Found something interesting on gnats today that I thought I would share. It involves the use of peroxide ...
Fungus gnats can also be easily controlled with a simple product you may already have at home. Fungus gnat larvae will die on contact with hydrogen peroxide. Be sure to use the ordinary 3% solution hydrogen peroxide with no added ingredients. First, allow the soil to dry out for a few days so the top two inches of soil is dry. This is where the larvae of fungus gnats reside, and they cannot survive in dry soil. During times of drought, the larvae will actually suspend their development, but once the soil is moist again, they will resume growing and eating.
Once the top layer of soil is dry, mix one part hydrogen peroxide with four parts water. Using this solution, water your plants as you normally would. The fungus gnat larvae will die on contact with the hydrogen peroxide, but the solution will not hurt your plants so long as it is mixed correctly. It would take an awful lot of hydrogen peroxide to harm the plants. When the hydrogen peroxide is watered in it will fizz a bit, but in a very short time it will naturally break down into nothing more than oxygen and water molecules which the plant will love.
Continue to monitor the fungus gnat population with yellow sticky traps, and reapply the hydrogen peroxide drench weekly as long as you continue to find adult fungus gnats buzzing around your plants. As extra insurance against the gnats, you can water your plants with the hydrogen peroxide mixture once a month to prevent further outbreaks of this little pest.