Impact. Dieback of major
limbs can occur within two years and progresses from the bottom
upward.
Damage. Needles on
infested branches dry up
and drop.
Infestations. Native
range of hemlock (green) and range of hemlock woolly adelgid
(red) in March 2002. They have shown up in
more places since then.
Nymphs. They settle at
the base of a hemlock needle, where they usually remain for the
rest of their life.
Predator.
Tiny but voracious, both the larva and adult (shown here) of
Pseudoscymnus tsugae attack all stages of the adelgid.
Eggs.
An easy way to identify infested trees, the woolly ovisacs can
contain up to 300 eggs.
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Sixty-seven years ago, a
Depression-era make-work project was launched near the
Virginia/North Carolina state line that would ultimately link
Shenandoah and Great Smoky Mountains national parks. More than a
half century elapsed before the 469-mile Blue Ridge Parkway was
completed. It’s taken less than a third of that time – a mere 14
years – for the two parks and the scenic corridor between them
to be connected in a far less happy way. In May, a minute,
non-native, sucking insect that has destroyed most of
Shenandoah’s hemlock stands was discovered for the first time in
the Smokies. Simultaneously, parkway personnel found hemlock
woolly adelgid infestations along a new 40-mile stretch of the
parkway in North Carolina – from Deep Gap to Linville Falls.
The arrival of the hemlock woolly adelgid in the Smokies is “a
major ecological disaster,” says GSMNP supervisory forester Kris
Johnson. “We have 5,000 acres in which hemlock is the dominant
tree – over 50 percent of the forest – and many more where it’s
a major forest component. The impact of the adelgid in this park
will be the equivalent to the chestnut blight, visually and
ecologically. I don’t think people realize how bad it’s going to
be. They should be taking their children to see old growth
hemlock now – while they still have a chance.”
“It was a dark, dark day,” says parkway resource management
specialist Lillian McElrath, of her discovery of adelgids
“pretty much everywhere” at Linville Falls: in the picnic area,
along the river, around the visitor center, on trails to
overlooks, and in Linville Gorge. “I think we’ll be seeing tree
death at Linville Falls within the next two years. Once the
adelgid hits, it’s a pretty quick thing.”
It was certainly quick in Shenandoah, where the adelgid was
first discovered in 1988. Between 1990 and 1994, the percentage
of hemlocks with crowns in good health nose-dived from 77
percent to 1 percent. A cold, snowy winter in 1995-96 allowed
some trees to recover temporarily, but drought and a string of
warm winters since 1999 erased that gain. Today, says
Shenandoah’s forest ecologist James Akerson, “you can walk into
every hemlock stand in the park and see gray crowns. At higher
elevations, there are still some needles attached. But at lower
elevations, most of the needles have fallen. All you have left
are dead sentinels. Along our very popular lower trails, like
White Oak Canyon, you have virtually no live hemlocks. Five
years ago, I predicted the park would lose its hemlock stands
within the next five to 10 years. Indeed, the loss has occurred
at the shorter end of that. I take no comfort in seeing my
prediction come true.”
To think that an insect the size of a pinhead can kill a
600-year-old hemlock in the space of a half-dozen years is
breathtaking. But the hemlock woolly adelgid is a formidable
pest, with a complicated biology, and a dizzying ability to
reproduce. It’s parthenogenetic, which means that all
individuals are female, so don’t have to find mates. Each
adelgid can lay 300 eggs; there are multiple generations a year.
With their capacity for exponential growth, offspring of a
single adelgid can kill a hemlock in as little as four years if
the tree is stressed by drought or other factors, in six to
eight years if it’s not.
A Tragedy in the Making
The hemlock woolly adelgid is native to Asia, but has been in
the United States for 80 years. It first showed up in the
Pacific Northwest, probably imported on infested nursery stock
from Japan. No one worried much about it there, because western
and mountain hemlocks, like Asian species, showed resistance to
it and suffered little mortality. In the 1950s, it turned up in
Richmond, Va. No one paid much attention to it there either,
because it was infesting hemlocks in landscape settings easily
treated with pesticides. Trees that died were simply replaced.
It wasn’t until the adelgid – carried by prevailing winds,
hurricanes, birds and mammals – reached the great natural
hemlock stands of Eastern America that the scope of the problem
emerged. Like other exotic pests (gypsy moths, for example), the
diseases and predators the adelgid co-evolved with had been left
behind in its native land. With no natural controls – and a huge
new food source to exploit – its population exploded.
Tragically, eastern and Carolina hemlocks – a rare hemlock
species that occurs only in isolated pockets in Virginia, North
and South Carolina, and Georgia – showed no resistance to
adelgids. Trees began dying – fast.
“The adelgid hit the Blue Ridge in the early 1980s,” says
USDA-Forest Service entomologist Rusty Rhea, a member of the
Hemlock Woolly Adelgid Working Group, an ad hoc coalition of
university, state, federal, private and corporate researchers
working on adelgid control. “By the mid-’80s, it had moved down
into the western side of the watershed. By the early ’90s, there
was significant mortality. The damage it does is highly variable
within a stand. It picks off one tree, then another. But over
time, I’ve watched it move down a watershed until all the
hemlocks – every one, large and small – are gone. Gray ghosts in
the landscape.”
In the half-century since they appeared in Richmond, adelgids
have impacted hemlocks in 15 states, from Georgia to New
Hampshire. More than half the hemlock’s natural range in the
East is now infested; the entire range is at risk. The adelgid
is showing up in new places so rapidly now that any story
(including this one) is soon out of date. The Smokies news
office issued a press release May 16 announcing discovery of
adelgids in two park locations; two weeks later, that number had
jumped to eight. Included on the expanded list were Stony
Branch, Laurel Falls, Greenbrier and Cataloochee – all areas
harboring old growth hemlock stands.
