How to control viburnum leaf beetles
Question: Help! We have some little slug-like creature eating one of our shrubs. I don’t know the name of the shrub, but it gets white flowers in the spring and then clusters of red berries in the fall. We’ve always called it a cranberry bush, but I don’t think it’s a real cranberry because the berries don’t look like actual cranberries. These bugs are all over the plant, eating the leaves until they’re full of holes.
Answer: It sounds as though your shrub is infested with viburnum leaf beetles. These recently introduced pests are specific to viburnums, and from your description of the shrub, it sounds very much like an American cranberrybush viburnum.
This pest attacks several other species of viburnums, too, especially Arrowwood and Blackhaw viburnums. Other species of viburnums are less prone to damage from this beetle.
The small, slug-like creatures you describe are actually the larval beetles, not the adults. Heavy infestations of this pest can cause significant damage and defoliation. Since there are very few natural predators of this new pest here in North America, their numbers grow quickly.
The viburnum leaf beetle (Pyrrhalta viburni) is a native of Europe. Adult females are around from early July through the autumn, when you can often find the females chewing holes in the young viburnum branches to lay their eggs.
It’s easy to spot evidence their egg laying behavior by examining the undersides of young twigs. You’ll see a series of chewed holes lined up all in a row.
Adult beetles are only a quarter-inch long, with antennae almost as long as their bodies. They’re brown with dark markings on their sides.
The eggs spend the winter inside the bark of the viburnum. They hatch in May, when the new larvae begin to feed and grow quickly. Eventually the larvae reach about a half-inch in length.
They’re creamy yellow grub-like critters with dark markings. Often the larvae are found on the undersides of the leaves.
By the time July arrives, the larvae are done feeding and will drop to the ground to pupate. A few weeks later the adults emerge to begin the cycle again.
Though viburnum beetle larvae and adults can cause significant damage, they are fairly easy to control. In the late fall, carefully examine your viburnum bush, flipping over the young branches to look for evidence of egg-laying on their undersides. It will look like little bumps of sawdust in a perfect row.
Use a sharp pair of pruners to clip off any twigs hosting eggs and throw them away in the garbage or burn them. I check my plants in November, after the leaves have fallen from the plant, making it very easy to spy the egg-laying sites.
If you find yourself facing an infestation of larvae again next spring, you can also spray the plant with a spinosad-based organic insecticide or insecticidal soap. However, it’s much more effective to prune out the egg-infested branches in the autumn.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.