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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

Resistant Hemlocks in Natural Stands

Research Issue

[photo:] Lingering hemlock in infested stand with putative resistance.During the course of a survey of 142 New England hemlock forests conducted for an unrelated research project, a few apparently healthy mature eastern hemlock trees persisting in hemlock woolly adelgid (HWA) -devastated hemlock stands were discovered. Each of these stands had >95 perent hemlock mortality (with the rest of the trees near death) and had not been sprayed or otherwise treated for HWA infestation. An evaluation of these trees is needed to determine if natural variation in the resistance of eastern hemlocks exists and if they can be propagated for use in restoring hemlock ecosystems devastated by HWA.

Our Research

  • Develop methods for efficiently propagating Tsuga canadensis and other Tsuga species through rooting branch cuttings
  • Develop standardized growing conditions for production and evaluation of hemlocks to allow testing adelgid resistance under identical conditions
  • Evaluate HWA survival on putatively resistant hemlocks relative to survival on plants of known resistance and susceptibility

Expected Outcomes

  • Identify eastern and Carolina hemlocks with resistance to HWA
  • Develop methods for propagating hemlock using branch cuttings

Research Results

Because the HWA susceptibility of hemlock is influenced by many factors including plant nutritional status and prior attack by HWA and other insects, we chose to vegetatively propagate cuttings from putatively resistant forest trees in order to grow and evaluate these plants for HWA resistance under standardized greenhouse conditions. We found that a combination of IBA and NAA rooting hormones gave the best rooting results of cuttings taken in midwinter. When 6-month old rooted plants were inoculated with adelgids, there was much lower settlement on putatively resistant plants than on control plants (collected from T. canadensis growing in northern Massachusetts).

Ingwell, L., Brady, J.; Fitzpatrick, M.; Maynard, B.; Casagrande, R.; Preisser, E. 2009. Intraspecific variation in Tsuga canadensis foliar chemistry. Northeastern Naturalist. 16: 585-594.

Caswell, Todd; Casagrande, Richard; Maynard, Brian; Preisser, Evan . 2008. Production and evaluation of eastern hemlocks potentially resistant to the hemlock woolly adelgid. In: Onken, Brad; Reardon, Richard, eds. Fourth symposium on hemlock woolly adelgid in the eastern United States: proceedings of the meeting; 2008 Feb. 12-14; Hartford, CT. Technology Transfer FHTET-2008-1. Morgantown, WV: U.S. Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team: 124-134.

Research Participants

Principal Investigator

  • Richard Casagrande, Professor, University of Rhode Island, Department of Plant Sciences
  • Brian Maynard, Professor, University of Rhode Island, Department of Plant Sciences
  • Evan Preisser, Assistant Professor, University of Rhode Island, Department of Biological Sciences

Research Partners

  • Michael Montgomery, Research Entomologist, U.S. Forest Service, Northern Research Station


Last Modified: 09/28/2010