Background

Hemlock woolly adelgid (HWA), or Adelges tsugae, was first detected in eastern North American in 1951. This aphid-like insect has killed hundreds of thousands of eastern and Carolina hemlock trees (Tsugae canadensis and T. caroliniana). HWA is native to Asia, where it rarely causes notable damage to native hemlock trees. It is also present in western North America, where the presence of specialized predatory insects and the lack of damage to native trees suggests HWA and other organisms in the ecosystem share evolutionary history.

Since HWA’s detection in eastern North America, scientists and other partners have researched and released biological control (or “biocontrol”) agents–insects from HWA’s native ranges that predate exclusively on HWA. Multiple species of beetles and flies have been released as biocontrol agents for HWA in eastern North America, some with more success than others. In a successful biocontrol campaign, enough predators or parasitoids of the invasive species are released in the invasive species’ introduced range to create self-sustaining populations that persist spread through the environment, keeping the invasive species in check. With thorough research guaranteeing biocontrols are host-specific and will not consume or harm any native species, this strategy is safe, effective, long-term, and landscape-level. This strategy has also been implemented for other invasive forest pests, such as emerald ash borer.

As biocontrol strategies continue to be researched and implemented, scientists and other partners are beginning to research another long-term, landscape-level strategy for hemlock conservation: resistance breeding. The Nature Conservancy, alongside expert partners, is heading a project called Tree Species in Peril. This project will support the development of pest-resistant eastern North American tree species that are severely threatened by invasive pests. It focuses on beech, hemlock, and ash trees. Because trees grow so slowly, it takes a long time for them to evolve in response to pest pressure and other stressors. Tree Species in Peril aims to assist in trees’ evolution through selectively breeding more resistant trees with each other.

The Lingering Hemlock Project is the component of Tree Species in Peril aiming to breed HWA-resistant hemlock trees. The first step in this process is finding hemlock trees that seem to be exhibiting more resistance to HWA than other hemlock trees. Once these trees are identified, partner organizations can test them to learn about their apparent resistance and interbreed them with each other to explore other, possibly stronger, HWA-resistant genotypes.

Hemlock Coalition Members

The Lingering Hemlock Project is made up of a vast team of academic institutions, nonprofit organizations, and government agencies. This enormous effort on behalf of hemlock conservation requires communication and collaboration for its success.

Project Oversight

The project is overseen by The Nature Conservancy and the U.S. Forest Service.

Locating “Lingering” Hemlocks

Most efforts by partners are currently focused on this phase of the project, as resistance testing and breeding cannot occur until we have candidate trees.

The first step in the resistance breeding process is to locate and identify hemlocks exhibiting signs of resistance to HWA. Generally speaking, HWA has already caused widespread hemlock mortality through much of its introduced range in the southeastern United States, while in northeastern North America, this level of hemlock mortality has not yet occurred. However, there is not yet a unified effort to track hemlock mortality; these geographic trends are based on select and sparse field reports, rather than an exhaustive survey of hemlocks throughout their eastern range. Through collaboration with other local partners, such as land trusts, foresters, academic institutions, etc., we hope to track hemlock mortality more systematically so as to better understand hemlock mortality trends and identify HWA-resistant hemlocks.

In locations where hemlock mortality has already reached 80% or higher (primarily the southeastern United States), project partners will search for “lingering hemlocks,” or hemlock trees that are still alive despite the deaths of many of their neighboring hemlock trees. These trees may not be in pristine condition, but even relatively higher survivorship may be important in understanding and breeding resistant hemlocks. If you are interested in learning more about how you can help with the Lingering Hemlock search, visit our Lingering Hemlock page for contact information and protocols for locating, surveying, and reporting lingering hemlocks.

In locations where hemlock mortality has not yet reached 80% (primarily northeastern North America), project partners will set up “hemlock health monitoring plots,” in which hemlocks will be monitored for decline alongside other environmental and site characteristics. Monitoring hemlock decline over time and space in this way will allow us to better understand patterns of hemlock mortality across the landscape. We will be able to understand HWA’s spread and severity across its leading edge and in doing, catch 80% hemlock mortality as it occurs. Once hemlock mortality reaches 80% in these plots, any remaining hemlocks can be documented as “lingering hemlocks.” If you are interested in learning more about how you can help with establishing Hemlock Health Monitoring Plots, visit our Hemlock Health Monitoring Plot page for contact information and protocols.

Project partners contributing to the search for lingering hemlocks are the New York State Hemlock Initiative, Hemlock Restoration Initiative, Blue Hill Heritage Trust, Harvard Forest, Pennsylvania Department of Conservation and Natural Resources, University of Kentucky, and The University of Tennessee Knoxville.

Researching Genetic Resistance in Hemlocks

Once potentially resistant hemlocks have been identified, research partners will run tests on candidate trees to learn more about the mechanisms behind HWA resistance in hemlock trees and to ascertain the degree to which they exhibit resistance to HWA. Through these tests and through cross-breeding of potentially resistant hemlock trees, we hope to accelerate hemlocks’ evolution of HWA resistance.

Partners at institutions such as North Carolina State University and the University of Connecticut will support research into hemlock genetic resistance to HWA.

Breeding HWA-Resistant Hemlocks

Using the knowledge gained by research genetic resistance and the genotypes identified as resistant, we can begin the process of developing nurseries of varied but resistant hemlocks. This genetic stock can be used to re-seed areas that have faced broad hemlock mortality, preventing the loss of the tree from the landscape.

Resistance breeding will be headed by Holden Forests & Gardens and the Ecological Research Institute.