Thank you for your response. We were assured that the tree would heal around the foam sealant. Ugh. The cavity is pretty big. Will provide photos as soon as I get home later this evening. Would you suggest we treat for the ants and leave the cavity open?

About all I would do is try to clean the cavity and remove the rotten wood. The ants are not doing any damage but, rather, removing the rot in their small way. They do not eat wood, they simply tunnel in rotten wood. Carpenter ants feed on proteins and sugars, they can't even digest wood, so I would ignore them and focus on what they indicate.

tj

Here are some photos. Any ideas how I might go about getting the problem diagnosed and (I hope) fixed?

The ants are in the dead part of the tree. They are cleaning it out for you.

Any human cleaning typically breaks the barriers the tree has set up against fungus. When that barrier is broken, more of the tree is adversely affected.

Leave it be.

If you're concerned about its safety and/or if it will blow-down, have several Certified Arborists (qualified to evaluate potentially hazardous trees) on-site for a look see. Search by zipcode at www.treesaregood.org -- the certifying agency.

Thank you, tj and Jean for your spot-on suggestions. I took your advice, and several certified arborists confirmed everything you said. The tree is not hazardous and leaving it be seems to be the best option. We were even advised not to try to exterminate the carpenter ants--apparently, they are happy where they are and not motivated to come into our home. An inspection of our home revealed no sign of carpenter ant infestation. I still cannot believe how out original arborist could be so wrong! Thank you both for putting me on the right track and saving me a lot of money and frustration, not to mention the loss of a good tree!