Incidence, Abundance, and Severity of Mites on Hemlocks Following Aplications of Imacloprid

Hemlock woolly adelgid (Adelges tsugae), is one the most serious and damaging insect pest of hemlocks (Tsuga spp.) in the eastern United States. The systemic insecticide imidacloprid has gained widespread acceptance and use by arborists to control hemlock woolly adelgid. In a residential landscape, we found populations of spruce spider mites and hemlock rust mites; their injury was found to be greater on hemlocks treated with imidacloprid than on untreated trees. A survey of hemlocks in gardens, parks, and residential landscapes revealed that hemlocks treated with imidacloprid were more likely to be infested with spider mites but not rust mites. Moreover, terminals on imidacloprid- treated hemlocks were approximately nine times more likely to have severe needle damage than untreated trees. Arborists and landscape managers applying imidacloprid to hemlocks should carefully monitor mite populations on treated trees and be prepared to intervene should mite populations increase. This study serves as an example of how a pesticide application for a primary pest, hemlock woolly adelgid, may contribute to the development of a secondary pestin this case, mites.