It sounds like your plant has been attacked by Viburnum Beetle. They are particularly fond of Viburnum opulus and the damage that you describe is quite characteristic of this pest.
The eggs hatch in May, and the cream/ yellow larvae (with black markings) feed on the leaves for about a month before dropping to the soil. They pupate in the soil, and when the adults emerge in mid to late summer, they return to the plant to continue feeding on the leaves and lay more eggs at the shoot tips.
Because both the larvae and the adults feed on the leaves of Viburnum they are capable of doing quite a lot of damage. You can pick off the infected leaves, however if your plant is severely affected then we would suggest that you spray your plant now with a contact insecticide containing the chemical thiacloprid.