The snowball bush, known scientifically as Viburnum opulum "Roseum," is a large shrub that produces showy clusters of white, sterile flowers each spring. This plant, which features dark green foliage, is susceptible to a number of pests, including aphids and the viburnum leaf beetle. Proper cultural practices will reduce the impact that these pests have on the bush.
Viburnum Leaf Beetle
The viburnum leaf beetle, Pyrrhalta viburni, feeds on snowball bush foliage both as an adult and as larvae. Adults are 1/4-inch long, brown, chew oblong holes in leaves and deposit eggs in square holes chewed into twigs. Eggs overwinter and hatch into greenish-yellow, .375-inch larvae that skeletonise the viburnum leaves. To control this pest, prune out twigs with overwintering eggs, monitor plants for damage in spring and pick off larvae. If necessary, address viburnum leaf beetles chemically with acephate, bifenthrin, imadacloprid or cyfluthrin.
A number of aphids may attack snowball bushes. These pests are tiny, generally dark-bodied insects that often exist in dense colonies on new growth and leaf undersides. A heavy aphid infestation can lead to distorted leaves and shoot tips, leaf yellowing and a decline in plant vigour. Aphids also excrete honeydew, which usually becomes covered with dark sooty mould. To address this pest, maintain plant health using proper care practices, ensure that aphid predators like lacewings and parasitic wasps are present, knock the aphids off the bush with a strong spray of water and, if necessary, treat aphids chemically with a number of oils or insecticides.
A few species of root weevil attack the roots and above-ground parts of the bush. Slow-moving, flightless adult weevils feed on leaves, stems and flowers at night, giving leaves or flower petals a ragged, notched appearance. The C-shaped, legless and white larvae feed on roots. Beneficial nematodes can be applied as a soil drench for some degree of control of this pest. Hand-pick adult weevils if they are visible, cultivate the soil in spring to eliminate larvae and use a chopped leaf mulch to attract birds. Control weevils chemically with acephate, azadiractin, bifenthrin, cyfluthrin or Beauveria bassiana.
A number of thrips species feed on viburnums. These pests are tiny, slender, dark and have fringed wings. Thrips feed between petals and on leaves using a scraping technique that leaves purplish spots on leaf undersides and causes leaf curl and premature drop. Flowers may be stunted and die. Naturally occurring predators will usually control thrips, so avoid insecticides that will reduce numbers of beneficial insects like green lacewings, minute pirate bugs and multiple species of parasitic wasps and predatory mites. If chemical treatment is warranted, avoid eliminating these predators by applying a narrow-range oil, neem oil, pyrethrins, spinosad or an insecticidal soap. Mow or remove weeds and grass around the snowball bush frequently. If it is deemed necessary, a number of insecticides may be applied to control thrips.
Mites are small, often reddish relatives of spiders that use piercing mouthparts to feed on leaves, causing leaves to develop a yellow stippling before eventually turning brown and dropping. Mites can be controlled with regular, strong sprays of water or applications of horticultural oil or insecticidal soap.
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- Pacific Northwest Insect Management Handbook: Spruce to Viburnum
- Pacific Northwest Insect Management Handbook; Common Landscape Pests; Sharon J. Collman, Neil Bell and Mike Bush; December 2010
- Clemson University Cooperative Extension Service; Viburnum Diseases & Insect Pests; Joey Williamson; Dec. 2007
- Pennsylvania State University Cooperative Extension; Viburnum Leaf Beetle; Silvia G. Barr and Gregory A. Hoover; Sept. 2010