See this link.
http://ipm.ucanr.edu/PMG/PESTN...
There a number of suggested non chemical alternative in this link above. The nematode mentioned is just one. There are others that can expand the range of their predatory effects. It takes only one nematode to kill an insect.
BTI will persist, it does not need to be reapplied as suggested. It was originally found on the shores of
a desert oasis. An extreme environment where nitrogen was locally deficient.
Like other Bacillus they all wear space suits so they
are very durable. Depending on what you are growing but the fungus gnat habits are very
wet and if a peat mix is your medium or in part
this holds the right kind of moisture. Since insects are
essentially leaky bags of water.
Drains are niches for other organisms and form communities of bacterial consortiums, fungus,
nematodes, algae, virus and insects etc. The bacteria can form plaques
similar to dental plagues on your teeth on the drain pipe surface. These form catchalls
for a mixture of debris composed of organic and nonorganic materials
moving into the drains. In horticultural areas great or
small, peat mix, fertilizer, paper, wood, plastics can make their way into the drains and accumulate.
providing enough infrastructure for capturing food for the drain biological community, including drain flies, fruit flies and fungus flies. Both having wings will venture out from their preferred habitats of origin to secondary locations
such as potted plant, hydroponics growing systems, kitchens, bathrooms and toilets or drains in the case of fungus gnats. They can carry
pathogens and introduce them from the drains into your uncovered
planting media and plantings. These microbial pathogens can damage
the roots of plants confined in the relatively small volumes given to
seedlings. It is a harsh environment in both locations, like an estuary, the drains and
seedlings growing media can be subject to periodic drying, overwatering and temperature fluctuations which can damage roots, compounded further by over fertilizing a common occurrence since the manufacturers of fertilizers make the recommendations.
A stabilized plant medium is not favorable to fungus gnat colonization and the secretive insectivorous nematodes are entertained by the great surface area and hiding places offered by this medium. Plants will survive better, the human errors and environmental extremes above. Average success rate will be in the 90 percentile. If you want to avoid damping off, I suggest using a stabilized media for your seedlings and cuttings. Stabilized media was developed for growing plants at zero gravity.
I forgot to take pictures (so much going on)! I will next time (if I have to do it again)! 
