Imidacloprid Treatments to Combat Hemlock Woolly Adelgid: A Metabolite Enhances Long-Term Persistence and Efficacy

Imidacloprid has been widely used in IPM programs against hemlock woolly adelgid (HWA), Adelges tsugae (Annand) (Hemiptera: Adelgidae).  Olefin, a toxic metabolite of imidacloprid, is persistent in eastern hemlock, Tsuga canadensis (L.) Carriére.  A retrospective study investigated the longevity and efficacy of imidacloprid treatments four to seven years after a single treatment and in different size classes of hemlock, focusing on the persistence of olefin.  Branchlets (0.5m) were collected from four size classes of hemlocks that received imidacloprid basal drench treatments four to seven years before sampling.  Olefin concentrations in foliage were determined by liquid chromatography-mass spectrometry, and HWA were counted on each branchlet. Chemical data were analyzed using a generalized linear model (p<0.05), and HWA data were analyzed by a zero inflated binomial model (p<0.05). Olefin was still present 4-7 years post-treatment and in all size classes. HWA populations were still suppressed.  Average olefin concentrations are greater than the LC₅₀ (i.e., the lethal concentration to kill 50% of the population) for HWA four years after treatment.  Higher overall concentrations of olefin were present in larger hemlocks.  Knowledge of the persistence of olefin will assist forest and land managers in future development and refinement of their HWA IPM programs.