Here in the UK there is a bilogical control agent available in the form of nematodes. They are sold for the
contol of vine weevils (Otiorhynchus sulcatus ). They kill the weevils at the larval stage by entering through
the pore in its skin. These nematodes are so small (microscopic) that they can also do the same in the case of
the fungus gnat larvae. This is a great control if you have kids or pets around. I am surprised that you have a
problem as years ago orchid growers use Pinuicula to control fungus gnat adults and so did not have a problem
with the larvae.
I have also them it as an expensive "Treat" for my U.alpina. No joke!
Barry