Hitching Rides
Remarkably, the adelgids popping up everywhere aren’t very
mobile. They can’t fly. Once they settle in at the base of a
hemlock needle and insert their feeding tubes into the plant
tissue, they’re there for life. They kill their host by sucking
its nutrients, literally starving the tree to death. Typically,
it takes adelgids a year or two to colonize a tree. After
hosting an established population for three years, a tree puts
out little or no new growth. The adelgid population crashes. The
tree recovers enough to put out a little new growth. The
remaining adelgids recolonize, and the tree soon dies. Adelgid
eggs and nymphal stages are concealed in white cottony tufts.
This “wool” is the visible sign that a tree is infested. It’s
most noticeable in spring, wearing away as the seasons progress.
To move from their natal tree, adelgids have to hitch rides.
Most often, they’re carried by birds. Adelgids go through a
highly mobile crawler stage that coincides with the spring
migration of neotropical songbirds, who ply hemlocks for hidden
insects to fuel their journey north. “The birds eat a few
adelgids,” Johnson says, “and get them all over their feet.”
Flitting from tree to tree, they deposit crawlers as they go.
Crawler-stage adelgids can survive 10-15 days off their hemlock
host, which helps explain why adelgid infestations have been
moving inexorably northward year after year.
The adelgid’s movement south has occurred much more slowly. It
wasn’t until 1996 that it reached North Carolina’s northernmost
counties. Then, last spring, isolated adelgid infestations were
discovered in Yancey, Macon and Graham counties. “People have
imported nursery stock from infested areas, probably from
Virginia,” Rhea says flatly.
“Commerce transplanted the adelgids
here. It’s likely that the whole Southern Appalachians will be
infested within five or six years.” The Yancey and Macon
infestations were relatively small, but the population in Graham
County was widespread. Damage to trees – some of them right
across Fontana Lake from the Smokies – was advanced. “As soon as
I heard that the adelgid was south and east of us,” says
Johnson, “I knew it was just a matter of time before it showed
up in the park.”
What We’re About to Lose
Eastern hemlocks, while not an abundant component of eastern
forest ecosystems, nonetheless occupy a vital ecological niche.
Many grow along streams, where their dark, dense, year-round
foliage moderates stream and ground temperatures year-round.
Hemlock stands are cooler in summer – and warmer in winter –
than hardwood forests. Stream monitoring in Shenandoah has shown
that ground not shaded by hemlocks undergoes extreme spikes of
temperature in March, before deciduous trees leaf out. “The true
significance of hemlocks’ ability to moderate temperature is in
winter and early spring,” Akerson says.
What effect the loss of the hemlock will have on native brook
trout, who are found more commonly in hemlock-shaded streams, or
on blackburnian and black-throated blue warblers, blue-headed
vireos and northern goshawks, who require hemlock forest
habitats, remains to be seen. Dozens more bird species use
hemlock as a food source, and for nesting and roost sites and
for shelter. Hemlock stands provide important cover for ruffed
grouse, deer, wild turkey, rabbits – and homes for innumerable
insects and other invertebrates, for salamanders and whole
complexes of plants.
Then there’s our loss. Though hemlock is not a valuable timber
species, it is widely used as a landscape tree (274 cultivars of
eastern hemlock have been developed), and glorious in natural
settings. In spring, bright new growth at the tip of every twig
offsets older needles’ deep and somber green. To walk beneath
old growth giants – on the way to an overlook at Linville Falls,
in Cataloochee, or in other places where remnants of the forest
primeval still exist – is to glimpse, however dimly, what
natural cathedrals our woodlands once were.
What Hope Is There for the Hemlock?
The search is under way for ways to control the adelgid. Seed
banking is planned, and biologists are looking for resistant
trees in the wild (so far, no one has found any). Researchers
hope one day to be able to breed western hemlock’s resistance
into eastern and Carolina hemlocks while maintaining as many
characteristics of the eastern species as possible.
Landscape trees can be protected using insecticide sprays,
though every needle must be thoroughly drenched annually to be
effective. Chemical control, in its current form, is impractical
to impossible in forest settings. Hemlock stands are scattered
and often inaccessible; full insecticide coverage is difficult
to achieve; many trees can’t be sprayed because of their
proximity to streams. Other chemical applications are being used
in some areas, including systemic insecticides applied through
stem injection, implantation and soil drenches. Shenandoah began
experimenting this year with an injectable systemic that may
allow protection of some large isolated trees not yet seriously
infested with adelgids.
Work is also under way on biocontrols, the most promising of
which is a tiny black coccinellid beetle (Pseudoscymnus tsugae)
discovered feeding on adelgids in Japan in 1992. Years of
quarantine and study convinced researchers that the beetle was
safe for release in the wild. (Other predatory beetles and mites
that prey on the adelgids in Japan and China are also being
studied, but haven’t been approved for release.) A lab in New
Jersey has been mass-rearing P. tsugae since 1997. Several
hundred thousand beetles have been experimentally released in
nearly a dozen states. Among the places the beetles have been
released are a few parkway locations north of Boone; in Ellicott
Rock Wilderness Area; and, in early June, in the Smokies at
Cataloochee, Laurel Falls and Panther Creek.
No one knows yet whether the beetles will reproduce in numbers
in the wild, or make a dent on the adelgid population. Even so,
demand for the beetles far outstrips supply – and will, for
years to come. “I wish I had a whole shoebox – or 10 shoeboxes –
of those beetles for Linville Falls,” McElrath says sadly. “But
I don’t.”
—Elizabeth Hunter
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