Optimizing dosage and preventing leaching of imidacloprid for management of hemlock woolly adelgid in forests

Hemlock woolly adelgid (Adelges tsugae Annand) threatens native hemlock (Tsuga canadensis and T. caroliniana) in the eastern United States. Imidacloprid used as a soil-applied systemic insecticide is effective against A. tsugae in natural forests and in landscapes. Non-target impacts are a special concern because hemlock is ecologically important; often growing next to streams which contain aquatic species that are sensitive to imidacloprid, some of which are protected or endangered fauna. Environmental risk can be mitigated by determining the minimum effective dosage in forests. Adelgid populations responded to imidacloprid dosage with approximately a linear relationship between the percent population reduction (probability scale) vs. log of dosage; 50% reduction in populations could be achieved with 0.15 g per 2.5 cm of trunk diameter at breast height (dbh), or 10% of the maximum labeled dosage. However, effectiveness was found to vary based on dbh; the dosage predicted to give ∼90% reductions in adelgid populations is given by the equation log(dosage) = 0.0153 × dbh − 1.074, where the dosage is g imidacloprid per 2.5 cm of trunk dbh, and dbh is measured as cm. For trees less than 82 cm dbh, these dosages are less than the typical 1.5 g imidacloprid per 2.5 cm dbh currently used. Formulation into tablets rather than as a wettable powder reduced the maximum imidacloprid concentration in soil solution at 5 cm depth from 80 to 1.6 ppm in columns constructed with the organic soil found beneath forest hemlocks. A combination of optimum dosing of trees and adoption of the tablet formulation when treating trees in sensitive habitats should minimize the risk of contaminating aquatic resources with imidacloprid.