Case Studies
Climate Change Gives Devastating Hemlock Pest Six Legs Up in New York State
When the tree-killing hemlock woolly adelgid spread northward, warmer winters in the Adirondacks led to unexpected outbreaks.
Many harmful organisms, particularly insect pests, cannot survive long, cold winters. But as historical winter climates shift to shorter winter seasons and warmer minimum temperatures, restraints on these pests’ survival and spread are weakening. This case study discusses the hemlock woolly adelgid (HWA), Adelges tsugae, an invasive insect in New York State that has expanded its path of destruction well beyond predictions.
Highlights
- Scientists and resource managers are revising HWA management strategies and ramping up surveillance across northern New York State.
- Widespread hemlock loss along headwater streams predicted to result in cascading effects including increased water temperature, decreased water quality, and loss of trout habitat.
- New HWA outbreaks offer a warning that other damaging pests could soon arrive in upstate and upslope regions where milder winters are now prevailing.
Background
Eastern hemlock (Tsuga canadensis) is the fourth-most common tree species in New York State.1 It is considered a foundation species, which means it is both locally abundant and uniquely supportive of biodiversity in forest ecosystems throughout the state. This conifer can live up to 400 years and forms large, old-growth stands that are valued attractions in parks and preserves.2 Hemlocks provide shade and habitat for wildlife, prevent erosion, and maintain temperatures along waterways.
The HWA feeds at the bases of hemlock leaves during fall and winter months. The HWA is minute, with no viable long-distance dispersal capability, possibly relying on wind and animals (likely birds) for travel. However, HWAs do not need to mate to produce eggs, and the introduction of just a few can quickly lead to a massive infestation. Infested trees in the northeast can succumb in as few as five years.3 Infested individual trees can survive with pesticide treatments, but due to the large number of hemlocks in the state, it is only feasible to treat priority trees such as a homeowner’s prized trees or a highly visible grove in a state park.3
Current Climate Impacts
Since the HWA feeds in winter months, cold tolerance is key to its success. Recent studies demonstrated that the HWA has the potential to develop increased cold tolerance,4 but the populations studied were well south of current known limits and only some individuals had adjusted. Projected warming trends are accelerating the HWA’s range expansion (Figure 1), and some models predict that the HWA will soon penetrate northward into Quebec.5 Until recently, most surveillance efforts focused on the northern Catskill range and warmer western regions of New York State under the assumption that cold winters would protect the Adirondacks. However, recent HWA infestations have occurred outside expected range limits, including along Lake George (Figure 2).6–8
Other Climate Hazards
In addition to warmer winters, climate-related factors such as more frequent droughts and permanent changes in hydrological conditions will accelerate the mortality of HWA-infested hemlocks.9
Due to the foundational role that hemlock trees play in the forest ecosystem, widespread hemlock loss causes cascading effects that exacerbate the impacts of climate change. Hemlock loss along waterways can cause fast changes in stream flow and flooding due to extreme rain events, and reduction in water quality due to erosion. The loss of shade and the decline of snowpack that contributes cooler water from gradual melting leads to warmer stream temperatures. Predictions indicate that warming streams will negatively affect valuable coldwater fish species such as brook trout. The ecosystem’s vulnerability to invasive species will also increase.
Opportunity for Adaptive Management
The New York State Hemlock Initiative at Cornell University coordinates community scientist efforts and research to increase early detection of HWA. The project develops biological control agents, evaluates their effectiveness, and prioritizes hemlock stands for chemical treatment while working to understand the impacts of hemlock loss on the forest ecosystem.
Moving Forward
The consequences of HWA for forest ecosystems and hemlock-dependent wildlife are still under investigation.10 Given HWA’s brief tenure in the state, it will take time to evaluate the aftermath of new infestations.
Although this case study focuses on HWA, the trend of forest pests moving northward also applies to other species that threaten important trees in New York State. The southern pine beetle is another forest pest that became established in the state due to milder winters, and others will likely follow.
For More Information
References
1. Albright, T. A., Butler, B. J., Crocker, S. J., Drobnack, J. M., Kurtz, C. M., McWilliams, W. H., Morin, R. S., Nelson, M. D., Riemann, R., Vickers, L., Walters, B., Westfall, J. A., & Woodall, C. W. (2020). New York forests 2017 (Resource Bulletin NRS-RB-121). U.S. Department of Agriculture, Forest Service, Northern Research Station. https://doi.org/10.2737/NRS-RB-121
2. Adirondacks Forever Wild. (n.d.). Trees of the Adirondacks: Eastern hemlock (Tsuga canadensis). Retrieved January 5, 2024, from https://wildadirondacks.org/trees-of-the-adirondacks-eastern-hemlock-tsuga-canadensis.html
3. Limbu, S., Keena, M. A., & Whitmore, M. C. (2018). Hemlock woolly adelgid (Hemiptera: Adelgidae): A non-native pest of hemlocks in eastern North America. Journal of Integrated Pest Management, 9(1), 27. https://doi.org/10.1093/jipm/pmy018
4. Elkinton, J. S., Lombardo, J. A., Roehrig, A. D., McAvoy, T. J., Mayfield, A., & Whitmore, M. (2016). Induction of cold hardiness in an invasive herbivore: The case of hemlock woolly adelgid (Hemiptera: Adelgidae). Environmental Entomology, nvw143. https://doi.org/10.1093/ee/nvw143
5. McAvoy, Thomas., Régnière, J., St-Amant, R., Schneeberger, N., & Salom, S. (2017). Mortality and recovery of hemlock woolly adelgid (Adelges tsugae) in response to winter temperatures and predictions for the future. Forests, 8(12), 497. https://doi.org/10.3390/f8120497
6. Lake George Association. (n.d.). Tiny bug poses giant threat to Lake George water quality. Hemlock Woolly Adelgid. Retrieved January 5, 2024, from https://lakegeorgeassociation.org/science-protection/hemlock-woolly-adelgid
7. Hall, A. F. (2010, October 10). Unprecedented collaboration assembled to “save our hemlocks.” Lake George Mirror. https://www.lakegeorgemirror.com/archive/unprecedented-collaboration-assembled-to-save-our-helmlocks/
8. Brown, J. (2020, December 11). Protecting hemlocks is important part of Lake George Land Conservancy’s work. Lake George Mirror. https://www.lakegeorgemirror.com/archive/protecting-hemlocks-is-important-part-of-lake-george-land-conservancys-work/
9. Jactel, H., Petit, J., Desprez‐Loustau, M., Delzon, S., Piou, D., Battisti, A., & Koricheva, J. (2012). Drought effects on damage by forest insects and pathogens: a meta‐analysis. Global Change Biology, 18(1), 267–276. https://doi.org/10.1111/j.1365-2486.2011.02512.x
10. Ellison, A., Orwig, D., Fitzpatrick, M., & Preisser, E. (2018). The past, present, and future of the hemlock woolly adelgid (Adelges tsugae) and its ecological interactions with eastern hemlock (Tsuga canadensis) forests. Insects, 9(4), 172. https://doi.org/10.3390/insects9040172