In Tucson, we have problems with Africanized bees swarming where they aren't wanted, but they can be killed quite effectively with a non-toxic solution of water and dishsoap. I suspect the same solution would work on yellowjackets. A gallon of water to a teaspoon of soap will do the trick, but you have to spray it directly on the bug, and pretty-well soak it. The soap breaks the surface tension of the water, so it runs inside the insect's breathing holes and drowns it--quite quickly. A hudson pump-up sprayer makes a good weapon!
Of course, this won't prevent new ones from coming to the tree, so if there are lots of them, it may not do much good. I've started protcting some figs (on an experimental basis) with "sleeves" of mosquito netting-type fabric (see photo in the post titled "eye opening experience???"). That would work as well, but may be more work than it's worth!