Treatment Strategies Using Imidacloprid in Hemlock Woolly Adelgid (Adelges tsugae Annand) Infested Eastern Hemlock (Tsuga canadensis Carrière) Trees

Abstract

Due to the widespread establishment of hemlock woolly adelgid (Adelges tsugae Annand) (HWA) across the range of eastern hemlock (Tsuga canadensis Carriere), woodland trees may be infested for many years before treatment is made. Symptoms of prolonged infestation include extensive dieback and thinned canopies. Imidacloprid, a systemic neonicotinoid insecticide, is a useful and effective tool to manage HWA. In this study, mature, large diameter trees in poor condition were treated with imidacloprid. Trees were treated once by trunk and/or soil injection in Asheville, North Carolina, U.S. Following application, changes in tree growth, HWA density and imidacloprid residues were measured for three years. Trees treated with imidacloprid recovered, whereas the untreated trees continued to struggle. Trees injected with imidacloprid accumulated compound in the canopy, facilitating refoliation and the imidacloprid persisted for three years. This extended activity of trunk-injected imidacloprid was attributed in part to slow upward movement through the restrictive tracheid vascular system and to perennial needle retention. The imidacloprid soil injection was slower to act systemically, but has potential for longer-term activity. Researchers suggest the combination of tree and soil injection for immediate and long-term (4+ years) activity as an effective and economic strategy to protect high-value trees.