Biological control of hemlock woolly adelgid in North America: History, status, and outlook

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The hemlock woolly adelgid (HWA, Adelges tsugae, Hemiptera: Adelgidae) is an invasive insect that threatens the ability to maintain eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana) as ecologically functional components of eastern North American forests. Since the early 1990 s, a classical biological control program for HWA has been pursued using insect predators from regions of Asia and western North America where the adelgid is native. Early efforts to establish ladybird beetles (Coleoptera: Coccinellidae) resulted in the establishment of Sasajiscymnus tsugae from Japan, but consistently poor field recovery of this species suggests it has not been an effective biocontrol agent in the introduced range. Two additional ladybird beetles, Scymnus ningshanensis Yu & Yao, and S. camptodromus Yu & Liu, were released but did not establish. Subsequent introduction, widespread establishment and spread of Laricobius nigrinus and La. osakensis (Coleoptera: Derodontidae) has provided substantial predation on winter stages of HWA, but has not provided overall regulation of adelgid populations below levels that cause hemlock shoot growth loss and mortality. Recently, efforts have focused on prey-specific genetic lineages of Leucotaraxis argenticollis and Le. piniperda (Diptera: Chamaemyiidae), which co-occur with La. nigrinus in their native range of the Pacific Northwest and show promise for eventual establishment in eastern North America. This review provides a brief introduction on the ecology, evolution, and population dynamics of HWA and its hosts, detailed coverage of the predators that have been (and are being) evaluated as biological control agents, and a discussion of the overall biological program with considerations for its future implementation.