Flowering Crab tree
I noticed some sawdust at the base of the tree. Upon looking closer, I found an area of rotting trunk with evidence of ant tunnels in the rotting area. Can I plug this cavity to help keep tree healthy and what can I use to fill this hole? The tree is approx. 30 years old.
It is not recommended to fill the hole in the tree trunk.
Ants can sometimes be an indication of a insect issue. The insects produce Honeydew.
You can treat the tree with Neem Oil as it acts as both an insecticide and fungicide.
Here is a few links for you.