Can you get your hands on Neem oil? This is safe to use for an insecticide, but not good to use in the hottest part of the year due to it being an oil. I've learned the hard way not to do that with any type of oil for plants in the warmer parts of the year.

Hope this helps,

Olyve...(O:
><>

They live on fungus and are not likely to be damaging your trees. If you can allow the surface of the soil to dry - the gnats will lose their food supply.

The adults only live a short time and spend it reproducing. Yellow sticky traps will catch 'em. And, I've read that the Bt for mosquitoes will kill 'em.

Steve