Posted: 18/09/2014 at 15:43
The only other outdoor pest of Viburnum is the Viburnum aphid which attacks other Viburnum, not normally V. tinus and they have usually done their damage and disappeared by this time of year.
There are black aphids which is what it might be, in which case the insecticide should clear them. However they do not normally do that much damage unless in huge numbers and then they are usually found on the growing tips rather than the underside of the leaves.
Sorry I got it wrong at first.
There are thrips which attack them under cover. There are also scale insects, but they are off white.