Establishment and Distribution of Laricobius spp. (Coleoptera: Derodontidae), a Predator of Hemlock Woolly Adelgid, Within the Urban Environment in Two Localities in Southwest Virginia

Abstract

Hemlock woolly adelgid (HWA), Adelges tsugae Annand, is a highly destructive non-native pest lethal to eastern hemlock, Tsuga canadensis (L.) Carrière, and Carolina hemlock, T. caroliniana Engelmann. Fifty years following the first observation of HWA in eastern North America, a predatory beetle, Laricobius nigrinus Fender, was evaluated, approved, and released as a biological control agent. Efforts were made to mass rear L. nigrinus with the goal of redistributing the beetles on public lands, typically in forest ecosystems, as a secondary option to silvicultural and chemical controls. The majority of L. nigrinus releases has occurred on public lands. Herein, we report the observation of L. nigrinus within the urban environment outside of known release locations. Two towns, Blacksburg and Radford, were divided into 0.40 km2 grids. A total of 27 and 19 grids were randomly selected from each town, respectively. Hemlocks were present in 44 and 42% of the grids surveyed in Blacksburg and Radford, respectively. In Blacksburg and Radford, 86 and 100% of the grids with hemlocks were infested with HWA, and of those infested hemlocks, Laricobius spp. was present in 100 and 75% of grids, respectively. A total of 154 Laricobius spp. (98% L. nigrinus and 2% Laricobius rubidus) adults were collected between each town. While it is unclear the level of control L. nigrinus has on reducing HWA's impact, the establishment of this biological control agent in the urban environment is an additional level of predation, that would otherwise not be present, for homeowners with HWA-infested trees.

Top-down regulation of hemlock woolly adelgid (Adelges tsugae) in its native range in the Pacific Northwest of North America

“…nigrinus that feeds on the overwintering sistens generation on HWA. That species has been widely established in the eastern U.S. (Mausel et al 2010;Foley et al 2019;Jubb et al 2021) and has caused significant mortality to HWA ovisacs (Jubb et al 2020), but the mortality is insufficient to regulate densities of HWA (Crandall et al 2020). Our results imply 16 that a combination of summer-active and winter-active predators will be required to suppress densities of HWA in eastern North America.…”

“…Given the economic and ecological importance of hemlock trees, the USDA Forest Service has devoted significant funding to the importation, mass rearing, and release in eastern North America of various insect predators of the Japanese HWA lineage. Some of these predators, such as Laricobius nigrinus Fender (Coleoptera: Derodontidae) from the Pacific Northwest, have established self-sustaining populations and are spreading from many release locations (Mausel et al 2010;Foley et al 2019;Jubb et al 2021). There has been very limited success with these introductions in terms of reducing HWA populations despite high predation rates (see Crandall et al 2020;Jubb et al 2020), suggesting that bottom-up forces might also be important in determining HWA abundance in its native and introduced ranges.…”

Subterranean Survivorship and Seasonal Emergence of Laricobius spp. (Coleoptera: Derodontidae), Biological Control Agents for the Hemlock Woolly Adelgid

“…The proportions of Laricobius spp. present from field collections showed a proportion of L. nigrinus and L. rubidus at 90 and 10%, respectively, in 2020 (n = 438) Foley et al (2019),. using the same collection location as this study (Blacksburg, VA), documented the proportion of Laricobius spp.…”

“…Deployment of L. nigrinus and L. osakensis for biological control of HWA has occurred through laboratory mass production efforts by universities and governmental agencies (Foley et al 2021), and through field collection and redistribution of L. nigrinus (McDonald et al 2011). Laricobius nigrinus has been released throughout most of the HWA-infested range of eastern hemlock and continues to spread from those original release sites to new locations (Toland et al 2018, Foley et al 2019, Virginia Tech 2019. The mass-production and redistribution efforts for L. nigrinus over the past 17 years has played a pivotal role in establishing this species in the eastern United States.…”

Impact of the introduced predator, Laricobius nigrinus, on ovisacs of the overwintering generation of hemlock woolly adelgid in the eastern United States

“…The first operational releases of the coastal strain of L. nigrinus began in 2003 and since then, over 400,000 beetles have been released from field and laboratory sources (Virginia Tech, 2019). The ability of L. nigrinus to successfully establish and disperse from select release sites is well documented (Davis et al, 2012;Foley et al, 2019;Mausel et al, 2010), but efforts to characterize the impact of L. nigrinus on HWA populations have been few and limited in spatial scope (Mausel et al, 2008;Mayfield et al, 2015). To help inform future management decisions regarding HWA biological control, this study evaluated the impact of L. nigrinus on the HWA sistens generation and their ovisacs at several sites in the eastern U.S. where the predator has been established for multiple years.…”