by Lindsay Bond Totten
Scripps Howard News Service
For a while I simply ignored it. Perhaps failing to acknowledge its six-legged presence would make it go away. Do gardeners really need another pest to worry about, I asked myself, especially one as pervasive as this bug could turn out to be?
If ignorance is bliss, here's my advice: Try to ignore the viburnum leaf beetle until the moment it arrives on your doorstep, or, rather, at your garden gate.
It won't wait for an invitation, and, once inside, it won't leave. So enjoy your ignorance while you can, for this unwelcome garden guest is most certainly on its way.
The viburnum leaf beetle is indigenous to Europe but entered the United States from Canada about five years ago. The discovery occurred in upstate New York, across the border from Ontario's Niagara Peninsula. The beetle has since spread southward into 26 New York counties and is now nearing the Pennsylvania line.
It probably will reach Pennsylvania in two more years. Midwestern gardeners can probably relax, at least for a while.
Because this pest is prevalent throughout the European continent, entomologists fear it will adapt easily to similar climates here, wherever viburnums are grown. The viburnum leaf beetle could have an impact equal to that of the Japanese beetle.
Darn it, anyway. Why couldn't it have been Daphne, or Kerria, or Weigela this destructive pest liked? What a group of plants as widespread and indispensable as viburnum? With over a hundred excellent varieties available to us, virtually every garden within the viburnum's range is likely to have at least one specimen.
From a bug's perspective, I guess the answer is pretty obvious.
Signs of feeding appear in spring, when the tiny caterpillar-like larvae hatch from eggs overwintered in the twigs. The worms are greenish-brown in color, with rows of black dots down the back. When mature, they reach about a half-inch in length.
So nondescript are the larvae, and, later, the small brownish-black adult beetles, that gardeners may have to rely on the insect's feeding habits to recognize it.
Look for holes in viburnum leaves, then turn a damaged leaf over. There you'll find the larvae, which are gregarious, so there will probably be several caterpillars feeding together.
If the infestation is heavy, viburnum leaves can be riddled with holes, practically skeletons by the time the voracious larvae are done.
After a month of serious munching, the larvae drop to the ground and pupate, only to emerge two weeks later as adult beetles. These, too, will descend on your shrubs, taking up where the others left off, eating and mating till frost.
Viburnums can be killed if plants are completely defoliated, so gardeners don't dare ignore this incorrigible pest once it sets foot in the garden.
Throughout late summer and fall, female beetles lay their eggs in viburnum twigs. Neat rows of small bumps, usually on terminal shoots, give the next generation's hiding place away.
Scientists from Cornell University are taking the lead in viburnum leaf beetle research. While they look for parasites and predators to thwart this nasty bug, they recommend relying on conventional control measures in the meantime.
Insecticides such as Pyrethrum will kill both larvae and adults, but sprays must be repeated several times throughout the season to account for the insect's extraordinarily long feeding period. Be sure to treat the undersides of leaves for maximum protection.
While plants are dormant, carefully examine viburnum twigs. Prune out shoots that harbor the eggs, collecting these for the trash instead of letting them fall to the ground, where the eggs would still hatch in spring.
A bright spot, if one could possibly be found, is that the viburnum leaf beetle prefers some viburnums more than others, and a few varieties not at all.
My favorite, the Prague Viburnum (V. pragense), is somewhere in the medium range, along with Doublefile Viburnum (V. plicatum), Blackhaw Viburnum (V. prunifolium), Wayfaring tree (V. lantana), and Linden Viburnum (V. dilitatum).
Preferred by the beetle, and thus most likely to be injured by heavy feeding, are American and European Cranberrybush Viburnums (V. trilobum and V. opulus), Sargent viburnum (V. sargentii), and Arrowwood Viburnum (V. dentatum). Varieties of these species, such as the wonderful Chicago Lustre Viburnum (V. dentatum Chicago Lustre) are also susceptible.
Fortunately, several of the best, and most popular, viburnums are resistant to beetle attack (we'll find just how resistant as populations build): Burkwood Viburnum (V. burkwoodii); closely related Koreanspice, Judd, and Carlcephalum Viburnums (V. carlesii, V. juddii, and V. carlcephalum); Leatherleaf Viburnum (V. rhytidophyllum) and its hybrid V. rhytidophylloides, Tea Viburnum (V. setigerum); and Siebold Viburnum (V. sieboldii).
(Lindsay Bond Totten, a horticulturist, writes about gardening for Scripps Howard News Service.)
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