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by Jim McGlone, Urban Forest Conservationist,
Virginia Department of Forestry
(Conservation Currents,
Northern Virginia Soil and Water Conservation District, Fall
2006)
The emerald ash borer (EAB) or Agrilus planipennis
is a non-native beetle that eats and kills ash trees. EAB was
first detected in the United States in 2002 near Detroit, Michigan
and probably arrived in solid wood packing material from its
native Asia. Since then, EAB has killed millions of trees in
Michigan, Ontario, Canada, Indiana, and Ohio. EAB has also been
detected in Illinois, Maryland and Fairfax County, Virginia.
Adult emerald ash beetles lay eggs on ash bark in the spring.
 The eggs hatch into larvae that bore into the tree and eat the
cambium under the bark, leaving S-shaped channels. The larvae
pupate and overwinter under the bark or in the sapwood of the
tree. In the spring, the adults emerge leaving behind a distinctive
D-shaped exit hole in the ash’s bark to start the process
again.
Although adults do eat ash foliage, it is the damage the larvae
do to the cambium that kills the tree. The cambium is the layer
of tissue that generates additional xylem and phloem. Food moves
from the leaves to the roots through the phloem, and water and
nutrients move from the roots to the leaves through the xylem.
As the cambium is destroyed, the xylem and phloem also deteriorate.
The leaves and branches in the canopy of the tree begin to die
and, eventually, the entire tree succumbs. Infected trees are
diagnosed by the canopy die back, distinctive D-shaped exit
holes, and S-shaped channels under the bark. Other signs include
bark splitting, sprouts at the base of the tree and increased
woodpecker feeding.
Although there are reports that EAB attacks walnut and elm
species in its native range of northwest Asia, the beetle is
only known to attack ash species in the U.S. An exception is
the mountain ash, a small tree of the eastern United States
that is actually a member of the rose family. Common ashes in
Northern Virginia include the green ash, Fraxinus pennsylvanicus,
an overstory tree in stream valleys and floodplains, and the
white ash, Fraxinus americana.
EAB is thought to spread in three different ways: flight of
adult beetles; transport of infected ash timber, particularly
firewood; and transport of infected nursery stock. Authorities
are targeting all three in order to contain the EAB. Quarantines
have been placed to prevent nursery stock and ash timber from
being moved from infested areas. Unfortunately, transport continues
to expand the range of this pest.
In 2003, nursery stock infected with the emerald ash borer arrived
in a retail nursery in Prince George’s County, Maryland.
To contain the EAB, the infected trees were destroyed along
with all ash trees within one mile of the nursery. (Adult beetles
are thought to move less than a mile from the trees from which
they emerge.) Some of the infested Prince George’s County
nursery stock was, however, planted near Wolf Trap Farm Park
leading to a similar eradication effort in Fairfax County.
In Maryland and in Fairfax County, sentinel trees have been
established around these eradication zones and are being monitored
for EAB activity. Sentinel trees are created by girdling each
tree to create stress. The stressed trees give off different
chemicals and are a different color in ultraviolet light. If
present, the beetles will recognize these signs of stress and
be drawn to the sentinel trees because they are less able to
fight off infection. In 2006, EAB once again appeared at the
Maryland site, but has not been detected again in Fairfax County.
Maryland has extended the quarantine of ash products originating
in Prince George’s County. Fairfax County has increased
its monitoring for EAB by establishing more sentinel trees near
the eradication zone and along the Potomac River.
You can help stop the spread of EAB by not buying firewood
except from a known source or transporting firewood into Fairfax
County.
Photo: David Cappaert, www.forestryimages.org. |
