Production and Evaluation of Eastern Hemlocks (Tsuga canadensis) Potentially Resistant to the Hemlock Woolly Adelgid (Adelges tsugae)
Todd Caswell, Richard Casagrande, Brian Maynard, and Evan Preisser University of Rhode Island, Kingston, RI 02881
Abstract. As the hemlock woolly adelgid (HWA) has spread throughout the forests of the northeast, it has killed countless eastern hemlocks while possibly sparing a small minority of trees with some degree of innate resistance. There are, as yet, no published records of HWA resistance in T. canadensis, but on rare occasions, a relatively healthy tree (referred to as ‘putatively resistant’) is found amidst a devastated stand. As HWA susceptibility 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 mid winter. 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 MA).
Keywords. Hemlock woolly adelgid, Adelges tsugae, Tsuga canadensis, vegetative propagation, pest resistance.
Introduction. The hemlock woolly adelgid (HWA), Adelges tsugae Annand, has become a serious pest of the native North American eastern hemlock species Tsuga canadensis (L.) Carriere and T. caroliniana Engelm.. Several Tsuga species resist HWA including western hemlock, T. heterophylla (Raf.) Sarg.; mountain hemlock, T. mertensia (Bong.); Chinese hemlock T. chinensis (Franch.) E. Pritz.; and Japanese hemlocks (T. diversifolia (Maxim.) Mast., and T. sieboldii Carriere) (McClure 1992, Pontius et al. 2002) Although these resistant species can be quite useful in managed landscapes, they are not suitable for replacing eastern hemlocks in forested settings. If HWA-resistant eastern hemlocks could be identified and propagated, they would be useful for both managed landscapes and stand-level reforestation of HWA-devastated areas. To date no one has demonstrated HWA resistance in eastern hemlocks. Although a few relatively healthy trees can be found in otherwise devastated areas, it is unclear if their health is linked to innate resistance or growing conditions (McClure and Chea 1992, Pontius et al. 2006, Preisser et al. 2008). HWA resistance should ideally be evaluated under standardized conditions in order to isolate the effect of innate plant resistance. Although it is possible to collect and grow seed from putatively resistant plants, this approach is problematic since the resulting seedlings may have at least one susceptible parent. For documenting resistance levels among selected clones, vegetative propagation quickly yields a population of progeny that are genetically identical to parent plants. Del Tridici (1985) and Jetton et al. (2005) successfully propagated hemlocks from branch cuttings and Butin et al. (2007) experimentally inoculated hemlocks with adelgids – two techniques that are central to our research. In this paper we discuss the selection, propagation, maintenance, and evaluation of hemlocks that may be resistant to the HWA. Materials and Methods Rooting Trial One. As part of a larger research effort (Preisser et al. 2008), we surveyed 142 hemlock stands in CT and MA in 2005 for potentially HWA-resistant eastern hemlock trees. In forest stands where the mortality rate of mature hemlocks exceeded 95%, we sought mature (>10 m) trees that appeared healthy and largely free of HWA and the elongate hemlock scale Fiorinia externa (Marlatt). We report data on six putatively resistant hemlocks identified in our 2005 survey – two trees at each of three sites near the towns of East Haddam, Madison, and Old Lyme in CT. We also sampled a tree from the Arnold Arboretum (Jamaica Plain, MA) that was identified as potentially resistant by Peter Del Tredici.