Viburnum under siege

Leaf damage inflicted on arrowwood by the viburnum leaf beetle

The shoots of Viburnum dentatum are thin, strong and straight. Native Americans used them for arrow shafts. That is why this shrub is often called arrowwood. Until recently it was considered as a plant free from serious pests. Now, the arrowwoods in my area are under attack by an invasive leaf beetle.

Last May, I began noticing leaves full of holes (skeletonized); something that I have never seen before. I finally spotted the culprits, little bugs hiding on the underside of damaged leaves. They look somewhat like caterpillars, but not quite; they are greenish, with black dots. A visit to Bugguide.net helped me identify them and provided links to several university sites full of information (see below). They are the larvae of the viburnum leaf beetle (VLB for short, Pyrrhalta viburni), an introduced pest from The Old World. Both the larvae and adults of this beetle feed exclusively on a variety of species of viburnum.

Viburnum leaf beetle larva and leaf damage

In its place of origin, probably eastern Asia, the beetle and the viburnum have been fighting an escalating war for perhaps as long as millions of years. The female lays its eggs in holes made on thin twigs of the plant and plugs the batches of eggs with sawdust and feces. The plant develops a thick and hard mass of scar tissue that crushes the eggs. The beetle fights back by producing larger and larger numbers of eggs, so that, at least, a few escape the crushing weapon. Also, several females lay their egg masses close together, overwhelming the capacity of the twig to develop scar tissue. The plant responds by getting better at making scar tissue. The struggle continues without serious damage to the plant and without either adversary going extinct. We could call it an armed truce.

In this scene enter the human horticulturists and their quest for exotic beauties to add to gardens, a noble endeavor it would seem. Who would have known the unintended consequences? It is easy to see how some beetle eggs were carried overseas along with nursery stock. When they arrived in this continent, it probably took them a number of years to make the transition from their familiar species of viburnum to our native ones; but, once they did it, they found a defenseless plant. None of our viburnum species had reasons to develop a defense weapon against an enemy that they had never met. They are easily overcome by the onslaught of the many hundreds of larvae that emerge from the eggs laid by each female. Healthy shrubs can be turned into gray skeletons in just a couple of years.

Adult viburnum leaf beetles

Annapolis Royal is a town in Nova Scotia that has been the port of entry for numerous kinds of horticultural stock, including some Asian and European viburnums, for more than a century. It has also been the site of a very important nursery. The viburnum leaf beetle was noticed for the first time, and soon forgotten, in the 1920s not far from this area. Not much happened until 1947 when it was collected and identified once again. And, for a second time, it was quickly forgotten, just a minor entomological curiosity. Another 40 years went by until the VLB and its damage to native viburnums began to be noticed in a number of places. Research and surveys began in the seventies and eighties with the object of studying the possibility of an invasion.

The spread of this pest has been gaining momentum since then. It is found in Canada and north eastern United States. Concern is growing, motivating some to say that this situation has the potential of “. . . verging on ecological disaster” (Small and Catling, 2005). The National Agricultural Pest Information System has been monitoring the advance of the beetle. They just added Montgomery county, PA, because of my finding. The story of the VLB is reminiscent to that of the chestnut blight and the hemlock woolly adelgid, to name just the best known examples of pests introduced with horticultural stock.

According to the USDA plant profile, there are 30 species of viburnum in the country, 15 of which are native. The others have been introduced from Europe or Asia. About half of the native ones are in the threatened or endangered category. Viburnum shrubs are present in our gardens and they also form part of the understory of many forests throughout North America. Their loss would be significant to wildlife. Their berries supply food for birds in the fall and winter. They are host plants for a number of moths, which in turn become food for baby birds. Among the moths that feed on viburnum we count the delightful hummingbird moth and the spectacular Io, imperial and cecropia moths.

In part II I will provide information on beetle identification and methods of control.

More Resources

Have you seen evidence of Viburnum Leaf Beetle in Your Wildlife Garden?

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Comments

  1. says

    Great Information Beatriz. I see that the chances of Florida being affected are diminished by the need for an extended cold period. That is a good thing.

    You know I love my bugs. Dare I say that the larva are rather attractive? Another reason not to put beauty ahead of the potential consequences. It’s so important to find appropriate native plants over showy invasive exotics that can have horrible affects on our collective gardens, but this shows that even imports of non-invasives can carry potential danger.
    Loret recently posted..The Blur of the Butterfly — Missed Opportunity

  2. says

    Great information, Beatriz. Thanks so much for sharing it with us. I’m looking forward to part 2. It would be so nice if the horticultural industry learned an important lesson from this and made significant changes in their practices, but since they have refused to learn any lessons at all from past mistakes, I don’t hold out much hope for that. Maybe they’ll prove me wrong this time.
    Carole Sevilla Brown recently posted..Ecosystem Gardening The Journey

  3. says

    Beatriz, thanks for sharing this important info. It hasn’t turned up here yet but I live in dread of the VLB because I grow 3 different varieties of native Viburnums…we do have the Hemlock Woolly Adelgid on our hemlocks but they have not caused complete devastation as in other places..I am hoping that the hemlocks are in the process of adapting to the adelgid pest using its own chemical defenses, and perhaps our most resilient Viburnum populations will be capable of doing the same against the VLB. Viburnum are so important to the New England woodland understory that their loss would leave a huge void..undoubtedly taken right over immediately by invasive buckthorn and non-native honeysuckles…

  4. says

    Thanks for the comments. As I mentioned hummingbird moths lay eggs on viburnums. I am happy to tell you that Pat Sutton posted an article on hummingbird moths in the sister blog, Native plants and wildlife gardens, http://nativeplantwildlifegarden.com/ive-just-seen-a-baby-hummingbird/.
    She has a picture of one egg on an arrowwood leaf. I hope that they keep finding healthy viburnums to continue feeding their babies.
    Beatriz Moisset recently posted..Mountain laurels instead of rose bushes?

  5. says

    Oh no! I love Rusty (V rufidulum) and would hate to see them under attack in my garden or in the nearby woodlands. Thank you for sharing this important info~I’ll watch for any of the critters in my garden. gail

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