The following are possibilities not experience:
1 I just recently read that voles really prefer to feed on grass not bark; if true, keeping areas at base of tree clear of grass may help. I used vacuum picked up leaves (1/2 rotted and matting)as mulch for years to control weed growth under and around trees and had minimal vole damage, but many of these trees were mature trees & not really so susceptible. Had little damage on new trees; all were in areas mowed like rough lawn.
2 Many years ago I heard a strategy( from Penn State?) but I never used or saw it put to use. Surround orchard with a band of crownvetch (3 feet wide?) , clear inner area of voles, keep area mowed, and more will not migrate in across vetch band.
3 Put your gravel down at the base of new trees when planting before backfilling all soil, imbed screen tube in gravel, use at least 1/2 bushel per tree.
4 Deter them with Plant Skydd ( a Swedish forestry developed mammal repellent, for deer, rabbits, voles etc. ), expensive, OMRI listed, long lasting, blood meal with a vegetable oil sticker.
5 "Poison them", with a vitamin D based poison which makes them accumulate toxic levels of calcium; but does not affect any animal predating them. I found it recently available again from Brandt Consolidated who now owns Monterey Chemical; the supplier. May also be on the VA supplier list (from the resources page of this website).
6 Encourage foxes: Anyone know how to do this? If you need to have deer fencing; how to let the foxes pass in and out?
Hope these ideas are helpful.
Dan Lefever
SE Penna
zone 7a