Certified GKH Gardening Expert
Is the spot in the soil near the trunk or is it part of the trunk? If it's not part of the trunk can you fill the spot with soil? Or can you add mulch around the tree that would fill it?
If it's part of the trunk you don't want to cut into it because that would allow pathogens and insects inside. And you wouldn't want to heap soil or mulch onto the trunk because it could cause stem rot or allow a place for insects or mice to hide.
If it's part of the trunk you can add mosquito dunk to kill the larvae.