The Snowball Aphid feeds on viburnum and causes sever leaf distortion.  As the new foliage emerges in early spring the aphid begins to feed.  The aphid uses it's piercing sucking mouth parts to puncture the young expanding cells and suck the juices out of them.  The result of the cell damage due to the feeding is a twisting and deformed leaf.  If the infestation is bad enough even the stems can be twisted.  The aphid can be controlled using horticultural in the spring.