Plant defense elicitors fail to protect Viburnum dentatum from herbivory by viburnum leaf beetle (Coleoptera: Chrysomelidae)

Pyrrhalta viburni (Paykull) (Coleoptera: Chrysomelidae), a new landscape pest in the United States, feeds in both the larval and adult stages on foliage of plants in the genus Viburnum. A field trial was conducted from 2004 to 2006 to examine the impact of several elicitors of plant defense on ability of arrowwood viburnum (Viburnum dentatum L.) to resist attack by P. viburni in both larval and adult stages. The treatments included jasmonic acid (JA), harpin, and paclobutrazol. For comparison, imidacloprid and untreated controls were included in the trial. The soil-applied treatments (paclobutrazol and imidacloprid) were applied once during the trial (spring 2004), and the foliarly applied treatments (JA and harpin) were applied each spring. Herbivory by viburnum leaf beetle larvae and adults was measured yearly in spring and summer, respectively, and plant height was recorded at the end of each growing season. The only treatment that decreased feeding by viburnum leaf beetle was imidacloprid; these plants were virtually untouched throughout the duration of the trial. Plants treated with JA and harpin actually suffered greater feeding damage at the end of the second growing season; other than this observation, the elicitors had no impact on viburnum leaf beetle. As expected, plant height was decreased for the shrubs treated with paclobutrazol, a plant growth regulator, and unaffected by JA and harpin. Plant height was increased for the shrubs treated with imidacloprid. These shrubs also seemed to be protected from viburnum leaf beetle after residues dropped below lethal levels.