Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 22, 2008
Publication Date: June 1, 2009
Citation: 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 inoculation. Journal of Economic Entomology. 102:1247-1254. Interpretive Summary: The native eastern hemlock, Tsuga canadensis, and the Carolina hemlock, T. caroliniaiia, suffer injury and death following infestation by the hemlock woolly adelgid (HWA),Adelges tsugae, an introduced pest. Hybrids, between T. caroliniana and T. chinensis and between reportedly resistant Asiatic species, T. chinensis, and T. sieboldii were developed in a U.S.National Arboretum breeding project in the 1990's. The hybrids and seedlings and self pollinated progeny of the parent species were infested with hemlock woolly adelgid in 2006 and 2007 in no choice feeding studies. T. canadensis, T. caroliniana, and T. sieboldii showed high levels of susceptibility and T. chinensis exhibited a high level of resistance. Hybrids between the resistant T. chinensis and T. caroliniana or T. sieboldii showed intermediate resistance to HWA, many with levels approaching that of T. chinensis. No associations were found between plant growth or vegetative budbreak and resistance. Further testing is planned to evaluate whether the hybrids will demonstrate similar and adequate field resistance to HWA under natural infestation conditions and to determine whether the potential for interbreeding with native populations exists. While considerable variability in resistance and tree growth was found among the hybrids, after six years establishment in the field planting, many of the hybrid plants are attractive and show promise as resistant landscape alternatives to the native hemlocks.
Technical Abstract: Hemlock species and hybrids were evaluated for resistance to the hemlock woolly adelgid, Adelges tsugae Annand. The adelgid was accidentally introduced from Asia to the eastern United States, where it is causing widespread mortality of native hemlocks, Tsuga canadensis and T. caroliniana. The two native species, the Asian species (T. chinensis and T. sieboldii) and the hybrids (T. chinensis x T. caroliniana and T. chinensis x T. sieboldii) were artificially infested with the crawler stage of A. Lsugae in early spring 2006 and 2007. After eight or nine weeks when the spring (progrediens) generation would be mature counts were made of the adelgid. In both years, the density of A. Lsugae 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 and of those that settled, fewer survived than on the susceptible species; thus, both non preference (antixenosis) and nutritional unsuitability (antibiosis) are possible mechanisms of host resistance. No associations were found between plant growth or vegetative budbreak and resistance. While there is considerable variability in resistance and tree growth among the hybrids, some show promise as resistant landscape alternatives for the native hemlocks.