I have 3 younger apples in a row…honeycrisp, galarina, and honeycrisp…and only the galarina got girdled by voles. The damn things dig under 1/2” hardware cloth fencing to get to it.
But I cut down a couple of saggy birches last fall and man did the voles have a feast chewing that up. Maybe that’s the secret? Give then some young birch to chew and they’ll leave most of my other trees alone?