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P R O V I D I N G T E C H N O L O G Y F O R F O R E S T H E A LT H P R O T E C T I O N
Hemlock Woolly Adelgid Biological Control
Hemlock woolly adelgid, Adelges tsugae Annand (HWA)
(Fig.1) is native to Asia and western North America.
HWA was first discovered in the eastern United States
(U.S.) in 1951 near Richmond, VA. It is believed to have
originated in southern Japan and introduced on nursery
stock. In the eastern U.S. HWA attacks two species of
hemlock native to eastern North America: eastern
hemlock Tsuga canadensis (L.) Carr., and Carolina
hemlock, T. caroliniana Engelm. In the late 1980s, HWA
began to spread rapidly at a rate about 20-30 km each
year and its population levels increased dramatically
to 18 eastern states, from Maine to Georgia (Fig. 2).
HWA is now a serious threat to the survival of hemlock
in eastern forests. Wind, birds, deer, and humans help
spread HWA throughout the Northeast, where hemlock
dominated forests encompass approximately 2.3
million acres. In the urban environment, hemlocks are
one of the most common backyard species planted by
homeowners. Eastern hemlock has a broad distribution
from New Brunswick in the north, to Alabama in the
south, and west to Minnesota, with isolated disjunctive
populations to the south and west of its main range.
Carolina hemlock has a very limited distribution and
with isolated pockets in Virginia, North Carolina, South
Carolina, and Georgia.
Figure 2. HWA distribution in 1991 (left) 2013 (right).
Figure 1. Micrograph of HWA nymph (left), white ovisacs (right).
Damage and Importance
HWA is a destructive pest of forest and ornamental
hemlock trees in the eastern U.S. Feeding by HWA
causes the needles on infested branches to desiccate,
turn a grayish-green color, and then drop from
the tree. Little, if any new growth is produced on
heavily infested branches and most buds are also
killed. Dieback of major limbs can occur within two
years and typically progresses from the bottom of
the tree upward. Trees may die within four years,
but some survive longer with only a sparse amount
of foliage at the very top of the crown. Since all
age classes of hemlock are vulnerable, HWA poses
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P R O V I D I N G T E C H N O L O G Y F O R F O R E S T H E A LT H P R O T E C T I O N
a significant threat to hemlock health, including
the reestablishment of hemlock stands. There are
no practical means available to manage HWA in
urban and forest situations therefore, long-range
management strategies are needed.
Biological Control
Biological control began in the early 1990s. Surveys
in the eastern U.S. indicated that the native natural
enemies associated with HWA consisted primarily of
generalist predators which did not effectively control
HWA populations. In Japan, where HWA is native, it
is a common but innocuous inhabitant of forest and
ornamental hemlock and spruce. Japanese hemlocks
are not significantly injured because of host resistance
and natural enemies. For this reason, early efforts since
1996 have focused primarily on the development of a
multi-agency biological control program as a method
to lessen the destruction by HWA in the eastern U.S.
Both Japan and China have been explored for natural
enemies. In 2003, a formal HWA initiative was begun
by the USDA Forest Service with the development of a
5-year program (2003-2007) to create and implement
management options to reduce the spread and impact
of HWA. A second 5-year program (2008-2012) was
funded to continue and to accelerate the development
of research and technology, management, and
information transfer. The biological control effort
has been greatly expanded with the onset of the
HWA Initiative and has involved 28 federal and state
agencies, 24 universities, and numerous private
industries in the U.S., as well as, 7 institutions in China
and Japan. Since no known parasitoids are associated
with the Adelgidae, the biological control program for
HWA has focused on prey-specific predator species
collected from East Asia and the Pacific Northwest of
the U.S., as well as entomopathogens.
The most widely released predator has been a
ladybeetle, Sasajiscymnus (=Pseudoscymnus) tsugae. It
was discovered in early 1990s in Japan. Since the first
release of adults in Connecticut in 1995, 2.5 million
adults were released into 15 states from the southern
Appalachians to Maine at more than 400 sites.
Although populations have established at a number
of sites, post-release recoveries of S. tsugae have
been sporadic and usually at low numbers. Recent
recoveries of S. tsugae in the Great Smoky Mountains
National Park were associated with the oldest release
sites, suggesting that this predator may take 5-7 years
or more to reach detectable levels in the landscape.
If this is true, many hemlock stands in the southern
Appalachians may become infested, decline, and die
before S. tsugae populations can begin to have an
impact, which may already be occurring in some areas
of the southern Appalachians. Over 30 generations of
S. tsugae ladybeetles have been lab reared from the
original genetic stock and released. A new genetic
stock of S. tsugae was recently collected in Japan and
mass reared at the North Carolina Department of
Agriculture and University of Tennessee. The original
genetic stock of S. tsugae has been phased out of the
overall program with the initiation of field releases of
the new genetic stock of S. tsugae.
