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Evaluation of Hemlock (Tsuga) Species and Hybrids for Resistance to Adelges Tsugae (Hemiptera: Adelgidae) Using Artificial Infestation

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Evaluation of Hemlock (Tsuga) Species and Hybrids for Resistance to Adelges Tsugae (Hemiptera: Adelgidae) Using Artificial Infestation PLANT RESISTANCE Evaluation of Hemlock (Tsuga) Species and Hybrids for Resistance to Adelges tsugae (Hemiptera: Adelgidae) Using Artificial Infestation 1 2 2 MICHAEL E. MONTGOMERY, S. E. BENTZ, AND RICHARD T. OLSEN J. Econ. Entomol. 102(3): 1247Ð1254 (2009) ABSTRACT Hemlock (Tsuga) species and hybrids were evaluated for resistance to the hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae). The adelgid was accidentally intro- duced from Asia to the eastern United States, where it is causing widespread mortality of the native hemlocks, Tsuga canadensis (L.) Carrie`re and Tsuga caroliniana Engelm. These two native species plus the Asian species Tsuga chinensis (Franch.) E. Pritz and T. dumosa (D.Don) Eichler and Tsuga sieboldii Carrie`re, and the hybrids T. chinensis ϫ T. caroliniana and T. chinensis ϫ T. sieboldii, were artiÞcially infested with the crawler stage of A. tsugae in the early spring 2006 and 2007. After 8 or 9 wkÑwhen the spring (progrediens) generation would be matureÑcounts were made of the adelgid. 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 slower. Thus, the nature of the host resistance is both nonpreference (antixenosis) and adverse effects on biology (antibiosis). Tree growth (height) was associated with resistance, but no association was found between time of budbreak and resistance that was independent of the taxa. Many of the hybrids grow well, have attractive form, and are promising as resistant landscape alternatives for the native hemlocks. KEY WORDS Adelges tsugae, hemlock woolly adelgid, hybrid hemlock, host plant resistance The eastern North American hemlocks, Tsuga cana- Only the hemlock species in eastern North America densis (L.) Carrie`re and Tsuga caroliniana Engelm., are believed to be susceptible to A. tsugae; however, are highly susceptible to injury by the hemlock woolly formal experimental comparisons of the resistance of adelgid, Adelges tsugae Annand (Hemiptera: Adelgi- hemlock species are limited. In a 1-yr-old nursery dae). The adelgid was Þrst observed in the eastern planting in Connecticut, adelgid survival was 74Ð89% United States in 1951 in Richmond, VA, and now on T. canadensis and T. caroliniana and only 1Ð4% on occurs in 17 eastern states from Maine to Georgia T. diversifolia, T. heterophylla, and T. mertensiana (Mc- (USDAÐForest Service 2008). A. tsugae also occurs in Clure 1992). Based on their own observations and western North America on Tsuga heterophylla (Raf.) those from several sources, Del Tredici and Kitajima Sargent and Tsuga mertensiana (Bong.) Carrie`re; in (2004) concluded that T. chinensis was highly resis- China on Tsuga chinensis (Franch.) E. Pritz and T. tant, but they were uncertain whether T. heterophylla dumosa (D.Don) Eichler; and in Japan on Tsuga di- and T. sieboldii were resistant. versifolia (Maxim.) Mast. and Tsuga sieboldii Carrie`re; Hemlock is the secondary host of A. tsugae on which however, these species are not damaged by the adel- parthenogenic generations occur; like all adelgids, its gid. Mitochondrial DNA analysis suggests a long evo- primary host is spruce (Picea) on which sexual repro- lutionary history between A. tsugae and hemlocks in duction and gall formation occurs. The primary host in western North America and Asia, whereas A. tsugae in Japan is Picea torano (ϭP. polita) (Koch) Koehne the eastern United States is a recent introduction from (Inouye 1953) and recently adelgids in galls on two southern Japan (Havill et al. 2006; Havill et al. 2007). spruce species in China have been conÞrmed as A. The lack of host resistance and/or natural enemies has tsugae by using DNA (Foottit et al. 2009). In North been suggested as a reason for the severe impact of A. America, A. tsugae migrants were observed to deposit tsugae on hemlocks in the eastern United States eggs on both native and exotic spruce species, nymphs (Cheah and McClure 1996). from these did not survive past the Þrst instar (Mc- Clure 1987). The hemlock woolly adelgid has two wingless (apterous) parthenogenic generations on hemlock each year. In Maryland, where the experi- 1 Corresponding author: USDAÐForest Service, Northern Research ments reported here were conducted, the spring (pro- Station, Hamden, CT 06514 ([email protected]). 