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Saturday, February 09, 2002

Q. I recently heard about a new insect in our area that attacks viburnums. I am concerned because I have an extensive planting -- a collection, really -- of many species of viburnums. Does it attack all viburnums? Does it attack any other plants? What does it look like? How do you control it?

A. You are probably referring to the viburnum leaf beetle (Pyrrhalta viburni). These European natives have not been identified in the Pittsburgh area yet, but it is likely they will arrive here eventually. Viburnum leaf beetles are widespread throughout continental Europe. They were first identified in Canada in the late 1970s. They have now been found in southern Maine, western New York, Erie County in Pennsylvania and northern Vermont.

	Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington St., Pittsburgh 15208. Due to volume of mail, personal replies cannot be provided.

According to entomologist Dr. Paul Weston of Cornell University, viburnum leaf beetles are known to infest only plants in the Viburnum genus. The good news is that some species of viburnums are more susceptible to attack than others. Weston's list of susceptible species includes arrowwood (V. dentatum); European cranberrybush viburnum (V. opulus); Rafinesque viburnum (V. rafinesquianum); Sargent viburnum (V. sargentii); and American cranberrybush viburnum (V. trilobum). Left untreated, viburnum leaf beetles can kill susceptible species in two or three years by repeatedly defoliating them.

Weston also lists moderately susceptible species, including mapleleaf viburnum (V. acerifolium); linden viburnum (V. dilatatum); wayfaring tree (V. lantana); nannyberry (V. lentago); doublefile viburnum (V. plicatum x tomentosum); Prague viburnum (V. x pragense); blackhaw viburnum (V. prunifolium); and Wright viburnum (V. wrightii). The beetles feed on these species of viburnum, but the plants seem to survive even large populations of them.

Finally, Weston lists species that are resistant to the beetle. They include Burkwood viburnum (V. x burkwoodii); fragrant viburnum (V. x carlcephalum); Koreanspice viburnum (V. carlesii); Judd viburnum (V. x juddii); lantanaphyllum viburnum (V. x rhytidophylloides); leatherleaf viburnum (V. rhytidophyllum); tea viburnum (V. setigerum) and Siebold viburnum (V. sieboldii). Species with thicker leaves and Koreanspice viburnum parentage seem most resistant to the beetles.

Adult viburnum leaf beetles are very small, roughly the size of a matchstick head. They are yellowish-brown to light brown in color. They resemble their close relatives, the elm leaf beetle, but are smaller and less brightly colored. Young larvae are off-white to greenish-yellow and mature larvae develop uniform black spots on their bodies.

Viburnum leaf beetles overwinter as eggs laid in twigs of host plants. Larvae hatch in May and feed voraciously on the underside of the newly expanding leaves. They skeletonize the foliage, leaving only the midrib and major veins intact. Mature larvae drop to the ground to pupate early to mid-June. Adults emerge mid- to late July and resume feeding on viburnum foliage. Plants often put on a second flush of growth to compensate for the foliage destroyed by the larvae, only to have it devoured by the adults. Adults feed, mate and lay eggs until the first frost. Adult females chew holes in small branches and twigs to create cavities where they lay their eggs. Afterward, they create a "cap" made up of excrement, chewed bark and shredded wood to protect eggs and maintain a humid environment to ensure their survival.

I understand your concern about these insects given your extensive collection of viburnums, but remember that they have not arrived in our area at this point. Scout your plants regularly, but do not jump the gun on making insecticide applications until it is necessary. An effective, non-chemical means of control is to prune infested twigs after egg-laying is finished in the fall. Burn infested twigs to avoid moving this pest into new areas. Hand destruction of larvae and adults can provide effective control in small plantings, but would be impractical with large ones.

Since this is a new insect in Pennsylvania, not many insecticides are labeled to control viburnum leaf beetle. That will change as the pests move farther into the state. According to research at Cornell, imidacloprid (Merit) provides the best control. It is sold to home gardeners under the name of Bayer Advanced Tree and Shrub Insect Control. While a number of broad-spectrum insecticides are effective against viburnum leaf beetles, they are not labeled for the pest. They also have the undesirable side effect of killing beneficial insects that may help control them.