We just listed a number of control options for fungus gnats.  Most of the registered pesticides and biological products for fungus gnat control are larvicides.  This means you have to apply the product preventatively, before you see a significant adult population.  While it is easy to monitor for fungus gnat adults with yellow sticky cards, monitoring for fungus gnat larvae is a little more difficult.  Here is one technique:

Cut a potato into several thin slices or small cubes.  Place these slices on the surface of the growing media in pots, flats or soil beds throughout the greenhouse. Fungus gnat larvae migrate to feed on the underside of potato pieces placed in media. To determine whether container media are infested, use 1-inch cubes or slices of peeled raw potato embedded about 3/8-inch deep into media. Pick up and examine the underside of each potato and the soil immediately beneath it about once or twice a week. Compare numbers of larvae before and after any treatment to determine whether larvae are being controlled.

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