Last May (2010), 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 the Bugguide site 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.
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.
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.
Viburnum Leaf Beetle (MSU)
Viburnum Leaf Beetle Survey Status (Purdue University)