Host acceptance by the hemlock woolly adelgid on hemlocks across a spectrum of host resistance

Eastern hemlock, Tsuga canadensis, is a foundation species in forests of eastern North America and is threatened by a highly invasive Asian pest, the hemlock wooly adelgid, Adelges tsugae. Resistance to A. tsugae varies among hemlock species, but the mechanisms associated with this resistance are unclear. T. canadensis is highly susceptible, whereas the western North American T. heterophylla and T. mertensiana are intermediate, and the Asian T. diversifolia and T. chinensis are resistant. I investigated the suitability of these hemlocks for A. tsugae crawler settlement and feeding in a laboratory host choice test and a field no-choice test. In the laboratory, ovisacs were placed equidistance from hemlock branch tips (1/species) in plastic rearing boxes (N=21) and settlement of newly hatched crawlers was observed after 10 days. Settled, active unsettled, and dead adelgids were compared across species. In the field, sleeve cages were used to infest hemlocks (6 branches/ tree, N=5 per species) growing in a common garden. Adelgid settlement of fully and partially formed ovisacs was observed after 10 weeks and compared across hemlock species. The highly susceptible T. canadensis showed the most settlement (p<0.0001). T. heterophylla and T. mertensiana had intermediate settlement with high proportions of dead adelgids in the laboratory test (p=0.0014). In the field experiment, T. diversifolia had a high proportion of partially formed ovisacs and a low proportion of fully formed ovisacs, indicating that A. tsugae crawlers settled but failed to complete development. The least settlement was found on the highly resistant T. chinensis.