I recently purchased a large (8 feet tall) multi trunked Spanish Daggar Yucca tree in a container, and have planted it. The tree came with ants ( on the inside), and although the tree appears healthy, one branch is beginning to sag. How can I get rid of the ants (and whatever else might be living inside), and treat the tree to stop the apparent decay inside.
How about asking the supplier, who shouldn't have sold you a badly infested, sagging specimen in the first place?
You can probably kill the ants with a pesticide (ask a nearby dealer), but if it is rotted out and falling over that won't help. And I doubt there is anything to be done about the decay, especially if it is to the point where it is starting to topple. I do know of antibiotic preparations being recommended for painting onto certain bacterial infections of plants to control their spread. What, if anything a nearby garden center might have of this nature I can't say. I also can't say how a paste or paint would be used for your problem - if it is even a bacterial decay.