We had the same problem. We have the house exterminated for centipedes, and the exterminator said to watch around plants, since there is a type of gnat that lives on molds and dead plant matter. She also said to run drain cleaner down all the drains, because there is another type that lives and breeds in drains (drain flies.) Something I found online was to put vinegar in jars and poke small holes in the lids. The gnats are drawn to the vinegar then can't get out. Put the jars around the house, and then when you see which jars have the most gnats, you'll have an idea of where they are coming from.
at 8:11 AM on Jul. 19, 2009