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Northern Research Station
11 Campus Blvd., Suite 200
Newtown Square, PA 19073
(610) 557-4017
(610) 557-4132 TTY/TDD

Hemlock Woolly Adelgid

Development of Resistant Hybrid Hemlocks

Research Issue

[photo:] Susan Bentz and Mike Montgomery standing next to a hybrid hemlock tree.The two hemlocks native to the eastern United States (Tsuga canadensis and T. caroliniana) have been severely impact in both forests and landscape plantings by the introduced hemlock woolly adelgid (HWA) (Adelges tsugae ), an insect pest  from Asia which was unintentionally introduced to eastern North America during the last century. Some hemlock species native to Asia including T. chinensis and T. diversifolia are very resistant to the strain of HWA in eastern U.S. while another Asian species, T. sieboldii is tolerant (the pest grows on it, but the plant is not substantially damaged). A plant breeding program at the U.S. National Arboretum successfully produced interspecific hybrids between T. caroliniana and T. chinensisT. chinensis, and T. diversifolia, and between T. chinensis and T. sieboldii. No hybrids between T. canadensis and the Asian species have been successfully produced.  Hybrids produced by these initial studies have been planted in multiple common garden settings to evaluate growth, resistance and tolerance to the adelgid, suitability to climatic conditions, and interactions with native fauna.


Our Research

  • Evaluate the growth and resistance to HWA of hybrid-crosses between T. chinensis and T. caroliniana established at the USDA ARS Beltsville Research Center
  • Produce clonal hemlock propagules of previously generated interspecific Tsuga hybrids and of T. chinensis seedlings of documented wild-collected germplasm accessions
  • Evaluate adelgid tolerance and growth characteristics in multiple environments of clonal propagules from F1, F2 and backcross populations along with native and Asian hemlock species.
  • Assess the utilization of the hybrid crosses by native arthropod fauna.
  • Assess potential ecological role of hybrid crosses in eastern forested systems.
  • Evaluation of suitability of hybrids to environmental conditions in northeastern North American forests.

Expected Outcomes

  • Demonstration that interspecific hybridization of T. chinensis with the susceptible species T. caroliniana and T. sieboldii is a promising method of producing hemlocks that are resistant to A. tsugae and have good growth characteristics.
  • Evaluate and select clonal propagules that for release to the nursery industry.
  • Initial evaluation of susceptibility/resistance/tolerance of hybrids to A. tsugae in a field setting.
  • Initial evaluation of suitability of hybrids to climatic zones 6-7.

Research Results

The resistance of the interspecific hybrids was confirmed by artificial innoculation of 1-3 meter tall seedlings in the field nursery at Beltsville in the spring of two successive years. In both years, the density of A. tsugae was highest on T. canadensis, T. caroliniana, and T. sieboldii; lowest on T. chinensis; and intermediate on the hybrids. On T. chinensis and the T. chinensis hybrids, fewer adelgids settled, fewer of the settled adelgids survived, and the surviving adelgids grew more slowly. Thus, both non-preference (antixenosis) and adverse effects on biology (antibiosis) are mechanisms of the host resistance.

Clonal propagules of these seedlings have been produced via rooted cuttings and outplanted in nurseries in North Carolina and Connecticut. These are being evaluated for survival and growth characteristics, appearance and the arthropod fauna is sampled periodically in the CT field nursery. Initial results suggest hybrids between T. chinensis and T. sieboldii may have less tolerance for low temperatures than other hybrid crosses.

Montgomery, M. E.; Bentz, S. E.; Olsen, R. T. 2009. Evaluation of hemlock (Tsuga) species and hybrids for resistance to Adelges tsugae (Hemiptera: Adelgidae) using artificial infestation. Journal of Economic Entomology. 102: 1247-1254.

Bentz, S.E., Montgomery, M. E. and Olsen, R.  2008.  Resistance of hemlock species and hybrids to hemlock woolly adelgid  In: Onken, B.; Reardon, R., eds. Morgantown, WV: Fourth hemlock woolly adelgid symposium, Hartford, CT,  U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team: 137-139.

Research Participants

Principal Investigators

  • R. Talbot Trotter, Research Ecologist, U.S. Forest Service, Northern Research Station
  • Michael Montgomery, Emeritus Scientist, U.S. Forest Service, Northern Research Station
  • Susan Bentz, U.S. National Arboretum
  • Richard Olsen, U.S. National Arboretum

Research Partners

  • John Frampton, Dept. Forestry & Environmental Resources, North Carolina State University
  • Mark Ashton, Professor and Director of School Forests, Yale School of Forestry and Environmental Studies, New Haven, CT

Last Modified: 05/08/2015