The leaves of a few of my arrowwood viburnums look like they have been shredded. Some of the remaining small pieces of leaves have turned completely brown. This has not happened in previous years. What is this problem and what should I do?
— Jennifer Baker, Morton Grove
Unfortunately, your description sounds like your viburnums are being damaged by the viburnum leaf beetle. This damage is commonly found on arrowwood viburnums because the insect favors its smooth leaves. At the Chicago Botanic Garden, we monitor our viburnum collection for this pest each year and treat it as needed. Thankfully, it can be controlled with treatment as well as supplemental water during dry periods this summer. The leaves should grow back this year if they were healthy before being damaged.
The viburnum leaf beetle is a heavy feeder that can completely defoliate viburnums. It happened very quickly to the viburnums in my garden a couple of years. Both larvae and adults feed on the leaves and remove leaf tissue between the midrib and larger veins. The leaves will then have a skeletonized appearance which will look like they have been shredded. Feeding is limited to viburnum species, so you do not have to treat other plants in your garden. However, you should monitor any other types of viburnums that are growing in your garden for damage.
The viburnum leaf beetle prefers viburnums with smooth leaves (little hair or pubescence). Arrowwood viburnum (Viburnum dentatum) and American cranberry bush viburnum (Viburnum trilobum) are commonly planted viburnums that are preferred hosts and will likely be killed over a course of two to three years if no control treatments are made. Other widely planted viburnums such as burkwood viburnum (Viburnum burkwoodii), blackhaw viburnum (Viburnum prunifolium), fragrant snowball viburnum (Viburnum carlcephalum) have varying degrees of susceptibility to damage but generally are not destroyed. Koreanspice viburnum (Viburnum carlessii), Judd viburnum (Viburnum juddii), doublefile viburnum (Viburnum plicatum) tend to be resistant to feeding.
Here is information about the lifecycle of the viburnum leaf beetle that can be helpful. They overwinter as eggs on twigs of the host plant. In May, larvae hatch and begin feeding on the host plants — this is the damage you are seeing on your plants. The larvae are greenish-yellow and develop dark spots as they age. They are usually found feeding in groups on the leaves. The larvae drop to the ground and pupate between early and mid-June and remain in the ground for about 10 days before emerging as adults in mid-to-late July so you may not see them at this time. The adult beetle is small, 1/4 to 3/8 of an inch long and is a golden-brown color with a sheen when in the sunlight. Adults will remain active until the first frost. Development from egg to adult takes eight to ten weeks. In late summer and fall, females will begin laying eggs. They chew holes in the bark of twigs to deposit eggs and then cover them with excrement and fragments of chewed bark. A female can lay up to 500 eggs.
One way to help control the viburnum leaf beetle is to prune and destroy infested twigs after egg laying has ceased in October up until April. You will need to spray insecticides such as spinosad, acephate, carbaryl or malathion when larvae first appear in early May next year for good control. You cannot rely just on pruning to control this insect. Acephate will have a degree of residual activity as it will move into the leaves. I was able to control this pest in my garden with one spray application in May and will not spray again until the damage gets to be extensive again.
A second application in mid-to-late summer on feeding adults may be helpful if damage is excessive but a strategy of just treating adults will not provide effective control. You should only spray your viburnums if this insect is present and follow all directions for proper and safe use of the product that you are using. Provide supplemental water to the affected viburnums during any dry periods for the rest of this year to help reduce stress on the plants.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.