The arbrex treatment may inhibit repair.Trees are able to recover the
wound by forming a calous in time which self seals the area.
look for other tree damage in the plot.
Our experience of tree damage is from deer and required the fitting of
6ft high deer fencing on all boundaries, costing £7000 for 1,100
metres.So maybe the high netting around each tree idea is your first
port of call. Would be interested in how successful you are. Steve.