2 USDAÐARS, U.S. National Arboretum, 3501 New York Ave., N.E., grediens) generation begins with eggs laid in April and Washington, DC 20002. ends in early July after the adults have laid the eggs 1248 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 102, no. 3 that begin the overwintering (sistens) generation. The tween the susceptible eastern North American hem- sistens eggs hatch quickly and the Þrst instar nymphs locks and the Asian species in the 1990s. InterspeciÞc (neosistens) enter diapause after settling at a feeding hybrids as well as intraspeciÞc crosses were success- site. The neosistens do not resume growing until the fully produced from controlled pollination of several cool months of autumn and complete their develop- hemlock species (Bentz et al. 2002). AmpliÞed frag- ment in early spring. A generation of winged adults ment-length polymorphism analysis conÞrmed Ͼ50 (alates), called sexupara, is produced along with the authentic hybrids between T. caroliniana and T. chi- progrediens generation, but matures about a week nensis, T. sieboldii, and T. chinensis, and between T. earlier and ßies away in futile search for a suitable sieboldii and T. diversifolia (Pooler et al. 2002). Several primary host. attempts to hybridize T. canadensis with the three The only mobile stage on hemlock is the Þrst-instar Asian species were not successful (Bentz et al. 2002). nymph, which crawls about on the tree and can be These controlled crosses and selfed progeny from pa- carried between trees by wind or animals (McClure rental species were grown in containers for 4Ð10 yr 1990). Once these crawlers settle at feeding sites at and out planted for long-term evaluation in fall 2002 the base of the needles, they do not change location. and by 2006, most trees in the Þeld planting were of The crawlers of the progrediens generation emerge sufÞcient size to evaluate for resistance to A. tsugae from eggs in the woolly ovisacs over a period of 4 and horticultural qualities. wkÑbeginning before and ending after most of the The objectives of this study were to 1) evaluate vegetative buds of T. canadensis have brokenÑand resistance of the hemlocks by artiÞcial infestation with usually settle on the terminal internode of branches A. tsugae progrediens crawlers in the early spring, 2) produced the previous year. There is some evidence determine whether nonpreference (antixenosis) or that the earlier hatching crawlers are more successful adverse effects on growth (antibiosis) are compo- in establishment (Butin et al. 2007). The crawlers of nents of any resistance observed, and 3) determine the sistens generation occur in late June and early July whether resistance is correlated with the growth and and settle on the new, current yearÕs growth. budbreak of the trees. T. canadensis is a popular ornamental landscape tree that is widely planted within and outside its native Materials and Methods range, which extends from Nova Scotia west to Wis- consin and south to Georgia. It has dense foliage that Plant Material. Seedlings from interspeciÞc and in- can be heavily sheared and is extremely shade toler- traspeciÞc controlled pollinations (see Bentz et al. ant. The somewhat smaller and slower growing T. 2002) were out planted in 2002 in sandy-loam soil at caroliniana, native to the southern Appalachian the USDA South Farm, Beltsville, MD. A randomized Mountains, is also a desirable landscape tree but is rare block design was used. Trees were spaced 3 m apart, in horticultural commerce and not widely planted, lightly fertilized initially, and drip-irrigated as needed. probably because it does not transplant well (Swartley Table 1 shows the hemlock taxa planted, survival as 1984, Dirr 1998). The horticultural industry would of 2007, and the number used in each year of the welcome an alternative for T. canadensis that is both experiment reported herein. Following convention, resistant to the hemlock woolly adelgid and retains its interspeciÞc hybrids are listed with the female parent shade tolerance and its esthetic form and foliage. The Þrst followed by the male. Because of high mortality western North American hemlocksÑfound at high of T. caroliniana and its importance for comparison elevations or in cool, moist areas of the PaciÞc North- with the hybrids, three additional trees of T. carolini- westÑare not well adapted to the eastern North ana of the same age and parentage, from another American climate (Dirr 1998, Del Tredici and Kitajima planting, were used in the 2007 trial. If trees of sufÞ- 2004), even though T. heterophylla from Idaho have cient size and in good health were available, up to 10 survived Ͼ30 yr at the Arnold Arboretum near Boston, replicates were chosen randomly for 2006; in 2007, MA. The southern and northern Japanese hemlocks additional hybrid accessions were added to better (T. sieboldii and T. diversifolia, respectively) have determine differences among the hybrids. T. diversi- been used to a limited extent in ornamental plantings folia and its hybrids were not included in the exper- in the eastern United States (Dirr 1998, Swartley iment because of limited numbers and their small size. 1984). Chinese hemlock, (T. chinensis) is showing Screening for Resistance. We evaluated 72 trees in good environmental tolerance as well as resistance to 2006 and 100 trees in 2007 and assessed adelgid density the adelgid and recently was suggested as a replace- in more detail in 2007. Before the start of the trials, the ment for eastern hemlock in landscapes (Del Tredici hemlocks in the plantation were carefully screened for and Kitajima 2004).
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