Arboriculture & Urban Forestry 35(1): January 2009 Table 6. Successful establishment of hemlock woolly adelgid on eastern hemlock by treatment. Treatment L100 H100 L1000 H1000 CNS z Data coded as 1 = noninfested; 2 = infested. Least squares mean z 1.77 bc 1.83 c 1.57 b 1.81 bc 1.13 a CONCLUSION This study demonstrates a procedure for successfully challeng- ing hemlock species with HWA ovisacs. Use of this procedure allows direct interaction between HWA and the hemlock species being evaluated. The use of protective sleeves was observed to be a major factor in challenging hemlocks with HWA. This chal- lenge procedure could be used to test additional hemlock species for resistance to HWA. Chinese hemlock has demonstrated considerable resistance to highest establishment occurred at the Lackawanna County site ( Table 3 ). To detect differences in the percentage of applications with suc- cessful HWA establishment, each inoculation site was classified as noninfested (1) or infested (2). Establishment was highest at Lackawanna and Center Counties ( Table 4 ). The least squares means of nymphs for the inoculation treat- ments had some differences ( Table 5 ). The lowest numbers of nymphs occurred on the control (CNS ) and L1000 treatments. Treatments with the high rate of inoculum or 100 µm mesh had higher levels of infestation than the CNS treatment. When quantified by the percentage of applications with suc- cessful HWA establishment, the CNS treatment had a lower least squares mean than the other treatments (Table 6). No live nymphs were found on any of the Chinese hemlocks. RESULTS AND DISCUSSION Eastern Hemlock The fact that HWA will readily feed on eastern hemlock is well established (Gouger 1971; McClure 1987). Failure of the HWA to establish on some eastern hemlocks in this study may have been the result of host resistance, handling and disturbance, envi- ronmental stress, or inoculum levels. One factor that supports the role of environmental stress in mortality is the difference in survival rates by location, which may be explained by temperature trends at the experiment sites. The temperatures at the Lackawanna County site were consis- tently lower than the other two sites, and prevailing winds con- tributed a wind chill factor to lower the temperature even more. The mesh cages provided enough protection from the winds to prevent the ovisacs from being damaged or desiccated, and the lower temperatures relieved stress. This cool environment may have contributed to the high level of infestation. Inoculum level and mesh size were not as critical as the pres- ence of a sleeve. The L1000 treatment resulted in less HWA establishment than the L100 treatment, although all sleeved treat- ments had higher percentages of applications with HWA estab- lishment than the nonsleeved CNS treatment. The unprotected ovisacs were exposed to more direct sunlight, wind, and poten- tial predators. The sleeved applications had a higher degree of protection, and subsequently a higher percentage of applications resulted in HWA establishment. Chinese Hemlock No HWA nymphs were found on the Chinese hemlocks in these field studies. All three sites yielded the same result. This suggests Chinese hemlock has substantial resistance to HWA. HWA and may be useful as a replacement for eastern hemlock in ornamental landscapes. Further research into the genetic factors involved in this resistance may also offer clues for breeding proj- ects with eastern hemlock. Acknowledgments. We thank David L. Sanford for assistance in monitor- ing the Berks County experiment site, Richard P. Marini for assistance with statistical analyses, and the USDA Forest Service for funding this research. LITERATURE CITED Bentz, S.E., L.G.H. Riedel, M.R. Pooler, and A.M. Townsend. 2002. Hybridization and self-compatibility in controlled pollinations of eastern North American and Asian hemlock (Tsuga) species. Journal of Arboriculture 28:200–205. Del Tredici, P., and A. Kitajima. 2004. Introduction and cultivation of Chinese hemlock ( Tsuga chinensis ) and its resistance to hemlock woolly adelgid ( Adelges tsugae ). Journal of Arboriculture 30:282–286. Gouger, R.J. 1971. Control of Adelges tsugae on hemlock in Pennsylvania. Scientific Tree Topics 3:1–9. McClure, M.S. 1987. Biology and control of hemlock woolly adelgid. Conn. Agr. Expt. Sta. Bul. 851. ———. 1991. Density-dependent feedback and population cycles in Adelges tsugae (Homoptera: Adelgidae) on Tsuga canadensis . Environmental Entomology 20:258–264. McClure, M.S., and C.A.S.-J. Cheah. 1999. Reshaping the ecology of invading populations of hemlock woolly adelgid, Adelges tsu- gae (Homoptera: Adelgidae), in eastern North America. Biological Invasions 1:247–254. Montgomery, M.E., D. Yao, and H. Wang. 2000. Chinese Coccinellidae for biological control of the hemlock woolly adelgid: Description of native habitat, pp 97–102. In McManus, K.A., K.S. Shields, and D.R. Souto (Eds.). Proceedings: Symposium on sustainable management of hemlock ecosystems in eastern North America, 22–24 June 1999, Durham, NH. GTR-NE-267. USDA Forest Service, Northeastern Research Station, Newton Square, PA. Swartley, J.C. 1984. The Cultivated Hemlocks. Timber Press, Portland, OR. Vidakovic, M. 1991. Conifers: Morphology and Variation. (trans. Maja Soljan) Grafickizavod Hrvatske. Zagreb, Croatia. Benjamin K. Hoover (corresponding author) Graduate Research Assistant Pennsylvania State University 310 Tyson Building University Park, PA 16802, U.S.
[email protected] Ricky M. Bates Associate Professor of Ornamental Horticulture Pennsylvania State University 303 Tyson Building University Park, PA 16802, U.S. James C. Sellmer Associate Professor of Ornamental Horticulture Pennsylvania State University 314 Tyson Building University Park, PA 16802, U.S. ©2009 International Society of Arboriculture 3
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