Distinguishing adult Laricobius osakensis Montgomery & Shiyake, 2011 (Coleoptera: Derodontidae) from Laricobius nigrinus Fender, 1945 and Laricobius rubidus LeConte, 1861 using pronotal morphology

Abstract

of the family Adelgidae (Franz 1958, Clark & Brown 1960, Leschen 2011, Montgomery et al. 2011). Two Laricobius species have been utilized as biological control agents for the management of hemlock woolly adelgid (HWA), Adelges tsugae Annand, 1928 (Hemiptera: Adelgidae). HWA is native to Mainland China, Japan, Taiwan, and western North America but was inadvertently introduced into the eastern United States from Japan (Havill et al. 2006) with its first detection being in Richmond, Virginia in 1951 (Gouger 1971, Stoetzel 2002). HWA causes significant damage and mortality to two native hemlock species in its introduced range: Tsuga canadensis (L.) Carriére (Pinaceae) and Tsuga caroliniana Engelmann. Operational releases of Laricobius nigrinus Fender, 1945 (Coleoptera: Derodontidae), a western North American native, were initiated in the eastern U.S. in 2003 as a part of a biological control program implemented by the United States Department of Agriculture, Forest Service (Mausel et al. 2010, Onken & Reardon 2011). This species has been highly effective at establishing and dispersing from original release sites (Mausel et al. 2010, Davis et al. 2012, Foley et al. 2019, Jubb et al. 2021), and has exhibited high rates of impact on the overwintering sistens generation of HWA (Mayfield et al. 2015, Jubb et al. 2020). After years of focused effort on the production and release of L. nigrinus, attention was directed towards a Japanese native congener, Laricobius osakensis Montgomery and Shiyake, 2011 (Foley et al. 2021). This species was of particular interest to this biological control program because it had coevolved with the lineage of HWA that was introduced into the eastern U.S. (Havill et al. 2006, Montgomery et al. 2011). The first releases of L. osakensis were conducted in 2012 (Mooneyham et al. 2016), and establishment at many locations has been confirmed (Toland et al. 2018, Virginia Tech 2022). Production and release of L. osakensis continues at two state rearing facilities: Virginia Tech and the University of Tennessee (Foley et al. 2021). A third species, Laricobius rubidus LeConte, 1861 is the only Laricobius species native to eastern North America (Clark & Brown 1960) and preferentially feeds on pine bark adelgid, Pineus strobi Hartig, 1839 (Hemiptera: Adelgidae), a specialist herbivore of white pine, Pinus strobus L. (Pinaceae) (Doane 1961). In areas where hemlock and white pine co-occur, this species is known to feed and develop on HWA (Zilahi-Balogh et al. 2005, Wantuch et al. 2019). Laricobius rubidus and L. nigrinus are closely related sister species that can hybridize and produce viable progeny (Havill et al. 2012, Fischer et al. 2015a). Laricobius nigrinus has become ubiquitous in many locations in the eastern U.S. due to consistent yearly releases and subsequent establishment and dispersal. As a result, it is often inadvertently brought into rearing labs with its native congener, L. rubidus, and associated hybrids on HWA infested hemlock branches used to maintain laboratory colonies of L. osakensis (Foley et al. 2021). Typically, this by-catch appears in March and April when larvae are active in the field. Larvae of each species and/ or hybrids are morphologically indistinguishable, which complicates considerably the Scientific Note