We have a large box tree about 90 years old in our front garden every year it seeds & we get scores of little box trees, my wife potted them up & planted them in a hedge. It was doing extremely well until last year when we got the dreaded box moth. The low hedges got infested with box moth caterpillars to the point we had no choice but to cut & burn them.
Surprisingly the old box tree has survived with little infestation, possibly the moths fly low to the ground? Blue tits love the caterpillars too & a pair have raised two clutches this year.