See the last paragraph on this page:

Here is a link that might be useful: Gnats

Thanks so much!!!

I would strongly discourage the use of larvicides or pesticides indoors because you probably don't want to breathe that stuff in.

If you are growing stuff like tomatoes, just try getting a cover, or a simple zip lock bag locked onto your pots with a rubber band to hold the bag in place.

I do it with my seedlings until they are big enough to handle themselves. Gnats only seem to go on egg laying sprees when they see small seedlings that are vulnerable.

steve, gnats could care less about 'vulnerable seedlings'. The larvae, which are the problem, wouldn't even do damage if they weren't confined to a little container. The adults will lay eggs in a container with no plants in it, as long as the potting medium is on the moist side.

ryse, be sure to read the attached label all the way through. It warns of several things, including that you must use enough water at each watering to insure that the active ingredient is distributed throughout the growing containers. Use it each time you need to water, but allow the mix to dry out somewhat between drenches. In other words, don't add to the problem by watering more often than you have to.