Recently a distraught gardener approached me with leaf in hand from his well-established snowball bush – it was riddled with holes, and he didn’t know what was chewing it up.
This problem is caused by a new pest called the viburnum leaf beetle (Pyrrhalta viburni), which originated in Europe and Asia, and was first reported in Eastern Canada in 1947. In the past year, I have seen leaf samples from across the Lower Mainland, but this insect has probably been here for a number of years.
The juvenile stage (larvae) of this insect is a black-spotted, worm-like, voracious feeder that can strip a leaf down to its veins, producing a skeletonized leaf by late spring or early summer. The plants most preferred by this beetle include Viburnum opulus ‘Roseum,’ European snowball bush, and to a lesser extent, Viburnum tinus (laurustinus) and other species.
The larvae drop or migrate into the ground to pupate. Adults emerge in late summer, and will also feed on leaves. A female beetle may lay 500 eggs up until first frost. Egg-laying sites on a viburnum are easy to see on one- or two-year-old twigs. These sites appear as raised bumps usually deposited in a straight line on the lower sides of the twigs. The viburnum leaf beetle has one generation per year.
Control measures are best aimed at pruning out egg-laying sites during the winter months. Handpicking the larvae is possible if there are only a few. If necessary, a contact spray may also be effective against the larvae early in the season. Check at your local nursery or garden centre for spray recommendations. However, spraying for adults is not effective as the insects will either fly off or drop to the ground.
Conway Lum is a Certified Horticultural Technician at Mandeville Garden Centre in Burnaby.