quality potting mix. A good potting mix will have plenty of organic matter, so that if the gnats do get in there, they will go for that before the plant itself. If you have larvae present, consider removing the top few centimeters of your media and replacing with fresh mix.
Another option, albeit a little gross, is to have a yellow sticky trap placed near the base of your plant to catch the adult flies.
As a last resort, a hydrogen peroxide mix (1 part peroxide to four parts water) can be used to drench your plants potting mix. The peroxide will kill the larvae instantly. Be cautious though, as the peroxide will also strip all the beneficial microbes and bacteria from your potting mix, so you will need to build that back up again.