Biological Control for Hemlock Woolly Adelgid: Where Do We Stand?
By Molly Darr, Ph.D., and David Coyle, Ph.D.
Hemlock woolly adelgid (Adelges tsugae) is a highly destructive invasive insect pest of hemlock trees in both urban and forested environments in eastern North America. Since its introduction more than 50 years ago, this aphid-like pest has caused significant mortality in eastern hemlock (Tsuga canadensis) and Carolina hemlock (T. caroliniana) in North America. This extensive loss of hemlock driven by the hemlock woolly adelgid (HWA) is having devastating repercussions on biodiversity and ecological functions, permanently changing the composition of eastern forests.
HWA has a complex lifecycle, is a prolific reproducer, and is difficult to control. For this reason, a variety of management strategies have been explored to bring HWA populations below the economic threshold. Options for HWA control include insecticides, silvicultural strategies, and biological control. Biological control manages invasive pest populations through the introduction of a natural enemy or predator.
Introducing a non-native species to a new environment is a complicated process, and biological control agents must be rigorously assessed in quarantine to ensure host specificity (i.e., that they only eat what they’re supposed to) and establishment potential (that they can reproduce and maintain a population) before release. Once a biological control agent is approved, large numbers are released into the invasive pest’s new range.
Biological control often acts as a secondary form of control to chemical and silvicultural management when these options aren’t feasible in an urban or forest setting. Two beetles that are native to North America, Laricobius nigrinus and Laricobius rubidus, are predators of adelgids in the western and eastern United States, respectively. Laricobis osakensis is native to Japan, where it feeds on HWA in its native range. Both L. nigrinus and L. osakensis were approved for release in the eastern United States, and over 400,000 beetles were released in forest settings from Georgia to Maine. Establishment and recoveries have been made in every release state in forest settings where these beetles were released, but it is unknown whether Laricobius spp. have become established at non-release sites within the urban environment.
Led by Ph.D. candidate Jeremiah Foley, a group of researchers at Virginia Tech surveyed for and reported L. nigrinus within urban environments outside of known release locations. Their findings were reported last week in the open-access Journal of Integrated Pest Management.
Blacksburg and Radford, two localities in southwest Virginia, were selected due to their proximity to known L. nigrinus release sites. Each town was divided into 0.4 square-kilometer sampling grids and surveyed in spring 2019. Laricobius adults were sampled using the beat sheet method. In a bit of good news, Laricobius spp. were present in 100 percent of the grids in Blacksburg and 75 percent of the grids in Radford. Of the recovered beetles, 98 percent were the introduced L. nigrinus, and 2 percent were native L. rubidus.
While L. nigrinus was commonly collected in urban environments during scouting missions in its native range of the Pacific Northwest, this is the first documentation of this biocontrol agent in an urban setting in the eastern United States. This means L. nigrinus can likely establish populations in urban areas within its newly introduced range—great news for biocontrol of HWA. Laricobius nigrinus beetles have been reared and released since 2003 to supply public land managers with biological control agents as an additional management option. When chemical or silvicultural treatments aren’t feasible due to ecological or economical limitations, this research shows that biological control can serve as an ecologically friendly, sustainable management option in the ongoing effort to preserve these keystone hemlock species.
Journal of Integrated Pest Management
Molly Darr, Ph.D., is a post-doctoral associate and David Coyle, Ph.D., is an assistant professor, both in the Department of Forestry and Environmental Conservation at Clemson University. Twitter: @DarrMolly and @drdavecoyle. Email: email@example.com and firstname.lastname@example.org.