Variable colonization by the hemlock woolly adelgid suggests infestation is associated with hemlock host species.
Studies increasingly indicate host resistance likely influences hemlock woolly adelgid (HWA) populations. We describe coexisting HWA populations in four hemlock species, three from HWA's native range (Tsuga chinensis, T. dumosa, and T. sieboldii) and one from its introduced range: T. canadensis. HWA populations were greater in T. canadensis and lesser or absent in T. chinensis and T. dumosa. HWA stylet insertion was observed the least in T. chinensis and T. dumosa, suggesting a physical and/or chemical feeding deterrent. While T. chinensis and T. dumosa consistently and distinctly differed from T. canadensis, T. sieboldii grouped with T. canadensis or T. chinensis and T. dumosa in different experiments, suggesting mechanisms influencing HWA populations differ by host species. HWA were found more often on tops of T. chinensis and T. dumosa branches but equally on tops and bottoms of T. canadensis branches, corresponding with locations of trichomes (which T. sieboldii lacks). Evidence from the literature about dissimilarities in hemlock hosts (e.g., HWA interactions and vulnerability, feeding damage response, cuticle thickness, terpenoid chemistry, arthropod and microbial communities) and native and introduced HWA populations (e.g., losses of sexual reproduction and primary spruce host) suggests a limit to applying information from native systems to management in introduced systems.