Snowball tree (Viburnum carlcephalum) is a hardy landscape shrub with bushy white flower clusters. Clemson University notes that viburnums are generally pest-free, and plants that do get pests are typically under stress. Stressed-out viburnums are susceptible to armored scale infestations, and these can be serious if not treated.
Scale are small insects, not disease. They come in two major types: soft shell and hard shell or armored shell. Those that infect snowball viburnum tend to have an armored shell, which makes them more challenging to remove. You can see scale with your eyes, although it can blend in with the bark. A scale resembles a small striated brown bug, about 1/8-inch long, according to Colorado State University.
Scale can remain on your viburnum for years before falling off naturally. Oystershell scale stay on the tree year-round. In the spring, young scale eggs hatch and the new scale crawl around. This is the only time they are mobile. The young scale soon fly to a new tree, hunker down there and feed off that tree until they die. So in the spring, you may see crawlers but at other times of year you'll notice only stationary bumps.
Try scrubbing the scale off snowball viburnum branches with a plastic scraper or scouring pad, taking care not to scrub branches too hard. This can remove minor infestations. You can also spray horticultural oil in temperate weather (between 40 to 85 degrees Fahrenheit), states Clemson University.
Clemson University recommends using only mild insecticides, since stronger products can kill the natural enemies of scale. They suggest using products containing acephate or malathion; Colorado State University suggests insecticides with permethrin or bifenthrin. Apply the manufacturer's recommended dose in the spring, repeating applications as often as recommended.
Viburnum left untreated from scale can eventually die. You will notice either dieback of heavily infested branches or a full-scale shrub decline from severe infestations.