The Hemlock Woolly Adelgid (Adelges tsugae Annand) is a destructive insect pest that feeds on eastern North American hemlocks (Tsuga canadensis and T. caroliniana). It is native to Japan, and was accidentally introduced to this continent earlier this century (McClure 1987). HWA first appeared in Connecticut in 1985, probably as a result of dispersal by hurricane Gloria from Long Island, where infestations had existed several years previously (McClure 1987).
HWA is dispersed by wind, birds, deer, and humans, and is moving north at a rate of about 30 kilometers per year (McClure 1990). The most obvious sign of infestation is the presence of white woolly egg sacs on the underside of hemlock needles, especially on new growth. Once established on a host, adelgids feed on twigs and new growth by piercing at the base of needles (Shields et. al. 1996). Shortly after infestation, this feeding has the effect of desiccating the needles and causing them to die and fall to the ground. Infested trees usually die within 9 years, and have been killed in as little as 4 years (McClure 1987). Because between 25 and 50 eggs are laid at each reproductive event (McClure 1991), and because HWA complete two life cycles per year, populations grow very rapidly. In areas such as southern Connecticut which have been infested for several years, massive hemlock die-offs are occurring. In contrast, it appears that in Japan hemlocks are protected by the presence of abundant and efficient natural enemies to HWA (McClure 1995, Sasaji and McClure 1997). Current research on HWA is focusing on the identification of a natural predator which will remain viable and effective at regulating HWA populations if introduced to the northeastern United States. The results of this research are promising. In small introductions of Pseudoscymnus tsugae, a coccinellid beetle, to infested Connecticut hemlock stands, the beetle appears to be successful at greatly reducing HWA numbers (McClure 1998). The next test for P. tsugae will be to determine whether or not it can maintain its population and keep pace with HWA's migrations northward at sufficient densities to regulate HWA populations.1996 Map of HWA distribution within the range of Tsuga canadensis
Hemlocks in southern Connecticut are dying rapidly. Since regular insecticide spraying of forest systems and attempts to completely eradicate HWA are impractical, the search for biological controls seems the most likely course for hemlock protection (McClure 1987). Until these controls are in place, foresters are struggling to limit the rate and scope of the damage.
HWA attacks trees under all growing conditions; no resistant eastern hemlocks have been identified. Although some authors have suggested a possible role of site quality in regulating the effect of HWA on hemlock stands (Evans et. al. 1996 and Suoto et. al. 1996) none has yet directly addressed the issue. If there is a relationship between stand susceptibility and site charateristics, then understanding it may prove useful in the development of a management plan to control HWA damage. In the current study, it was hypothesized based on observational and anecdotal evidence that hemlock trees growing on sites with higher water availability would be more effectively able to resist mortality from HWA infestation than trees on drier sites.
This study focuses on a single large 80 to 100 year old forest in the Crooked Brook uplands just north of Lake Gaillard, in North Branford, Connecticut. The New Haven Regional Water Authority owns a number of large forests in the New Haven Area, including the Crooked Brook site. These forests are managed primarily for watershed protection, with secondary uses including small, sustained yield timber sales and firewood harvesting by local citizens. The forest slopes upward from a reservoir (Lake Gaillard) to the ridge of Totoket Mtn. to the north and west. Average slope is approximately 22%, and the soil is generally very rocky (angular basalt), xeric, and very shallow to bedrock, with abundant rock outcroppings.
Two upland plots and two riparian plots, all with canopies dominated by hemlock, were selected for investigation. Each plot had an area of 314 m2. Hemlocks within each plot were assessed for diameter, height, age, time since infestation, and canopy vigor. The vigor of each tree was estimated based on the following scale: 1 = no foliage present, 2 = between 0 and 25% of foliage present, 3 = between 25 and 50% of foliage present, 4 = between 50 and 75% of foliage present, and 5 = between 75 and 100% of foliage present (see photos below). It was predicted that the time of infestation would be manifested as a reduction in ring width related to the inhibition of new growth by HWA (McClure 1991).
Using this information, hemlock basal area and biomass were calculated. An analysis of variance (ANOVA) was performed to determine the relationship between site type and biomass, and between site type and canopy vigor (see below).
The site is described in some detail at the FES 519b main page. Crooked Brook drains the watershed east of Totoket Mountain and feeds into Lake Gaillard in North Branford, CT. The reservoir and forest are owned and managed by the New Haven Regional Water Authority and managed primarily for watershed protection through the maintenance of a healthy and diverse forest. The study site is a hemlock-mixed hardwood (primarily Tsuga canadensis, Quercus rubra, Q. alba, Q. prinus, Acer saccharum, A. rubrum, Carya spp., Fagus grandifolia) forest on Totoket Mountain's gentle southeast-facing slope. The ridge's shallow, rocky soil overlies a basaltic till. Ridges of exposed bedrock occur throughout the forest. The forest is in the stem exclusion stage, and has a relatively open understory. The understory contains musclewood (Carpinus caroliniana), hop hornbeam (Ostrya virginiana), witch hazel (Hamamelis virginiana), and flowering dogwood (Cornus florida). Though our study began in January, the warm, wet winter of 1998 has kept a few ephemeral streams flowing across the broad slope. Some depressions on the mountain were actually flooded enough to be impassable on foot. The abundant hemlocks on the site have been infested by the HWA for about ten years now, and are dying fast. This spring, salvage harvesting began on several large, heavily damaged hemlock stands.
The New Haven Regional Water Authority manages this forest primarily for watershed protection. Some small-scale logging operations do occur here. As previously mentioned, foresters are harvesting some dead and unhealthy hemlock stands. Though generally seen as one of the less desirable forest products, hemlock boards are used for non-load bearing construction and other purposes. In addition to these salvage and other harvests, the water company allows local permitees to cut firewood for personal use on marked plots scattered throughout the forest.