From 1995 - 1998, over 50 species of
ladybeetles were collected from China, 21 of which
were new to science. Three species of Scymnus, S.
camptodromus, S. sinuanodulus, and S. ningshanensis,
the most abundant ladybeetles feeding on HWA in
China were investigated. Over 30,000 S. sinuanodulus
beetles were released in eight states from 2005-2009,
mostly in southern states where it is believed to be a
better climate match. However, this predator provided
no evidence of becoming established and was since
abandoned. Likewise, two small experimental release
of S. ningshanensis occurred in North Carolina, but
was also abandoned because of rearing difficulties
and lack of establishment.
In the late 1990s, Laricobius nigrinus beetles
were collected from British Columbia, and successfully
established on HWA infested hemlock branches in
quarantine facility at Virginia Tech. It was approved
for release in 2000, and the first release was in 2003.
Additional surveys and collections in Washington
and Oregon resulted in the shipment of thousands
of L. nigrinus (Seattle biotype). Over 250,000 have
been released and are now becoming widely
established in plant hardiness zones 6a and 6b. These
establishments span 11 states from the southern
Appalachians to New England and have become
the first potential biological control agent of HWA to
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establish in such numbers that beetles can be fieldcollected and redistributed. Since 2007, L. nigrinus
(Inland type) were collected from northern Idaho
and northwest Montana, and released at a limited
number of sites in New England. Small numbers of
this biotype have since been recovered, indicating its
adaptability to the more northerly climate and colder
plant hardiness zones.
A related species, Laricobius osakensis, was
collected in Japan in 2005 and approved for release
from quarantine in 2010. Extended studies in Japan
determined that L. osakensis was the most abundant
and frequently encountered predator of HWA in
Japan, sharing many similarities to L. nigrinus in
western North America. Study of the life histories
of L. osakensis, HWA, and their interaction in Japan
indicated a good synchrony between L. osakensis
and HWA in the native habitat. L. osakensis has great
potential as a biological control agent of HWA as
the HWA found in the eastern U.S. originated from
Japan. Because L. osakensis occurs in a broad range
of climates in its native range, it should be well
adapted to both southern and northern climates in
the eastern U.S. L. osakensis was first released in 2012
and is currently being reared a VA Tech and University
of TN.
Since mid-2000, one survey in China and
several surveys in the eastern U.S. were conducted
for entomopathogens of HWA.
One fungus,
Lecanicillium muscarium (=Verticillium lecanii ) was
recovered from HWA populations in the East and it
has been investigated. Laboratory, ground, and aerial
application trials to document its efficacy on HWA
have been promising but the rapid degradation of
the fungus once applied remains a challenge. The
commercially available formulation of the fungus
Mycotal® (Koppert Biological Systems, a Netherlandsbased company) along with MycoMax, a whey
byproduct, has been added to the tank mix in an effort
to stimulate fungal growth and spore production
FO R MORE I NFORMATION CONTAC T: Richard Reardon
Biological Control Program Manager
USDA Forest Service
180 Canfield Street
Morgantown, WV 26505
in the field. Meanwhile, registration of Mycotal® in
the U.S. is being considered by Koppert pending
completion of successful field trial demonstrations.
Outlook
High mortality of hemlocks in the eastern U.S. has been
attributed to a combination of host tree susceptibility
and lack of effective natural enemies of HWA.
Significant effort has focused on classical biological
control to increase the population of predators of
HWA. In the northern part of the HWA range, the
establishment of the predators, along with periodic
HWA population reductions due to cold winter
temperature may provide the additional mortality
necessary to effectively suppress HWA populations
below damaging levels. In the southern part of HWA
range, due to lack of the cold temperature that reduce
HWA populations exceptional high mortality from
natural enemies may be needed to maintain HWA
populations at innocuous level. Rearing and releasing
of predators should be continued to promote their
natural spread. The number of field insectaries
should be expanded to allow increased harvesting
and redistribution of the climate-adapted “wild”
individuals. A biological control program integrated
into management programs with early detection
surveys, monitoring of pest density throughout the
management effort, and a combination of control
methods consistently applied through time will
be necessary to obtain management objectives. A
strategy of applying insecticide on a select number
of large higher value hemlocks, and at the same time
releasing and allowing the predators to become
established on understory and non-treated trees
is urgent for management of HWA in the southern
U.S. This idea is that by protecting the health of and
reducing HWA population density on the larger higher
value hemlock trees, predators will have more time to
populate sufficiently to offer long-term suppression
of HWA populations.
Phone: 304-285-1566
Fax: 304-285-1564
E-mail: rreardon@fs.fed.us