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

Hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), is an invasive pest causing significant mortality to eastern and Carolina hemlock in eastern North America. Since 2003, management of HWA has included targeted release of the HWA predator Laricobius nigrinus Fender (Coleoptera: Derodontidae), native to western North America. Establishment of L. nigrinus at release sites is well documented, but investigations of its impact on HWA populations have been limited. A four-year (2014-2018), two-phase study using predator exclusion cages to assess the impact of L. nigrinus on HWA was conducted at nine previous release sites in the eastern United States. Significantly more HWA sistens ovisacs were disturbed on no-cage and open-cage branches than on caged branches where predators were excluded. Mean disturbance levels on cage, no-cage and open-cage branches was 8, 38, and 27 percent, respectively. Seven of nine sites had a mean HWA ovisac disturbance greater than 50% for at least one year. Winter temperatures were also a significant factor in overall mortality of the sistens generation with a mean of 46% on study branches. Six of nine sites had a mean overall mortality (winter mortality and predation) greater than 80% for at least one year. Larvae of Laricobius spp. were recovered at all sites during this study. Sequencing of the COI gene from recoveries in Phase One (2015 and 2016) indicated that 88% were L. nigrinus and 12% were L. rubidus LeConte. Microsatellite analysis performed during Phase Two (2017 and 2018) indicated that approximately 97% of larval recoveries were L. nigrinus, 2% were hybrids of L. nigrinus and L. rubidus, and 1% were L. rubidus. Results of this study suggest that L. nigrinus can significantly impact the HWA sistens generation ovisacs and continued investment in the use of this species as a biological control is recommended.