For each tree within one of the four study plots, crown vigor was estimated based on a five point scale. The scale is explained in Methods above.
|Vigor 1: No Foliage||Vigor 2: <25% Foliage Present|
|Vigor 4: 50-75% Foliage Present||Vigor 5: 75-100% Foliage Present|
Site characteristics for two upland and two riparian hemlock stands in North Branford, CT. Canopy vigor values are based on a scale of 1 (dead) through 5 (undamaged). See text above for details.
|Plot||hemlock total basal area per hectare||estimated hemlock biomass per hectare||estimated total biomass per hectare||mean hemlock canopy vigor||mean tree height|
|Upland 1||53.26 m2||19841 kg||21705 kg||2.53||19.04 m|
|Upland 2||51.29 m2||19989 kg||21795 kg||2.32||14.40 m|
|Riparian 1||47.52 m2||22837 kg||22849 kg||3.96||18.34 m|
|Riparian 2||53.37 m2||22592 kg||23159 kg||3.16||14.48 m|
Analysis of variance tables based on
data collected from two upland and two riparian stands
A statistically significant relationship (p<0.025) was found between total biomass per plot and primary site characteristic (upland xeric vs. riparian mesic). The relationship between site type and average tree vigor was less conclusive (p<0.08), but nevertheless suggestive of a correlation between water availability, site quality, and susceptibility to HWA. All four of the sites were approximately the same age and showed no statistically significant variation in canopy height; all currently suffer from some degree of HWA infestation, and have been infested for approximately 4-9 years, according to tree ring analysis and reports from water company foresters.
Although the results of this study show no statistically significant relationship between tree vigor and site type, they do suggest that a relationship may exist. Geographically separated by only approximately 500 meters, there appears to be a difference in the way that the riparian and upland plots are being affected by HWA. While upland stands are already suffering significant losses due to tree mortality, infested trees in sampled riparian stands have for the most part not succumbed.
Sounds good, right? Well, the results of this project need qualification. First, because we did not identify any statistically significant relationship between site quality and mean tree vigor within infested stands, we can draw no conlusion about the relationship. Second, our site characterizations are based on assumptions about the hydrology of the sites, and are based on no empirical evidence. We have assumed that the sampled upland sites are lower quality than the riparian sites. This assumption is borne out by informal observations (soils on the upland sites are shallow to bedrock and dry, while those on the riparian sites are deeper and moister) and by conversation with water company foresters, but not by rigorous sampling. As a consequence, even a statistically significant result would be questionable without confirmation by a more rigorous study. Third, we have assumed equal times of infestation for upland and riparian stands. Again, this assumption is supported by conversation with water company foresters and a qualitative examination of tree cores from each site, but not by any quantitative analysis. Fourth, the spatial scale in which our sampling occurred was small enough that our finding may reflect the normal patchy nature of HWA dispersal (and different times of infestation among sites) and not differences related to site characteristics. These limitations to our results are important and need to be addressed before drawing reliable conclusions about the validity of our hypothesis. Nonetheless, our results suggest that further study of this hypothesis would be justified. For foresters, nursery owners, or homeowners living in regions not yet affected by HWA (see map above) and for those who wish to prolong the lives of trees in affected areas, water availability and site quality may prove to play an important role.
Evans, R.A., E. Johnson, J. Shreiner, A. Ambler, J. Battles, N. Cleavitt, T. Fahey, J. Sciascia, and E. Pehek. 1996. Potential impacts of hemlock woolly adelgid (Adelges tsugae) on eastern hemlock (Tsuga canadensis) ecosystems. pp. 42-57 in Proceedings of the first hemlock woolly adelgid review, Charlottesville, VA. United States Department of Agriculture Forest Service. FHTET 96-10.
McClure, M.S. 1987. Biology and control of hemlock woolly adelgid. Bull. Conn. Agric. Exp. Stn. 851.
----. 1990. Role of wind, birds, deer, and humans in the dispersal of hemlock woolly adelgid (Homoptera: Adelgidae). Environ. Entomol. 19:36-43.
----. 1991. Density-dependent feedback and population cycles in Adelges tsugae (Homoptera: Adelgidae) on Tsuga canadensis. Environ. Entomol. 20:258-264
----. 1995. Diapterobates humerales (Oribatida: Ceratozetidae): An effective control agent of hemlock woolly adelgid (Homoptera: Adelgidae) in Japan. Environ. Entomol. 24:1207-1215.
----. 1996. Biology of Adelges tsugae and its potential for spread in the northeastern United States. pp. 16-25 in Proceedings of the first hemlock woolly adelgid review, Charlottesville, VA. FHTET 96-10.
----. 1998. Lecture, Yale University. 16 March, 1998.
Sasaji, H. and M.S. McClure. 1997. Description and distribution of Pseudoscymnus tsugae sp. nov. (Coleoptera: Coccinellidae), an important predator of hemlock woolly adelgid in Japan. Environ. Entomol. 90:563-568.
Shields, K.S., R.F. Young, and G.P. Berlyn. 1996. Hemlock woolly adelgid feeding mechanisms. pp. 36-41 in Proceedings of the first hemlock woolly adelgid review, Charlottesville, VA. United States Department of Agriculture Forest Service. FHTET 96-10.
Suoto, D, T. Luther, and B. Chianese. 1996. Past and current status of HWA in eastern and carolina hemlock pp. 9-15 in Proceedings of the first hemlock woolly adelgid review, Charlottesville, VA. United States Department of Agriculture Forest Service. FHTET 96-10.
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