Bugs and crud is my weak suit.
What I do know is that there are few fungus growths found on trees that are good news.
Occasionally the tree can grow over the decayed portion. This isn't done by scattering fertilizer like a potted plant of the turf.
If the tree is growing far from a target you may consider having the canopy removed and leave the trunk/branches for wildlife habitat.
Before a removal is planned a diagnosis should be done. Using a shovel or even doing a root crown excavation with an Air Spade or Air Knife to see what is going on would be in order.
Don't rely only on a diagnosis by Internet.