this article about gnats in tomato plants says   to cover the top with foil. 

 Would this also work to deter those bastards from the little trees, while they are in cups or even in pots. it is a lot easier to wrap the entire thing in aluminun foil. 

http://homeguides.sfgate.com/gnats-tomato-plants-37429.html

Knee high nylon stockings (queen size will encase up to a 3/4 gallon pot) has resulted in zero gnat issues for me this year. Previous years the cups and pots were ravished by the gnats. Not one lost this year using these. I just use a twist tie around the cutting to close off the opening. It lets the pots breathe and easily bottom watered.

Tyler

So far the only things that have worked for me are the stick tape, BT and doing what Tyler does in making it impossible or more difficult for them to get to the soil to lay eggs, a layer or perlite on the top helps that...but the fingernail polish and the dryer sheets didn't deter the ones I have around here at all.

I finally got my gnat problem under control but it took Bti, nematodes (a little over $20 for 10 million which I used to treat my plants twice) and yellow and blue sticky traps. Finally. I lost more cuttings then ever before by a factor of 3 and will now pray my single back up cuttings will root and will have to hunt down at least half a dozen varieties where I lost all my cuttings.
Lesson: act faster. Don't wait until cuttings are affected - as soon as I see any signs of these retched creatures I'll dose with everything I can find. I will also try using pure perlite for the top 2 inches as this is supposed to deter the darn things.

Hi Grasa,
IMO : Aluminum foil works as long as there is light. So under grow lights it does work well.
Be careful to turn the bright side to the light and it seems that gnats loose their way to the dirt.
I did that, and this is what happened.
The problem then is that aluminum foil does not let the dirt breath ... So I had to remove the aluminum foils to release some humidity from the surface of the dirt...