Evaluating the benefits and risks of biological control agents for managing hemlock woolly adelgid in North America

The hemlock woolly adelgid (HWA), Adelges tsugae Annand, is an invasive pest devastating hemlock ( Tsuga ) forests in eastern North America. Importation biological control efforts have involved the introduction of insect predators from Asia and western North America, where HWA is native. Releases of Coccinellid ( Sasajscymnus tsugae , Scymnus ningshanensis ) and Derodontid ( Laricobius nigrinus , L. osakensis ) beetles have provided limited control of HWA, while more recent efforts with Chamaemyiid flies ( Leucotaraxis argenticollis , Le. piniperda ) show potential but have not yet established in eastern regions. This study conducts a benefit-risk analysis using native biodiversity as the common metric to assess the ecological impacts of these biological control agents (BCAs). We calculate a benefit-risk index (BRI) to quantify the benefits and risks of introduced BCAs. In developing the BRI, we consider four categories: (1) invasive HWA populations, (2) the introduced BCA species, (3) native species benefiting from HWA suppression, and (4) native species potentially at risk from BCAs. A positive BRI indicates benefits outweigh risks, while a negative BRI suggests the opposite. Our findings show that BCAs have not as yet resulted in a net ecological benefit under current suppression levels, but stacking multiple BCAs could lead to a positive BRI under estimated suppression levels. Though our results are sensitive to the perceived conservation value of eastern hemlock. We discuss how BRI could be used in the selection of suitable BCAs for long-term management of HWA, supporting sustainable hemlock conservation in North America.