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Hemlock Restoration at CMLC: Biological Control of Hemlock Woolly Adelgid
Healthy stand of Carolina Hemlocks on Wildcat Rock in the Hickory Nut Gorge
HEMLOCKS & HEMLOCK WOOLLY ADELGID:
Over the last decade, the exotic forest pest, Hemlock Woolly Adelgid (HWA) has ravaged our southern Appalachian forests. Since the “arrival” of HWA in our area, Carolina Mountain Land Conservancy (CMLC) has been working to protect our native Eastern and Carolina hemlocks from this attack.
Hemlock Woolly Adelgid is a destructive pest that gravely threatens the eastern hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). Accidentally introduced to North America from Japan, HWA was first found in the eastern United States near Richmond, Virginia in the early 1950s. The pest has now been established in eleven eastern states from Georgia to Massachusetts, causing widespread mortality of hemlock trees. As of 2015, 90% of the geographic range of eastern hemlock in North America has been impacted by HWA.
CMLC has taken a lead in initiating biological control efforts to protect both Eastern and Carolina hemlocks on conservation easement properties over a multi-county area. Over the past 5 years, CMLC has released over 15,000 predator beetles on 40 different conservation properties.
Hemlock Woolly Adelgid (tiny black dots within white ovisac) on an Eastern hemlock in Henderson County
BIOLOGICAL CONTROL of HWA:
Biological control of an invasive insect pest, such as Hemlock Woolly Adelgid, involves establishing a predator population that can consume and reproduce on this pest, so as to reduce its population burden on our native hemlocks. Because HWA has two consecutive reproductive cycles in spring and summer, it is critical to have a summer-feeding predator that will attack HWA over both reproductive cycles (sistens and progrediens). But it would also be beneficial to add a winter-feeding HWA predator to extend this HWA predation over an entire 12 month cycle.
The foundation for CMLC's biological control program has been the summer-feeding HWA predator beetle Sasajiscymnus tsugae (aka Sasi). This beetle is the native ladybug predator for our HWA “import” from southern Japan and has been USDA-approved for release in the eastern US. Sasi has been widely released by US Forest Service on federal lands (National Parks and Forests) and state lands in the eastern US. And this predator beetle can be commercially lab-reared to provide the significant quantities of beetles needed to protect large private hemlock properties.
Sasajiscymnus tsugae predator beetles feeding on Hemlock Woolly Adelgid
The search for a winter-feeding predator beetle (with summer diapause) leads to the Laricobius genus. The native Laricobius rubidus has proven ineffective as an HWA predator. And so the current candidates for use as winter HWA predator are two non-native species: Laricobius nigrinus from the Pacific NW of North America and Laricobius osakensis from Southern Japan. Each of these beetles is a specialist adelgid predator with a summer (May to October) diapause. However, neither can be reliably lab-reared on a commercial scale. And as a result, these winter-feeding predators are not readily available for release on private properties.
WHAT IS HEMLOCK RESTORATION?
Hemlock restoration is the process of restoring hemlocks and hemlock habitats to health and long-term recovery. The mechanism for this process is biological control of the invasive insect – Hemlock Woolly Adelgid (HWA) using a USDA-approved predator beetle (Sasajiscymnus tsugae, aka Sasi) that is the native HWA predator for this insect in its Japanese homeland.
Hemlock Restoration, which begins with production of new hemlock growth on HWA-damaged hemlocks, will be clearly evident on some trees within 1-3 years after predator beetle releases. And this restoration process will continue for surviving hemlocks until a biological balance between HWA and Sasi is achieved. Note that long-term biological control requires continued HWA presence, for predator beetle feeding and reproduction. And so applying insecticides after a predator beetle release are both wasteful of your resources and counterproductive for hemlock health and restoration.
