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