By swarm do you mean they just hang out at the tree all day or are the hornets building a nest in it? Do you only want to cut it down because of this? Is it in good health otherwise?

If they are building a nest. You can do one of two things, try some sort of chemical or orange oil to wipe them out, OR, get a big garbage bag, put it around the nest, cut the branch they are attached to and seal up the bag and leave it in the sun for about 3 days. That should bake them.

4-21-2011

I have noticed 3 dark circles, forming a triangle, on the head of yellow jackets, bees,cicada, can you tell me what these are?. If there is a charge for this, forget it.

You might want to get advise on the fruit & orchard forum.
It seems the tree has aphids under the leaves, wasp and hornets feed
on these, milking the secretion or eating these,..which would be a good thing,
several soap spry under the leaves with a hose adapter can work, more
info would be helpful.