The impact of imidacloprid on the subterranean survivorship of Laricobius (Coleoptera: Derodontidae), a biological control agent of Adelges tsugae (Hemiptera: Adelgidae)

The invasive hemlock woolly adelgid (HWA), Adelges tsugae (Annand) (Hemiptera: Adelgidae), has spread throughout most of the range of eastern hemlock, Tsuga canandensis (L.), and the entire range of Carolina hemlock, Tsuga caroliniana (Engelman), in the United States. Integrated pest management (IPM) of HWA combines chemical applications with the release of biological control agents on untreated trees within the same stand. Laricobius spp., Rosenhauer (Coleoptera: Derodontidae), have been used as biological control agents of HWA since 2003 and have subterranean and arboreal life phases that are synchronous with HWA's lifecycle. When utilizing IPM tactics, there is potential for Laricobius spp. to settle below an insecticide-treated tree for its subterranean phase. Field investigations assessed the impact of historical (five years post treatment in 2017) and recent imidacloprid soil treatments (via soil injection, soil drench, and tablet in November 2020) on the subterranean survivorship of Laricobius spp. by quantifying concentrations of imidacloprid and its metabolites to determine its potential impact on percent adult emergence from the soil. We observed a significant treatment effect on mean soil concentration among application methods at the recent treatment site, but not the historical treatment site. Additionally, at the more recently treated site, significantly lower mean percent emergence was observed from soil drench and tablet imidacloprid applications after one year (2021), but by the following year (2022), that effect was no longer present. This study supports recent recommendations to delay releases of Laricobius spp. for one-year post-treatment with imidacloprid. Furthermore, these data suggest when applying imidacloprid to a stand established with Laricobius spp., soil injection techniques pose the least risk to their subterranean survivorship.