Management of the Viburnum Leaf Beetle at the Morris Arboretum

Pyrrhalta viburni (Coleoptera: Chrysomelidae), or the viburnum leaf beetle (VLB), is an invasive pest on viburnums in North America, where native species of the plant have little natural resistance. Resistance can be conferred by leaf texture, leaf chemistry, or a wound response that crushes VLB eggs. The beetle does not immediately kill host plants, but repeated defoliation is fatal after several years. Because viburnum is a common forest and landscape plant in the eastern United States, VLB is a serious concern. The Morris Arboretum has a large collection of viburnums, including many native and non-native species. While VLB had already been observed in passing, this project included a thorough baseline survey of VLB damage throughout the Arboretum. Data were collected for the number of twigs infested with VLB, the number of cavities on each twig, and whether a wound response had been produced. This information was compared to existing evaluations of viburnum susceptibility based on defoliation, and previously unlisted species were evaluated. In the future, VLB populations should be managed by annually clipping and destroying infested twigs between October and March. If this window is missed, young larvae can be sprayed with horticultural oil, and adults shaken off plants into soapy water. However, if regional VLB populations become denser in coming years, pest pressure will still continue to increase at the Arboretum, as beetles come from surrounding properties. Therefore, highly susceptible viburnums should be avoided in new plantings, and the understory of natural areas should be diversified. Pest pressure should be reevaluated every 3-5 years, with a repetition of the survey in this project.