Native Plant Seminar Discussions: Viburnum Beetle Alert…

Healthy Viburnum trilobum foliage and flower buds

Over the weekend, I presented a talk on native plants for ornamental landscape use to a fantastic group of New England gardeners, and some very good questions were raised about alien pests and diseases of North American native plants. One of the gardeners participating in the workshop mentioned die-back on the cranberrybush viburnum, (V. trilobum, as pictured above), in her yard. Without looking at the actual shrubs, it is impossible to know whether the die-back is being caused by viburnum beetles or by late frost or some other defoliating disease. However, at this time of year, I am concerned about the appearance of viburnum beetle larvae. It is very important for gardeners in the northern/north central United States and Canada to be on the lookout for small, greenish-yellow, caterpillar-like larvae on viburnum. If you grow shrubs in this genus -particularly cranberrybush and arrowwood viburnum- and you begin to notice skeletonization of foliage, check the undersides of the leaves -using a magnifying glass if necessary- for beetle larvae, (see Cornell University photos below). Spraying larvae directly with insecticidal soap at regular intervals will help contain the infestation. But be sure to avoid spraying beneficial insects, such as ladybugs, when applying any pesticide, including organic oils and/or soaps.

Photo Kent Loeffler/via Cornell University – click link here for CU article

The viburnum beetle, (Pyrrhalta viburni) -shown above in its spring larval stage- is a highly destructive, non-native beetle introduced to North America. In North America, the beetle was originally spotted in Canada, and it has been moving southward ever since. The beetle was first seen in the United States in 1994 along the New York State, US/Canada line. This voracious insect has been slowly, but steadily expanding its territory in the Northeast and the Pacific Northwest, and has now been spotted on viburnum, (particularly the native arrowood viburnum, V.dentatum and cranberrybush viburnum V. trilobum), in a number of northern states in the US. Both larvae and adult viburnum beetles feed on plant leaves, robbing the shrub’s ability to photosynthesize. Unchecked, this insect has the potential to destroy the native populations of a number of viburnum species throughout the North American continent. Arrowwood viburnum, (V. dentatum), and cranberrybush, (V. trilobum), fruit is an important source of food to North American songbirds. Horticulturalists hope that over time, our native viburnum will develop resistance to these insect invaders, though widespread destruction of wild viburnum populations is feared.

Some viburnum appear to be more resistant than others to this invasive pest. However, the shrubs most vulnerable to destruction are viburnum with smooth, broad leaves, including many natives. For more information on this beetle, including varieties most susceptible, and other ways to spot and manage the pests, visit the Cornell University “Citizen Science” Viburnum Beetle Information Page, linked here. If you suspect viburnum beetle in your area, be sure to report the sighting to your local university extension service, in order to help with tracking and management of these insect invaders.

No viburnum beetles have been spotted on my compact cranberrybush, (V. trilobum ‘Bailey Compact’), yet this spring, though the insect infestation is here in North America to stay. All gardeners should be on the look-out for this destructive pest, so that it can be properly managed.

I would like to extend a special thank you to this weekend’s seminar participants at Walker Farm for their great native plant questions. I will be posting more articles and information from the seminar over the coming weeks. Please scroll down the blog to find additional native plant posts, information and resources…

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Top photo is copyright Kent Loeffler via Cornell University Online. All other photos are copyright TGE.

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