My Arch Nemesis!
I live near a small brook, farm and open/wooden area and as such the fungus gnat is alive and kicking
They are attracted to wet areas of soil that is full of decomposed material, so anywhere with a small brook running through natural land is the ideal setting
In my early days of growing chillies, I made the fatal mistake of watering from the top, while growing indoors on the windowsills
This provided the perfect environment for fungus gnats to invade
They like to lay their eggs near the roots of plants surrounded by wet soil
The young then start to eat the root matter before emerging from the soil
At which point they start to crowd the corner of your window to find a mate
And they breed! Like all tiny insects, hundreds at a time and will annoy the dickens out of you
How to get Rid of Them
Take your plant out of its pot, and using luke-warm water wash all the soil off the roots, and then give the roots another good washing to get rid of as many eggs as possible
Re-pot your plant in fresh compost in a clean pot - repeat for all plants in the area
For a belt and braces job, you can get Silver Sand, this is sand that has been treated to make sure there are no insects or eggs, and it is also very fine sand grains
Place this on top of your soil and pat down to form and even coating around the top - about 1inch thick
The Gnat has a very hard time getting through the sand, to both lay eggs and emerge to find a mate, so is excellent at slowing down the reproduction, while protecting from future invasions
The Gnat should look elsewhere to find better conditions to breed
As a total secure job, you can purchase an insect zapper to hang around the window frame at night - blue light that attracts the insects then zaps them dead
Prevention
The Gnat as I stated above is attracted to wet top soil, so watering from the top only provided them the environment they want
Over watering so the soil soaked all the way through is also a means that will achieve this
Both of which is no good for the plant - please see the watering section for a full explanation, but for now, chilli plants do not like wet top soil, so you should always water from the bottom
After years of protecting chilli plants from fungus gnats there is no finally a product on the market that targets these annoying gnats
NilNat works by targeting the insects eggs and killing them off, so after a week the problem is gone after the adults die out!