I was folding my lace tablecloth that I air-dried outside after washing out a stain as a result of coffee spilled during a weekend get together. And as I was bringing two ends of the fabric neatly together, the sun shone through my back patio doors and beamed through the holes in the lace.
If you've ever seen these neat pinpoints of light come through the intricate shaped holes in this delicate fabric, then you have an idea what viburnum leaf beetle damage looks like on the shrub's velvety emerald leaves. The reason it's on my mind lately is because the pest is particularly bad this year, according to Greg Mazur, one of our many arboricultural gurus (or officially, technical service advisors) at The Davey Institute.
The term used to describe this damage done by the beetle larvae in spring is skeletonized. Then irregular holes are chewed into the leaves by the beetle adults in summer. Unfortunately, branch dieback follows the rapid defoliation. In one to three years, viburnums are toast.
And Mazur now realizes he was being generous when he gave viburnums one to three years against this pest. For he knows all too well the swift damage they cause.
He knew the beetle was coming. It is an invasive species that came from Europe to North America in the 1940s and then into the U.S. in the mid-1990s. Mazur has native viburnum growing in the woods behind his home. In 2008, as he was mowing the lawn and spreading the grass clippings in the woods, he noticed the viburnums were being nibbled on. "But I didn't think too much of it," he says. "I thought I'd get around to spraying them later in the summer."
Then by the end of summer, every leaf on every viburnum was skeletonized. "The viburnum stuck out because they were all bare twigs," he says. "So I said, 'I blew my chance but I'll get them next year.'"
But in the spring of 2009, the viburnum didn't even leaf out. Mazur's only leftover option was to get out a pair of loppers and cut them down.
And our expert scientist isn't alone. He remembers a Davey client calling last year. "She said her viburnums looked OK on Monday," he says. "But by the Friday before Memorial Day - five days later - they were nearly dead."
How can the beetle cause such swift destruction? "Because there are no natural predators, viruses or bacteria in U.S. to slow down this introduced species," Mazur explains.
The pest is moving throughout the U.S. rapidly. "It's like when you throw a stone in a pond and the ripples move outward from the point of impact," Mazur says. "The beetle will continue to spread as long as there are viburnums in its path."
Though scientists are still trying to understand the true extent of this species, they are getting good control of the pest in mid-May with an insecticide application. Then when the adults emerge, a follow-up application can prevent them from laying eggs on the branches. As always, strong cultural practices are important, and your professional arborist can take good care of your shrubs with proper irrigation, mulching and fertilization to keep them strong enough to resist the beetle.
Holes in your viburnums? Call your local arborist today to save your plants. There's no second chance with this pest. "If you do nothing this year, you might not have to worry about it," Mazur says, chuckling softly remembering his own backyard, "because you may not have any viburnums left."
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