They feed on the roots of the tree and will do so until the tree dies.
As I do not like using Chemicals on my trees, I soak the pots to kill the grubs.
if you suspect that you have an infestation, Immerse the pot completely, and leave submerged for at least 1/2 and hour. Most times the Grubs will climb to the surface of the soil and will expire there. Feed them to the birds

While you are soaking the pot you can add some Charlie Carp or Seasol to the water to fertilise at the same time.
Ken