I use neem oil to get rid of mine. You have to use three to four applications (I use it as a root soak whenever I water my orchids until the little critters are 'no more') and it works a treat. I think it also acts as a mild fertiliser too, so an extra bonus! I find that fungus gnats are far more of a problem in the winter than in the summer. I guess because in the winter orchid potting mix takes longer to dry out after watering.
can you tell me exactly how to use the neem oil. thanks
I put a 5ml spoonfull of neem oil into a two litre bottle of water, give it a good shake and then dispense the liquid into various containers that I place my orchid pots into for a root soak. I leave the orchid pots in the solution for five minutes. As I have quite a few orchids in my collection, I make up quite a few bottles of neem solution! Afterwards, I put all the leftover solution onto the plants in the garden, so none is wasted!
I also meant to say, don't give up with the neem oil if there are still flies after the first application! Just keep using it weekly until the flies are gone. I think because some of the flies are at different stages in their life cycle, it takes a little time to kill them all! With mine, they were gone after four applications.