They are not too good for seedlings, but relatively harmless to mature plants. Put up your sticky trap and cover the surface of your soil with foil. If you have HPS or MH lamps be sure it's dull side up- with my CFLS and fluoros, I put it shiny side up. It seems that the gnats get confused when light comes from both "up" and "down" and they are busy looking for dark dirt to lay their eggs in. Their poor little brains can't handle light from 2 directions! They are more of a nuisance than a problem. - Granny:hippy: