Management strategies to reduce hemlock woolly adelgid (Hemiptera: Adelgidae) damage

Abstract

The goals of this work were to improve control of hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae). The first project focused on manipulating tree health and adelgid populations using low rates of imidacloprid insecticide and fertilizer to provide an adequate, uninterrupted supply of high quality adelgids to support long-term predator population growth. After two years, more ovisacs and eggs were found on trees that received imidacloprid in the order 0% > 10% > 25%. Fertilized trees had greater adelgid fecundity which was positively correlated with total foliar N in both winter generations. Trees treated with low rates of imidacloprid were healthier than untreated trees, but only trees treated with 10% of the recommended rate of imidacloprid and fertilizer had sufficient adelgids to possibly sustain predators over extended periods of time. The second project looked at how A. tsugae is distributed within hemlock trees and how their distribution changes over time. We found that adelgid density tended to be high in the upper tree crown after multiple generation sampling. The third study evaluated host tree resistance to hemlock woolly adelgid among various hemlock species, whether or not fertilizers affected resistance, and to determine whether fertilizer and host tree species affected prey quality. Adelgids were denser on Tsuga mertensiana (Bong.) Carrière than on other hemlock species (T. canadensis, T. heterophylla (Raf.) Sarg. or T. chinensis (Franch.) E. Pritz). Application of fertilizer did not increase adelgid density on reportedly resistant T. heterophylla; density was higher on unfertilized plants. The fourth project focused on evaluating eight insecticides for controlling hemlock woolly adelgid on eastern hemlock. Although dinotefuran-treated trees demonstrated an average range of 63-100% adelgid suppression, residual activity diminished 26 months after spring application as adelgids re-colonized treated trees. Clothianidin and imidacloprid were slower acting and provided more long-term adelgid suppression. The fifth study looked at how odors of T. canadensis branches affect the behavior of the adelgid predators, Laricobius nigrinus Fender (Derodontidae) and Sasajiscymnus tsugae (Sasaji & McClure) (Coccinellidae). Neither S. tsugae nor L. nigrinus demonstrated attraction to T. canadensis branches in Y-tube olfactometer studies.