Infested Snowball Tree

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Q: I have a large, well-established white snowball tree that has been infested with little tiny bugs on the back side of the leaves, basically stripping the tree of foliage in about two weeks. I sprayed the tree with a insecticidal soap. It is starting to establish a second growth of leaves, should I spray again? The tree is probably 20 years old and has always been beautiful with lots of blooms; will this disease spread to nearby trees? I have a pink dogwood close by and a small pink snowball tree approximately 50 ft away, should I spray these as well? Would a winter dormant spray be helpful? I’m really worried that I’ll lose the tree and the wonderful shade garden planted beneath it. I live on the Sunshine Coast.

Viburnum leaf beetle (Pyrrhalta viburni) is the insect causing this “shot hole” feeding on your snowball tree (Viburnum opulus ‘Roseum’). It’s become very widespread in the Lower Mainland since it was first identified back in 2001. Other notable viburnums, including V. tinus and V. davidii, are very susceptible. This insect only feeds on viburnums.

The beetle only has one generation a year. Egg-laying sites are usually noted on the underside of first and second year twigs and look like bite marks laid in a straight row. The insects emerge in May as caterpillars, mainly on the underside of leaves. In early summer, the larvae drop to the ground and emerge as an adult beetle by mid-summer. Adults continue to feed on the foliage and eventually mate. Females have the potential to lay up to 500 eggs from late summer to first frost.

Cut out first and second branches during winter. You may not have very many flowers. Monitor new growth for further chewing (May). At this time you can either spray (use a product listed for beetles) or continue to cut infested branches. Feed, mulch, and water plants to re-establish growth. Further information is available on Cornell University website. For whatever reason, maybe our climate, the insect loves V. tinus and V. davidii, despite both of these trees being rated only moderately susceptible and resistant respectively by Cornell.