Multi-trophic interactions and niche partitioning in the native range informs biological control in forests invaded by the hemlock woolly adelgid

Abstract Understanding natural enemy communities in the native range of pest insects is critical for implementing management decisions where such pests are invasive. The hemlock woolly adelgid (HWA) is a destructive invasive insect, causing decline of hemlock forests in eastern North America. We studied patterns of predator coexistence and prey suppression among native predators in western North America to inform biological control strategies in the invaded eastern range. In particular, we examined the structure and interactions of HWA’s specialist predator complex (a beetle Laricobius nigrinus , and two flies Leucotaraxis argenticollis and Le. piniperda ) across five western populations over two years. Field surveys quantified spatial and temporal overlap, while a laboratory experiment assessed competition between the two Leucotaraxis species. We employed species-specific RT-qPCR to quantify populations of cryptic Leucotaraxis immatures. Supporting the notion of complementary predation, predators exhibited clear temporal and spatial niche partitioning: (1) the two Leucotaraxis species were temporally separated, (2) while La. nigrinus overlapped with both Leucotaraxis species, they showed negative spatial co-occurrence on twigs, and (3) no evidence of competition between Leucotaraxis species was observed in the lab. These native predators appear to partition resources facilitating prey suppression, and they may be complementary in biological control. Nonetheless, during our study we observed only one HWA pest generation annually in the native western range, contrasting with bivoltine populations in the invaded east. This difference in pest voltinism, along with climatic differences between native and invaded communities, will likely influence predator–prey synchrony as the natural enemy complex establishes in invaded range.