Viburnum leaf beetle is a relatively new pest to Chicagoland gardens. Here’s how to treat an infestation.

“The leaves of my arrowwood viburnums look like they have been shredded, and the small pieces of leaves that remain have turned completely brown. This has not happened in previous years. I had always thought that this viburnum was trouble-free and easy to grow. What is this problem and what should I do?”

— Gail Westerman, Highwood

Your description sounds like your viburnums have been attacked by the viburnum leaf beetle, a relatively new insect pest for the Chicago region.

I have been seeing lots of arrow wood viburnums in home gardens, with lots of damage this year, including my own plants, since the arrowwood viburnum is a favorite of this insect. The Chicago Botanic Garden monitors its viburnum collection for this pest each year now and treats it as needed.

You will need to control the insect on your viburnums, as well as provide supplemental water during dry periods this summer if you want to save them. The plants should leaf back out 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 on the viburnums in my garden. 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.

Plants may die after two or three years of damage from very heavy infestations. This insect’s feeding is limited to viburnum species, so you do not have to treat other plants in your garden. Monitor any other viburnums that you are growing, though, for this insect.

The viburnum leaf beetle prefers viburnums with smooth leaves (little hair or pubescence). Arrowwood viburnum (Viburnum dentatum) and American cranberrybush 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) and 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) and doublefile viburnum (Viburnum plicatum) tend to be resistant to feeding. There generally will be more damage on any viburnums that are grown in the shade.

Viburnum leaf beetles overwinter as eggs on twigs of the host plant. In May, larvae hatch from the overwintered eggs and begin feeding on the host plants — this is the damage you have recently noticed 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, ½ to ⅜ of an inch long and is a golden-brown color with sheen when in the sunlight. Adults will remain active until the first frost. Development from egg to adult takes eight to 10 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 effective way to control viburnum leaf beetle is to prune and destroy infested twigs after egg laying has ceased in the fall in October, up until April. But pruning alone will not be enough. You will need to spray an insecticide such as spinosad, acephate, carbaryl or malathion when larvae first appear in early May for good control. Acephate will have a degree of residual activity, as it will move into the leaves.

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 you find this insect 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.