Answer: Bt is the abbreviation for Bacillis thuringiensis, a bacterium that is used to control caterpillars, mosquito larvae, and a variety of pests. For fungus gnat control, you would use a soil drench product such as Gnatrol -- it contains Bt and is intended for this purpose. Please contact your local store to see if they have the Gnatrol in stock.
Neem is a separate unrelated product based on oil from the neem tree. It is used as an insecticide and fungicide.
Be sure to read and carefully follow all of the label directions on any product you use.
However, fungus gnats are most often controlled by improved cultural methods. Fungus gnats breed in wet soil and are usually an indication that you are watering too often so the soil surface is not drying out as it should. Since overly wet soil can damage plant roots, watering less often will also benefit most plants. To stop the fungus gnats from breeding, simply allow the soil surface to dry out between waterings. It will then take about two weeks to see them disappear completely.
If you have a plant that requires constantly wet soil, you can cover the soil surface with a thin layer of clean coarse sand or fine gravel. This fast draining layer will dry out quickly and prevent the gnats from breeding. Again, it will take about two weeks for total control.
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