Gnats in the GH...any ideas?

I guess these gnats had been a problem outdoors, but I didn't notice them until I brought the plants into my little GH!!
My DIT, Tracy said that she was able to get rid of hers with using a bowl of apple cider vinegar with dishwashing soap. I tried that but it didn't seem to do anything.
I also heard of putting sand on the surface of the containers...
Does anyone have any ideas? If I let the containers dry out, will that kill them?
I NEED HELP, PLEASE !!!

Bt Gnatrol.
totally safe. totally effective.

for a more potent kick (on non-edible plants, and of course with proper protection) an acephate (orthene) drench works well.

sand on the soil works, but is unfeasible for anyone with a few thousand plants....
Fungus gnats are easier prevented than gotten rid of.

few things to think of for prevention:
1. air movement in the greenhouse, ie fans (normally a gnat outbreak in my greenhouses occurs in December in the corners of greenhouses)
2. not overwatering, specifically peat based media
3. not overpotting
4. destroy nesting areas, clean all debris under the benches and keep them clean. Ridding gnats from pots is pointless if there is a layer of dirt on the floor that they live in.

dp

I've been using Knock out Gnats for over a year, and I have no gnats in my greenhouse. Here is the link for it:
http://www.gardensalive.com/item_display.asp?ProductNumber=1962


Blessings,
Awanda

This probably won't appeal to many but I allowed a hive of white faced hornets to build their nests in the greenhouse. They returned each year and they took care of any insect problems for me. In 20 years I was never stung. Jessamine

Thank you for your suggestions...it seems that the Bt is the most recommended treatment. I went out and got some mosquito dunks, they are the only thing I could find in the local stores. I was told that if I soak 1/4 of a donut in a gallon of water and water with it, it would be effective in getting rid of the lavrae.
If that doesn't work...I will try and get another product on-line.
Thank you for your help.

Just be sure you get the correct variety of Bt.

What you want for gnats is Bt var. Israelensis. That will cover you also for mosquito larvae and blackfly larvae.

Other varieties of Bt will only work on other types of pests

I was in a rush and failed to mention another solution that will control the gnats that are already hatching out. Sticky traps are very effective, If you can't locate them you can make your own with yellow plastic or cardboard strips. Coat them with Tanglefoot and place them at soil level. They will quickly collect them as they hatch. To remove tangle foot from hands or fly coated traps, rub some cooking oil on it. Jessamine