Try carbaryl or a pyrethroid. They will kill your bees and have no effect on your tree. If the big fat ones are living in a colony, then they are not likely carpenter bees. Carpenter bees have a bare abdomen while bumble bees have a hairy abdomen. Bumble bees often live in old mouse nests in the ground or tree holes and are colony formers rather than solitary like carpenter bees. Sometimes one sees a lot of carpenter bees in one place if there are a bunch of nesting holes in the same wooden structure.
