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When you wash food down your drain, garage disposal a film builds up and the gnats lay there eggs in this film. Bleach doesn't kill the eggs. There is a foam called hot spot that you can get from a company called Univar or another company called Solutions They are at various locations around Houston and Dallas.Very drunk, not at all Texans, but fits the past few posts ... how in the hell did I get gnats and how to I get rid of them? I have no fruit or veggies to have brought them in (neighbors certainly do), but I can't seem to kill them fast enough or at the source.
Use a fogger to kill the existing gnats. Then put the hot spot in all of the drains in the house and that should do the trick. Depending on how bad they are you may need to do this again in thirty days.