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Not All Birches are the Same
Recent research has presented compelling evidence
for returning white-barked birch trees to the
landscape. Many birch trees suffer damage from
the bronze birch borer, a destructive and often difficult
pest to manage. Long term research conducted by
Ohio State University has provided evidence to support
what our own observation has told us: The bronze
birch borer can be managed effectively, and white
barked birch trees can be successful in the landscape.
Bronze birch borer is a native pest which feeds primarily on vascular
tissue in the
trunk and
branches of
trees. Infested
trees usually
decline over
time, but can be
killed in just one
season. In native
forests, bronze
birch borer acts
as a secondary
insect, affecting
only previously
stressed trees
with weakened
natural defense
systems. For
example,
drought stress
has been shown
to predispose
birch to bronze
birch borer attack
Research initiated in 1979 consisted of 1,200 trees of
seven birch species, including European white birch,
Asian species, as well as three species native to our
region: paper birch, gray or white spire birch, and river
birch. The results are striking. All European and
Asian species were killed by 1999, however the native
species fared much better, particularly river birch,
exhibiting a 98% survival rate. Although river birch is
the most borer-resistant, it is not truly 'white barked',
as its exfoliating bark can be cinnamon brown to
salmon pink in color.
The high survival rate of the native species is remarkable, as it occurred in the face of several severe drought
and massive borer outbreaks over the 20-year study. The tested trees were also without the benefit of irrigation or insecticide controls. These results stand in stark
contrast to historic claims of bronze birch borer resistance in Asian species, and recent claims of resistant
European varieties.
As is often the case, proper plant health care is the key to effective insect
management. Providing ecological requirements of plants is the first critical
step in successful landscape design. Birch trees
planted in sites they are not adapted to will be
stressed, creating prime candidates for borer
attack. For example, paper birch is an ecological pioneer species requiring full
sun. Therefore, planting paper birch in the shade may cause as much stress as
long-term drought. Successful long-term survival of white-barked birch trees
is a function of matching the physical requirements to local environment and
selecting the proper species to meet the goals of the landscape design.
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