Sarah Fraser releases Sasi beetles on a young eastern hemlock
In 2007 hemlocks in a portion of CMLC’s Florence Preserve were treated with imidacloprid, a systemic insecticide approved for use on HWA. In 2010 CMLC volunteer Patrick Horan donated 1,200 Sasajiscymnus tsugae predator beetles for release on these same trees at the Florence Preserve (see green dot on map below). Converting from insecticide treatments to biological control gave CMLC an opportunity to test an integrated pest management approach to controlling HWA. While some hemlocks at Florence have succumbed to HWA, there are also trees that are surviving with HWA present. We continue to monitor the health of this stand.
In 2012, CMLC Stewardship Director Julia Brockman initiated a program to release 500 Sasi predator beetles and 400 Laricobius nigrinus larvae to help protect an old-growth Carolina hemlock colony on Weed Patch Mountain, a large CMLC conservation property in Hickory Nut Gorge. In 2013, her successor, Sarah Fraser, followed up on this initial biological control effort by releasing 2000 Sasi predator beetles on additional Carolina hemlock colonies, located on this same property and several adjoining properties (yellow dots).
Sarah expanded CMLC’s 2014 campaign to get predator beetles distributed to conservation properties throughout the Hickory Nut Gorge (blue dots). CMLC’s newly acquired Wildcat Rock, an inaccessible Carolina hemlock rock bluff, required four separate expeditions with CMLC staff and volunteers to get 2500 Sasi predator beetles out to all the Carolina hemlocks.
In 2015, private CMLC conservation easement landowners with significant hemlock stands were invited to participate in Sasi predator beetle releases, resulting in a release of over 6,000 predator beetles across five counties (red dots), including several Carolina hemlock sites. To date, CMLC has released over 15,000 predator beetles on 40 different conservation properties.The map below shows locations of CMLC’s Sasajiscymnus tsugae beetle releases from 2010 – 2015.
SAVING CAROLINA HEMLOCKS IN THE HICKORY NUT GORGE:
The Hickory Nut Gorge is one of Carolina Mountain Land Conservancy’s three major environmental focus areas. And it is also home to one of the largest concentrations of Carolina Hemlock (Tsuga caroliniana) habitats in the US. As indicated in the map below, a majority of known Carolina Hemlock habitats are located in western North Carolina. And a significant portion of those Carolina hemlock habitats are situated in the Hickory Nut Gorge.
The Carolina hemlock is not a close relative of the more familiar and widely distributed Eastern or Canadian hemlock (Tsuga canadensis). It exhibits different size and growth characteristics, as well as distinctive foliage (below). And over its limited geographical range, the Carolina hemlock occupies and anchors a very different type of hemlock habitat and ecosystem.
US Distribution of the Carolina Hemlock Distinctive foliage of the Carolina hemlock
Like the Carolina hemlocks themselves, Carolina hemlock habitats are special! These hemlocks are typically found on remote, elevated rocky bluffs. Such locations may delay their exposure to the HWA avalanche that has swept through lower elevations. But without biological control measures, Carolina hemlocks are still vulnerable to damage and destruction by HWA. So CMLC’s HWA control efforts for Carolina Hemlock habitats in the Hickory Nut Gorge is intended to help protect this reclusive hemlock species in its native elevated habitats and ecosystems.
A Typical Top-of-the-Rock Habitat Area for our native Carolina Hemlocks
WHERE WE ARE NOW:
In the fall of 2015, CMLC was awarded a grant from the Asheville-based Hemlock Restoration Initiative (HRI). This award will be used to support CMLC’s Hemlock restoration efforts, helping us assess health of hemlock stands on conserved properties and identify appropriate sites for establishing a field insectary for Laricobius predator beetles. We’ll be working with HRI and other grant recipients to locate sources of Laricobius nigrinius beetles to complement our Sasi releases. Throughout 2016, the CMLC stewardship team will be distributing Sasi beetles to several landowners throughout our focus region. We will be recording data to monitor the progress and assess the success of biological control. Stay tuned!