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The
bronze birch borer (Agrilus anxius) has been a serious
pest of ornamental birches for many years. The first
sign of borer attack is usually the die back of the
uppermost branches of the tree. Over the following 2 or
3 years, the tree will gradually decline and die.
Vein-like ridges or swollen bands occur in the bark and
trunk. The presence of "D" shaped holes indicates where
the adult borer emerged from the wood.
European
white birch (Betula
pendula) and its cutleaf varieties are especially
susceptible to borer attack. Paper or canoe birch (Betula
papyrifera),
yellow (Betula
alleghaniensis) and gray (Betula
populifolia) birch are less susceptible. River birch
(Betula
nigra)
appears to be resistant to borer attack.
Adult
borers emerge from infested trees in early to mid-June
and may continue to emerge for another 5 to 6 weeks. The
adult beetles are attracted to the sunny side of trees
for feeding and egg laying. They lay their eggs in
cracks and crevices and in areas that have been recently
injured in some manner. The young hatch in about two
weeks and begin to tunnel into the wood.
The
tunnels behind active feeding larvae become packed with
excrement and wood particles which turn a dark brown.
They feed through the following fall and spend the
winter in the tree. In April or May, they pupate and
emerge as adults. There is one generation per year in
the Midwest.
Resistant
Trees: Perhaps the best control is to plant
resistant birches. European white birch is extremely
susceptible to this pest. As a non-native species, this
tree often becomes stressed during the hot, dry summers. This makes them more attractive to
the borer. The native gray birch is also susceptible
when planted in urban/suburban sites.
Paper
birch seems to have fewer problems with borer. It is a
white birch species but has a slightly different growth
habit than European white birch.
River
birch is considered resistant to borer. However, it does
not have the white bark but does have a salmon-pink
papery bark. Also, this tree tends to get larger than
the white barked types. The variety, 'Heritage', has
lighter bark which more closely resembles the paper
birch.
Several
white birch species are touted as resistant to the borer
but most have not been tested widely.
Cultural
Controls: Proper fertilizing and watering can help
minimize the attack of borers. Deep watering during hot,
dry weather is very important.
Also, it
is important to keep a circle of mulch around the trunk
of trees in the landscape. This prevents damage from
lawnmowers and weed whips that can be prime entry points
for the borers.
Chemical Treatment: Chemical control is difficult,
if not impossible, once the larvae have entered the
trunk itself. Current strategy is to spray the foliage
and bark with an insecticide to control the egg laying
adults or newly hatched larvae before they enter the
bark.
Apply the
first spray to the bark and foliage about the first week
of June. Two additional sprays at 3 week intervals are
needed to cover the 6 week flying period of the adults. |
