As I was preparing to seed one of my media beds I noticed thousands of fungus gnat larvae in the media. The larvae eat plant roots and seedlings and have contributed to the failure of my plantings in the recent past.
There are several ways to deal with fungus gnat infestations including the use of predatory nematodes and potassium salts of fatty acids (insect killing soap). However, this infestation was so bad that I decided to do something more extreme.
After clearing the bed of all vegetation, I fired up a blow torch and went over the top of the media with the flame. Then, I scooped out the top 2 inches of the media (which is a mix of cinder rock and hydroton) into a bucket and rinsed it. I left it to soak for a few hours to drown any remaining larvae. While the media was soaking I applied the torch again to the top of the media in the media bed to kill off any surviving larvae. As this was the top of the "wet zone" I used the torch after the water had drained from the bed.
Finally, I rinsed off the media that was soaking in the bucket and added it back to the media bed. This morning I dug around in the bed and could not find any surviving larvae.
One concern about using such an extreme method of pest control is that I probably killed off some beneficial bacteria. I'm not too worried about this. Most of the bacteria exist in the wet zone of the media bed and these beds are about 16" deep. The loss of bacteria is negligible and given that this is an established system with 3 other media beds, the bacteria will quickly reproduce.