Experts and BHG readers answer.
Bug Damage to Tree
Sadly, filling a tree gap such as the one you describe is not recommended mainly because it doesn't help the tree. You don't say what kind of tree this is or how old it is, but it sounds like it's an older specimen that might be near the end of its lifespan. Once a tree, of any species, has a giant gap like this, there's not much you can do to really save it."