A common aphid on viburnums is the snowball aphid.  These brownish aphids can cause noticeable leaf curling that is often mistaken for herbicide injury. In many cases, only a small number of leaves become curled and the aphid numbers often drop off in June. You can often find ants tending the aphids as well.  Control is usually not suggested, but insecticidal soap sprays directed to the undersides of the leaves can reduce numbers.