Adelges Tsugae: Ecosystem Impacts & Avenues For Management
Eastern hemlock (Tsuga canadensis, L. ) is a tree that has inspired artists and scientists alike. One need only walk through a hemlock forest to experience a sense of quiet and awe, as the canopy envelopes those beneath its green ceiling. One need only walk through a hemlock forest, as the canopy envelopes those beneath its green ceiling, to experience a sense of quiet and awe. One of the biggest threats to this majesty, and Tsuga canadensis in general, is the insect hemlock woolly adelgid (Adelges tsugae Annand; HWA). Hemlock woolly adelgid were first introduced in 1951 when some were inadvertently shipped to Virginia on a Japanese hemlock host. Since its introduction, it has gradually expanded throughout the range of Eastern and Carolina hemlock (T. caroliniana Engelm. ), decimating their populations in the eastern United States. In 2012 and 2013, HWA were identified in southern Ontario, near Etobicoke and the Niagara Gorge, though swift action by the CFIA apparently eradicated the insect from both sites. It was discovered in 2017 that HWA are well established in southwestern Nova Scotia, where first arrival is estimated to have occurred around 2007. Once established on a host, the tree will almost certainly decline and die, though timescale of mortality varies greatly by region. This essay will describe hemlock woolly adelgid, the importance of Hemlock and effects of its loss, and explore proposed avenues for management.
Adelges tsugae is an aphid-like insect that feeds on a host tree’s phloem, attaching itself to the base of the needle and inserting a feeding tube into the twig. It has been suggested that death is the cumulative effect of Hemlock’s defensive reactions, which may include localized cell death as a means of depriving the insect of food. HWA has no natural predators in eastern North America, though there is a related species in the Pacific Nortwest kept in check by native predators. HWA populations are made up of females that reproduce parthenogenetically, producing two generations of wingless adults per year. A winged adult stage seeks out suitable spruce trees for sexual reproduction, though establishment on native spruce species has proven unsuccessful. HWA cannot travel very far but are likely spread by highly unpredictable vectors of dispersal such as birds, humans, and other animals. Once established in an area, HWA attack trees of all ages and sizes, decimating the population of hemlock. The speed of mortality varies, with certain southern sites exhibiting almost complete loss of hemlocks in three to four years, whereas further north the effects of HWA are slowed considerably. Kim et al. (2005) noted that on New England sites after an infestation of 10-12 yrs, the trees had experienced 50% defoliation. This marked difference is due to lower HWA survival in colder winter temperatures. Thus far, cold has been the limiting factor in the spread of HW but this is shifting as average temperatures increase, creating the possibility for it to expand throughout hemlock’s northern range. Furthermore, research suggests that northern populations of HWA are adapting to colder temperatures.
Eastern hemlock has been described as a foundation species, in that it fundamentally shapes its environment through structural or functional traits. For a species to be considered foundational it must be ubiquitous within the ecosystem; the basis of the local food web; and must be associated with many of the other species in that web. Furthermore, the species must be viewed as being important to the particular ecosystem. This last point is somewhat nebulous, but is described by Baiser et al. (2014) as the perception of the species as an intrinsic and defining part of its ecosystem.
Eastern hemlock is one of the few climax conifer species, creating forests that are almost exclusively made up of its own species. These nearly pure hemlock stands break up forested landscapes, primarily dominated by deciduous hardwoods in the Great Lakes/Saint Lawrence ecoregion. Hemlock is incredibly shade tolerant and long-lived, outlasting other species and creating conditions favourable for its own regeneration; heavily shaded understories with damp microclimates that moderate temperatures throughout the year, making them particularly good shelter during winter months.
In order to predict the spread of HWA and its influence, Albani et al. (2010) created a forecasting model of the distribution of HWA, predicting that it would have reached the eastern edge of Lake Michigan by 2025 and that it would have established itself throughout eastern hemlock’s range in the USA by 2040. By modelling the influence of forest dynamics on the carbon cycle, they predicted that the loss of hemlock would have large impacts at local scales but that regional productivity would not be severely affected. They suggest that because of its relatively small contribution to forest composition, and the predicted establishment of fast-growing hardwoods, the loss in productivity would largely be compensated for by 2050. It is assumed that as hemlock dies, it will make way for a mixed-hardwood ecosystem, but this prediction may not hold true. For example, one study predicted that transformed ecosystems in southern New England would be dominated by fast-growing black birch. Abella (2018) did not find the expected compensatory growth of deciduous species after the death & toppling of hemlock. Rather, there was an observed decrease in species richness, as many of the deciduous trees that were expected to take over were outcompeted by blackberry and rhododendron, which are hardy and aggressive short-lived species.
