Why Are the Hemlocks Dying?
You might not be able to pinpoint any identifying details of the Eastern Hemlock. But you’ve definitely seen them because they’re all over the Shenandoah Valley and the Blue Ridge Mountains, and they range into the deep South, way up into eastern Canada, and as far west as Wisconsin. And chances are if you’ve seen one recently, it’s dead. Or in the process of dying. WMRA’s Jessie Knadler has the story.
The Eastern hemlock is one of those trees that are so ubiquitous you don’t even notice it. It’s a tall evergreen with soft, silvery green needles and teeny tiny pinecones. They can grow up to 150 feet high and live up to 800 years. They’re a mainstay of Eastern forests and they’ve been dubbed the “Redwoods of the East.”
JENNIFER HUGHES: It’s an absolutely irreplaceable tree in our native ecosystem. There is no other tree that can step up to the plate and do what this tree does in this area.
HUGHES: I can say for a fact – I have a background as a park ranger, I worked in Shenandoah National Park – and over 80 percent of the hemlocks just in Shenandoah National Park are gone because of the adelgid.
The hemlock wooly adelgid -- or, for all you bug nerds out there, the HWA -- first came to the Eastern United States from Japan in the early 20th century and has run amok ever since because it really doesn’t have any native predators in this part of the country.
Adelgids feed on the tree’s vascular system. They basically starve the tree, robbing it of its nutrients. As it’s being slowly killed, the tree sends out a distress signal, which in a cruel twist of irony, only draws more adelgids
Hughes takes me to a small row of hemlocks between two parking lots near the area food bank in Lexington to demonstrate an infestation. She flips over a branch. It’s covered in small white woolly balls the size of peppercorns.
HUGHES: …You don’t need to get too close to notice this white cotton-y, fluffy substance on it, and that’s the adelgid.
To save this mighty tree from extinction, chemical controls have been used. Imidacloprid is a commonly used insecticide that kills the adelgid but is very toxic to honeybees and other beneficial insects. And it can allow mites to take hold.*
Hughes has recently begun using an organic, non-petroleum based horticultural oil to treat diseased trees of Lexington. She shows me another row of hemlocks at the back of Hopkins Green that have been treated with the oil. The needles look great. There’s no white, cotton-y adelgid residue.
HUGHES: You can see they’re doing a lot better. There’s no signs of infestation, a little bit of bronzing.
The problem with the organic oil is that it’s expensive -- $80 per tree, depending on its size, and it needs to be used every year by a certified tree care expert.
There is a third option, one that Jennifer is very hopeful about, but it’s still a way’s off: the beetle.
Dr. Scott Salom is an entomologist at Virginia Tech and he and his team have been studying the adelgid for twenty years trying to establish some kind of biological control—meaning, something to eat it. Researchers found one. The Laricobius nigrinus was found in the Western United States. Since 2003, more than 200,000 of these beetles have been put into the wild to try to save the hemlock.
The problem is, you can’t just release a bunch of beetles into a new habitat and hope they do what you want them to do.
SCOTT SALOM: It’s a very, very difficult effort and project because there’s so much variability that occurs in a natural forest setting.
Extreme winter temperatures kill off the beetle. They kill off the adelgid too. But it makes tracking success of this project very difficult. It’s also really hard pinpointing the right beetle to adelgid ratio.
SALOM: You can’t build up a population of beetles fast enough to control the adelgid once [their numbers] get high….If we could rear them like we could rear mosquitos or fruit flies, or something like that…then it would be possible. But in our system we can’t produce enough beetles to do that so what we do is inoculate them into locations and give them time for populations to build up and eventually they will control but because the trees can decline more quickly than the populations are allowed to build up we are proposing to integrate the use of chemical and biological controls.
In other words, it might be a few years before the beetles can be used in any kind of large-scale municipal setting. Until then, it’s a mad scramble to save the hemlock.
SALOM: You just can’t watch them die. If your job is to manage and protect trees, you can’t just watch these things die.
*An earlier version of this story incorrectly reported that imidacloprid is an antibiotic. It is actually an insecticide. The story now reflects this correction.