Hello. I have just received a reminder that your question needs an answer. My apologies if you have not heard from me--I did send a reply days ago. If this tree is important to you and you want to save it please have it evaluated by a certified tree arborist in your area. You can find one at this website: http://www.treesaregood.org. Since your tree lost a major limb and it shows symptoms of an insect presence (perhaps borer larvae) you are right in being concerned about the structural integrity of the tree.