D3449: Radial growth of eastern hemlocks following systemic insecticide treatment to control hemlock woolly adelgid in recently infested stands in Michigan

Hemlock woolly adelgid (HWA) (Adelges tsugae Annand) (Hemiptera: Adelgidae), an invasive sap-feeder, has killed thousands of hemlocks ((Tsugae canadensis) in the eastern US.  Localized HWA infestations, first detected in Michigan in 2015, threaten >170 million hemlocks, which provide essential wildlife habitat and drive ecological processes in upland and riparian forests. We launched Study 1 in 2016/2017 (70 trees) and Study 2 in 2018/2019 (70 additional trees) to evaluate HWA impacts and efficacy of a dinotefuran basal trunk spray, imidacloprid applied via a soil drench, slow-release tablets, a trunk spray or trunk injection, and a dino+imi trunk spray. Canopy condition of trees was ranked annually from 2018 to 2023. Two increment cores were collected from all treated and control hemlocks in 2022 (Study 1) or 2023 (Study 2), along with ten Quercus rubra trees, for dendrochronological analysis. Radial growth of untreated hemlocks dropped substantially from 2018-2022, reflecting HWA impacts, while radial growth of red oaks on the same site remained steady or increased during this period. Hemlock radial growth from 2018 to 2022 and annual growth increments were higher for trees treated with a dinotefuran or dino+imi trunk spray or with an imidacloprid soil drench than trees treated with imidacloprid applied as slow-release tablets, a trunk spray or trunk injection. Growth of untreated hemlocks was 3-4 times lower than growth of trees treated with a dinotefuran or dino+imi trunk spray.  Dinotefuran is less persistent than imidacloprid, but its high solubility and relatively rapid translocation better facilitated recovery of infested trees.