How to treat snowball beetle infestations

“The leaves on my Arrowwood snowballs look like they have been shredded and the little leaves that are left have turned completely brown. This has not happened in previous years. I had always thought that this viburnum would be hassle free and easy to grow. What is the 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 area.

I have seen many arrowwood viburnums in home gardens with a lot of damage this year including my own plants as arrowwood viburnum is a favorite of this insect. The Chicago Botanic Garden now monitors its viburnum collection for this pest each year and treats as needed.

You’ll need to control the insect on your viburnums and provide extra water during dry spells this summer if you want to save them. Plants should sprout again this year if they were healthy before being damaged.

The snowball leaf beetle is a heavy feeder that can completely defoliate snowballs. With the snowballs in my garden, things went very quickly. Both larvae and adults feed on the leaves, removing leaf tissue between the midrib and larger veins. The leaves then have a skeletonized appearance that looks like they have been crushed.

Plants can die after two or three years of damage from very heavy infestations. Feeding this insect is limited to viburnum species, so there is no need to treat other plants in your garden. However, monitor any other viburnums you grow for this insect.

The snowball beetle prefers snowballs with smooth leaves (few hairs or hairs). Arrowwood viburnum (Viburnum dentatum) and American cranberrybush viburnum (Viburnum trilobum) are commonly planted viburnums that are preferred hosts and are likely to be killed over the course of two to three years if control treatments are not applied.

Other common viburnums such as snowball viburnum (Viburnum burkwoodii), viburnum viburnum (Viburnum prunifolium), and scented viburnum viburnum (Viburnum carlcephalum) vary in susceptibility to damage but are not generally destroyed. Koreanspice viburnum (Viburnum carlessii), Judd viburnum (Viburnum juddii), and doublefile viburnum (Viburnum plicatum) tend to be resistant to feeding. In general, snowballs that grow in the shade take more damage.

Snowball beetles overwinter as eggs on branches of the host plant. In May, larvae hatch from the overwintered eggs and begin feeding on the host plants – this is the damage you’ve been noticing on your plants recently.

The larvae are greenish-yellow and develop dark spots with age. They usually feed on the leaves in groups. The larvae fall to the ground and pupate between early and mid-June and remain in the ground for about 10 days before becoming fully grown in mid to late July, so you may not see them at this time.

The adult beetle is small, ½ to ⅜ inch long and golden brown in sunlight with luster. Adults remain active until the first frost. Development from egg to adult takes eight to ten weeks.

In late summer and autumn, the females begin to lay eggs. They chew holes in the bark of twigs to lay eggs, then cover them in excrement and chewed bark fragments. A female can lay up to 500 eggs.

An effective method of controlling the viburnum leaf beetle is to prune and destroy infested branches after oviposition has stopped in October through 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 to ensure good control. Acephate will have some residual activity as it migrates into the leaves.

A second application in mid to late summer when feeding adults may be helpful if damage is excessive, but a strategy of treating adults only will not provide effective control. You should only spray your viburnum when you find this insect and follow all directions for the proper and safe use of the product you are using. Give affected viburnums extra water during dry spells for the rest of the year to reduce stress on the plants.

For more plant advice, contact the Chicago Botanic Garden’s Plant Information Service at [email protected]. Tim Johnson is Senior Director of Horticulture at the Chicago Botanic Garden.

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