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Hemlock Preservation at Biltmore Estate

In 2001, hemlock woolly adelgid outbreaks started impacting Western North Carolina. At Biltmore, exhaustive resource assessments were conducted in order to decide which hemlocks were the most important to begin treating and which would be left untreated. The early outbreak years were a time of difficult decisions, knowing that we were not going to save all the hemlocks. Treatment with neonicotinoid products was (and still is) the primary tool on the frontlines for saving hemlocks. Like any other tree, hemlocks need to be evaluated for structural soundness and health; they fail in storms and have to withstand the forces of nature.  Now add to this the hemlock woolly adelgid (HWA) that our Eastern and Carolina Hemlock did not evolve with and are defenseless against, and we have trees that are now on a chemical-dependent life support system. Although rewarding and very worthwhile, this adds significantly to the overall tree management regimen. At Biltmore we have ended up with around 1000 hemlocks on “life support”. Tending our hemlocks has become just another part of our management program. We have been able to stretch our chemical treatment intervals, through monitoring, from 3 to 5 to even 6 or 7years. Now it’s the year 2015. For the past 14 years, we have managed to keep our core hemlock population alive and in decent condition. So this raises a question for many of us on the frontlines of hemlock management: how long will we be dependent on these chemicals until we decide it’s time to stop injecting neonicotinoids, or let the tree go? Is this the only way?  

When HWA was discovered at Biltmore, we knew what the options were. Chemical control would guarantee protection at the landscape level. Predatory beetles looked like a great option, and certainly a great way to go, but the research results were not at all promising to keep pace with the overwhelming HWA outbreak we were experiencing. Research toward a biocontrol solution with Sasajiscymnus tsugae and Laricobius nigrinus was well underway in the late 1990’s and early 2000’s. For practical purposes however, applying the chemical was and remains the sure way to go.   

In recent years the research with L. nigrinus is showing that populations are establishing in the release areas, and this is promising compared to previous results.  L.nigrinus feeds nearly exclusively on the hemlock woolly adelgid, is native to the Pacific Northwest, and recent research indicates that Ln has established itself in release areas.  A hypothesis could be that it may just be a matter of time, perhaps decades or more, until the predator will have measurable impact against the HWA. That being the case, we really need to start somewhere to establish the predator in place. Proceeding and investing with cautious optimism is how I describe this effort. In October of 2013 we released 850 L. nigrinus into an area with hemlocks that have not been treated and are still alive and infested with HWA. The beetles were shipped overnight from the Pacific Northwest and released within the hour of their arrival at our facility.  We returned to the release site one year and two months later in early December 2014 to see if we could recover any beetles by using a beat sheet sampling method. Not expecting to find any, I was very encouraged when our group found 6 L.nigrinus in about 15 minutes. Entomologist Dr. Richard McDonald was on site and confirmed that the recovered beetles were indeed L. nigrinus, and not our native Laricobius rubidus. This demonstrated that our beetles had survived the winter and were still on site. And if they are here, they must be eating hemlock woolly adelgid- right?  

Reliance on the predatory beetle as a control method is a very long way off, and there is concern from biologists about possible Ln hybridization with the eastern native Laricobius rubidus species reducing prey specificity. That being said I remain cautiously optimistic that a biocontrol solution may be establishing on our site. Is it possible that due to our efforts there may be a day in the distant future where the predator-prey balance may exist? A strategy for transitioning to biocontrol is an ongoing consideration.  We should be doing everything that makes sense to protect the hemlocks from the devastating impact of this destructive pest. We will continue to closely monitor and evaluate our hemlock population and maintain our established preservation program. We will continue to monitor the release sites and through results-driven decision making we may expand our efforts to establish biocontrol in the ongoing fight against hemlock woolly adelgid. Establishing these predators against HWA might be likened to planting an acorn; we will likely not be around to see the giant oak, but future generations will get to appreciate it.