| Pest Alert! The Viburnum Leaf Beetle |
Once again, Whatcom County Master Gardeners are on the frontline of the exotic pest invasion. Recently, a sample of Viburnum came into the MG clinic with the Viburnum Leaf Beetle, Pyrrhalta viburni. This beetle is native to Europe and most likely came to North America on nursery plants in the early 1900’s. It was first found in 1947 in Ontario Canada and later discovered in New York State in 1996. Interestingly the beetle did not occur in very high numbers for many years; only in late 1970’s it became a viable pest problem. In the Eastern US, the beetle has spread to neighboring states, including Maine, New Hampshire, Vermont, New York and a small part of Pennsylvania and Ohio.
In 2001, British Columbian gardeners began to find Viburnum leaf beetles in southern Victoria Island and the Fraser Valley. It appears to be spreading rapidly southward. Our Washington State Department of Agriculture expected the eventual spread of Viburnum leaf beetle into Whatcom County and did a great job in making us aware of this pest.
Description and History Life: The Viburnum leaf beetle overwinters as eggs. Eggs hatch in spring and the small larvae begin to feed on the foliage. The leaf beetle larvae have a characteristic feeding habit of eating the foliage along the leaf veins. After the larvae are finished, all that’s left is the skeletonized leaf. The newly hatched larvae are very small (about 1-2mm long) and greenish-yellow. Small larvae graze on the undersurface of the leaf. As the larvae mature, their bodies become cover in a pattern of dark spots and darken in color. Mature larvae are 10mm long and migrate to the soil to pupate. The pupal stage lasts for about ten days. The span from egg hatch to adult can be as quick as two months.
Beetles emerge from the soil and begin to feed on foliage of Viburnum. Adult beetles chew oblong shot holes into the leaves. Adults not very fancy for an exotic leaf beetle; they are about 5-6mm long, brown with filamentous antennae. Adult beetles can survive until the first frost. During this time, they migrate to new bushes, feed and lay eggs. An adult female can lay up to 500 eggs. The eggs are deposited in rows usually on the new growth stems; they appear as neatly arranged nodules. A hole is dug out from the stem and up to five eggs are deposited. The hole is capped with cement made from plant fiber, spit and a little excrement.
Damage: Viburnum leaf beetles feed on many species of Viburnum in both adult and larval stages. The beetles are very damaging because of this successive feeding by larvae followed by adults; bushes do not have time to re-vegetate between beetle stages. Two or three consecutive years of defoliation can cause significant die-back of the canopy and kill a bush.
The beetles appear to prefer some species and cultivars over others. New York State entomologists are developing a host preference list. Viburnum dentatum (arrowwood viburnums), V. opulus (European cranberrybush), V. opulus var. americana (American cranberrybush), V. rafinesquianum (Rafinesque Viburnum) and V. sargentii (Sargent Viburnum) are the most susceptible to infestations. A complete host list can be found at: http://www.hort.cornell.edu/vlb/suscept.html.
Management: Begin monitoring for eggs after the first frost in fall and winter. Prune out or otherwise physically destroy the eggs. Begin monitoring for larvae when the first leaves begin to form in the spring. Pick off the small larvae as you find them. Continue to physically remove larvae when you regularly inspect bushes. While this management hasn’t been tested, NY entomologists suggest applying a sticky barrier such as Tanglefoot® to the base of the bush stems. They found that the larvae do not drop directly to the soil to pupate but crawl down the stem. The application of Tanglefoot® will be identical to the methods you would use for foiling adult rootweevils. After the adults have emerged, regularly remove and destroy the adults. It is best to remove them in the morning before they become too active.
If you plan on planting Viburnum plants into your landscape, consider using a species or cultivar that shows some resistance. Known resistant species include V. plicatum var. tomentosum (doublefile viburnum), V. carlesii (Koreanspice viburnum), V. burkwoodii (Burkwood viburnum), V. × juddii (Judd viburnum), V. × rhytidiophylloides (lantanaphyllum viburnum), and V. rhytidiophyllum (leatherleaf viburnum).
There are no known natural enemies that are specific to Viburnum leaf beetle. New York has identified some bird species, ladybug larvae, green lacewing larvae and predatory bugs as good general predators of the beetle larvae. Perhaps when the beetles settle into Whatcom County, something will decide to eat them regularly.
WSU does not have a specific pesticide recommendation as of yet; physical controls previously mentioned are the best bet for managing this emerging pest. As we learn more about Viburnum leaf beetle, pesticide recommendations can be made. Entomologists at Cornell are currently researching ‘softer’ insecticides for managing this pest.
But don’t feel helpless as this pest comes into town and munches our Viburnums! For those Master Gardeners that are computer savvy and expert Viburnum-philes, sign up as a “Citizen Scientist” and help to discover the solution to this pest problem. Cornell has organized a citizen science program and has enlisted a number of community members throughout the affected areas to research the Viburnum leaf beetle. Learn more about this great program at: http://www.hort.cornell.edu/vlb/index.html
To reach Todd Murray please call (360) 676-6736 or e-mail.