Wednesday, September 15, 2010

Adirondack Invasive Alert: Viburnum Leaf Beetle

I had a request over the weekend to write a piece about an invasive species that has been in the news off and on over the last six to eight years: the viburnum leaf beetle (Pyrrhalta viburni). A native to most of Europe, it first showed up in Ontario, Canada in 1947 and has since made its way into the northeastern United States. Today it is found in Maine, New York, Vermont, Pennsylvania and Ohio.

Normally considered a pest to ornamentals, it seems that this small beetle, which is no more than about a quarter inch long as an adult, is now making some headway into our native viburnums. It is, therefore, time to bring it forward into the limelight once more.
Viburnums are plants with which many an Adirondacker has come into contact, whether he knows it or not. Consider, for example, witchhobble (V. alnifolium)3. This is the understory plant with the large, heart-shaped leaves that are mottled a burgundy color now that fall is on its way. Sometimes the plants grow so thickly that they make a nearly impenetrable barrier. Their flat-topped clusters of white flowers are some of the earliest bloomers in the spring. By late summer, bright red berry-like drupes have replaced the flowers. As fall progresses, the fruits turn blue. Game birds, like the ruffed grouse, and songbirds, such as pine grosbeaks and thrushes, eat the fruit as part of their regular diet. Mammals, like deer, eat the fruit, twigs and leaves. Birds and mammals alike shelter in the dense understory witchhobble creates.

Imagine now a forest without witchhobble. Sure, it is a whole lot easier to walk through, and bushwhacking is a breeze. But now the forest is devoid not only of an important wildlife food source, but also of an entire forest layer. Vertical stratification is important in a balanced forest – when there are only tall trees, there is nothing below to provide shelter for mice, rabbits, snakes, frogs, weasels, deer, bears, ground-nesting birds. The only shelter is high above, a layer accessible to only a few animals. It is a forest that is bereft of wildlife.

Other native viburnums that can be found in the Adirondack Park include witherod/wild raisin (V. nudum and V. n. var. cassinoides)1, American cranberrybush (V. opulus var. americanum)1, Rafinesques’s viburnum (V. rafinesquianum)1, mapleleaf viburnum (V. acerifolium)2, and nannyberry (V. lentago)3. All off these shrubby plants provide shelter and food for wildlife. Now picture them gone, eaten away by a tiny brown beetle.

So, here is the point. The viburnum leaf beetle is here. It has moved on from its strictly ornamental diet to one in which our native viburnums are part of the menu. Should the progress of the beetle go unchecked, we could, in theory, see a drastic change the character of our northern forest.

It is, therefore, imperative, that we, as responsible residents of the Adirondack Park, do our bit to patrol for this insect. Here’s is what you can do.

If you have any viburnums in your yard, native or ornamental, take a good long look at them. Right now the adult beetles are out and about, but they are tiny and difficult to see. You might find small oval-shaped holes in the leaves. This is a sign of adults feeding on your shrub(s). What is most easily spotted, though, are the results of their endless pursuit to propagate their species: the egg sites. Look carefully along the twigs of your viburnums, especially this year’s growth. If you see a row of odd looking bumps along the length of the twig, you probably have the beetles.

When the female is ready to lay eggs, she chews a hole in a twig and deposits about eight eggs inside. Then she plugs the hole with a “cap” made from chewed bark, excrement and a special cement she makes just for this purpose. The cap absorbs moisture, which keeps the eggs from desiccating during the long dry winter. She can lay up to 500 eggs.

If you find egg-filled twigs, you want to prune them off your shrub(s). Either bag them up or burn them – you want to destroy them.

Eggs that are not destroyed overwinter. When April and May roll around the following year, the surviving eggs hatch, and little yellow-brown larvae begin their assault on the plant. The skeletonized leaves they leave behind are pretty easily spotted. You can try to remove the larvae at this stage, but those that survive your mission to end their lives mosey down to the ground after they complete three instars (they shed after each instar, which is a stage of growth). Once in the moist soil below, they pupate.

It isn’t until the adults emerge in late summer to early fall (8-10 weeks after hatching) that we can once more go after their eggs. Eggs are probably the easiest stage of the beetle’s life cycle for us to manage, for they are easily spotted and, because they are non-mobile, they are easily removed.

So, do your part this season. Monitor the viburnums on your property and around your neighborhood. Destroy any VLB eggs, adults and larvae that you find. And continue to do so for the next few years. It is only through constant vigilance that we can try to keep this invasive menace from destroying our beloved Adirondack forest. We want to be sure that thrushes, grouse, deer and bears will be able to hide and eat in the woods for many years to come.

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Ellen Rathbone

Ellen Rathbone is by her own admission a "certified nature nut." She began contributing to the Adirondack Almanack while living in Newcomb, when she was an environmental educator for the Adirondack Park Agency's Visitor Interpretive Centers for nearly ten years.

Ellen graduated from SUNY ESF in 1988 with a BS in forestry and biology and has worked as a naturalist in New York, New Jersey, and Vermont.

In 2010 her work took her to Michigan, where she currently resides and serves as Education Director of the Dahlem Conservancy just outside Jackson, Michigan.

She also writes her own blog about her Michigan adventures.

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