Have not had this myself, but upon looking at info, it may be the European shot hole borer Scolytus rugulosus, which is a pest in orchards. Not sure that cutting bits off would help much.
The type of insecticide that might be of use may be one that contains bifenthrin. like Bug Clear by Scotts or B&Q Insecticide For Flowers and Vegetables. This is a contact insecticide with some residual effect. Maybe squirting some into the hole would kill the larvae.
Also, I understand that growers use sticky traps to see if the pest is active, but not sure what sort - maybe grease bands would trap the beetle, but placed at higher level on the tree than normal.
