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University
of Rhode Island GreenShare Factsheets
Bronze
Birch Borer Agrilus
anxius
Coleoptera: Buprestidae
The bronze
birch borer is a native insect which occurs throughout the range
of birch in North America. It is a serious pest of ornamental birch
plantings. The
bronze birch borer is a slender, olive to copper-bronze colored
beetle, 6-12 mm (1/4-1/2 inch) long. Females are slightly larger
than males. The larvae are white, slender, and legless with a light-brown
head capsule that is retracted somewhat into a wide, flattened first-thoracic
segment. The remaining body segments are smaller and ribbon-like
in appearance. Two brown, hardened, pincer-like structures are located
at the tip of the abdomen. Mature larvae may be up to 12 mm (1/2
inch) long. The
bronze birch borer attacks paper (canoe), European white (especially
cutleaf variety), gray, yellow, and other birches, as well as poplar,
cottonwood and willow. The larva bores in the trunk and larger limbs,
often girdling them. Trees weakened by drought or injured are most
susceptible to attack.Tree injury is caused by larval tunneling
in the inner bark or cambium. The girdling of the trunk or branches
interrupts sap flow downward to the roots and destroys the tree's
cambium tissue. The interruption and subsequent accumulation of
sap flow above larval tunnels often causes characteristic swollen
bands or vein-like ridges in trunks and affected branches. The galleries
range to approximately 1.5 meters (5 feet) in length and are packed
with a dark brown sawdust-like frass. The first sign of
bronze birch borer attack is usually a die-back of the uppermost
branches, followed by a gradual decline and eventual
death of the entire tree in two to three years. The presence of "D" shaped
adult-emergence holes in the tree trunk is a positive sign of borer
activity. Attack usually begins in crown branches
19 to 25 mm (../images/4 to 1 inch) in diameter, progressing downward
year after year. In healthy, vigorous trees, the larvae do not
complete
development and the galleries heal over. Adults
begin to emerge through "D" shaped holes in the bark in early to
mid-June and may continue for five to six weeks. They feed on the
foliage of alder, aspen, birch, willow or poplar for at least a
week before beginning to lay eggs in bark crevices or beneath loose
bark. Like other flatheaded borers, the bronze birch borer is attracted
to the sunny side of trees for feeding and egg laying (oviposition).
Areas of recent injury, mechanical or other, appear most attractive
to the borer. Females lay up to 76 eggs, which hatch in two weeks.
The larvae bore into the inner bark (phloem) and cambium to feed.
Mature larvae construct oblong cells in the thick bark or wood
in
the fall and pupate the following spring. There is only one generation
per year.
Cultural: Avoid planting susceptible (white-barked) birch
species. European white birch and its cutleaf cultivar are most
susceptible to attack. River birch, Betula nigra, has salmon-pink
bark and is resistant to the bronze birch borer. 'Heritage' is a
relatively white-barked cultivar of river birch which is readily
available in clump form and as single stems. The U.S. National Arboretum
and others have breeding programs to develop resistant birches with
desirable landscape characteristics. Improper site
selection for planting birch trees often contributes to the borer
attack and death of the trees. Under natural conditions, birch grows
best in cool, moist, shaded situations. It is not adapted to open,
sunny, exposed locations, such as the middle of a large, open yard
or the exposed south or west side of a building. Trees planted in
such sites will lose vigor and become weakened, increasing their
susceptibility to borer establishment. Adult borers also prefer
to lay eggs on trees in full sunlight. It is possible
to prolong the life of susceptible trees by reducing stress. Deep
watering may be helpful during dry periods. Trunk injury by lawn
mowers, etc. can also weaken the tree and provides an attractive
bronze birch borer oviposition site. Mulching around the tree base
eliminates the need for close mowing and potential trunk injury
and retains soil moisture. Bronze birch
borer-infested firewood should be burned before May to stop adults
from emerging. Chemical:
Chemical control is difficult once larvae have bored into the trunk.
Use foliar and bark insecticide sprays to control egg-laying adults
or newly-hatched larvae before they enter the bark. Apply the first
spray thoroughly to the bark and foliage in early June when Kousa
dogwood blooms. Since the adult flight period may last for six weeks
or more, two additional sprays at 3-week intervals are suggested.
Most effective insecticides for the bronze birch borer are restricted
to professional applicators. Homeowners may find permethrin available
at some garden centers. Adapted
from the University of Vermont Cooperative Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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