The dense canopies present in hemlock forests intercept a large part of precipitation year-round, regulating the flow of water into streams and watersheds while preventing erosion (Ellison et al. 2005). When these trees are lost, they can have multiple and potentially interacting effects on the abiotic characteristics of the area. Kim et al. (2017) found that peak evapotranspiration (ET) decreased by 24-37% over a year, which they conclude was due to a 25-50% foliage loss. The investigators also found an increase of 15% in water that flowed out of the deforested area. Stadler et al. (2005) assessed the trophic interactions of HWA on microenvironments, and used their findings to extrapolate effects at the landscape level. It was found that HWA changed the nutrient composition of hemlock leaves and the forest leaf litter. Specifically, they found an increase in nitrogen under infested trees after rainfall. This N-rich litter could be a potential boon for incoming species, though it is highly soluble and could quickly drain out of the soil if water flow increased through the area. Ultimately, this means that as hemlock forests die out, the regulatory effect of hemlock forests on water flow will be lost, resulting in less available water for use by the ecosystem and possible increased erosion and nutrient run-off. Hemlocks are a commonly riparian species, adding to the productivity of streams through the slow release of nutrients from woody debris as the dense canopy regulates the temperature of streams, resulting in particularly good habitat for salamanders, fish and other animals that require consistently moist environments. Snyder et al. indicated that hemlock forest streams support greater species richness of aquatic invertebrates –10% of taxa were strongly associated with T. canadensis – when compared with mixed-hardwood stands. Beyond the potential loss of important habitat for aquatic species, the loss of hemlock could have interesting effects on local mammal populations. In one study examining the effects of HWA and salvage logging, it was found that while species richness increased, animals with more particular habitat requirements - such as the southern flying squirrel – were less common. Several responses to HWA have been explored or considered, including biotic controls, pesticides, hybridization, pruning, salvage logging, as well as seed collection and storage. Multiple introduced predators have been tested in the USA, the most successful of which was Laricobius nigrinus Fender from the Pacific Northwest, which preys on one of the life stages of HWA and has been known to hybridize with a native Laricobius species. L. osakensis Montgomery and Shiyake is a Japanese beetle that is being considered as a more aggressive alternative to L. nigrinus. In order to successfully control HWA, it would be necessary to use a predator with a complimentary feeding pattern; for this, several species of Leucopis, and some species of Scymnus, are currently being considered. The main problems with biological controls are that their establishment can be difficult and slow, while there is also the potential for cascading effects resulting from the introduction of a new native species into an ecosystem. Close monitoring of the introduced species is therefore essential.
Two neonicotinoid insecticides have been successful in protecting trees, though their effects are relatively short, only protecting individual trees for 4-7 years. In Canada, these pesticides can only be applied via injection, making their use quite expensive and labour-intensive. These barriers make them unsuitable for a wide scale approach, though they have been used effectively in the context of parks, campgrounds, and other areas where the trees have high aesthetic or social value. A recent study suggests that a reduction in shade could increase HWA mortality, which could be an interesting method of control if it were integrated into current salvage harvesting practices. Salvage logging is carried out to recover as much value as one can from a threatened stand, which precludes the possibility of recovery in that area. Given the observed effects of reducing shade on HWA, however, if a selection system were implemented, it might be possible to retain high quality trees, improve their growing conditions, and expose HWA to adverse conditions. Salvage logging could also serve as a physical barrier to HWA spread. Emilson et al. (NRC, 2018) also suggest that pruning and cutting of infested trees in managed stands should be implemented in order to limit the possibility of dispersal via humans or animals.
Tsuga caroliniana has the potential to hybridize with Chinese hemlock, which is resistant to HWA. Unfortunately, for reasons of practicality, it is unreasonable to look to hybridization for a large-scale solution, but it is encouraging that T. caroliniana could be conserved in some form.
Emilson et al. advocate for the collection and preservation of eastern hemlock seeds in order to ensure a genetically diverse seed bank. They also suggest that greater federal investments will be necessary in order to fund research efforts into resistant genotypes, and the breeding thereof. Successful management of HWA will require a multimodal approach with strategies selected on a case-by-case basis, determined by the extent of infestation, the probability of near-future infestation, as well as the value of the forest in terms of ecosystem services, economic, and social benefits. Furthermore, because we exist in a world where funding is limited and social/political will is ephemeral, conservation efforts must do their best to maximize effectiveness of resources. Plans must be developed for the possible spread of HWA throughout the range of eastern hemlock, as there is no turning back the clock on climate change. Because of its role as a foundation species, loss of eastern hemlock would fundamentally and irrevocably change the landscape and the composition of the communities from which it is removed. The loss of these conifer ecosystems would not only deprive future generations of their benefits and magnificence, but would result in more homogeneous landscapes, rendering them vulnerable to invasive species and further transformation.
