Just found this and am going to try it on Gnats. If it will do the job on mosquito larvae it should work on Gnats, right??
http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=12263&N=62728+62730
It dont leave anything on the surface. so you should be able to just mix it and spray or mix it with the water to feed.
New product to use on Knats
Hmmm. It's Bti, I've heard that talked about a lot, as a "natural" contorl for mosquitos, but don't really know much about gnats. Hmmmm.
(Hi Ted! Merry Christmas!)
It should kill the larvae if it works on mosquito larvae, right. Need someone that knows more then me. LOL
I found these when I googled dirt gnats. There was more sites. I hope this helps.
http://www.greenmethods.com/incl_pages/bb_soilpests.php
Fungus gnats are four to five millimeters in size, delicate and mosquito-like, and have a distinctive Y-shaped vein on their wings (the top, open end of the 'Y' is towards the wing-tip). They readily get themselves stuck on yellow sticky traps. The traps can be useful in reducing adult numbers in addition to their usefulness as scouting aids. Yellow traps will be most effective if laid sticky side up on the medium's surface; just peel back one side at a time. Do expect the traps to become littered with debris from watering and other common activities, though, what with being so close to the action. The debris may also make bug-counting more difficult. Potato disks, which have been previously mentioned, are very useful in detecting the gnats' larvae, isolating problem containers, and at reducing larval numbers — assuming you frequently change the quarter-inch disks. Moreover, potato disks are useful in playing a part of a control program and for population monitoring where sticky traps would be impractical, such as in many interiorscapes.
also
http://www.dirtdoctor.com/view_question.php?id=253
Put apple juice or apple cider in a mason jar, cover the opening with plastic and secure with a rubber band. Stab a small hole in the plastic. The small gnats are attracted th the apple juice, enter the small hole and can't get out.
Adding ceder chips to the soil and layering as a mulch I started to do.
On the link above also said to add some apple cider vinegar to one gal of water to water with.