The bronze birch borer is a greenish-bronze beetle 1/4 to 1/2 inch
long with a rather blunt head and a slender pointed body. The larvae
are slender white grubs about 3/4 of an inch long and have a slight
brownish enlargement of the body just behind the head.
In May and June, the adult female lays tiny white eggs, one
at a time, either in cracks in the bark or in crevices made
by her jaws. The eggs hatch in about 10 days, and the larvae
bore into the bark. The larvae remain active until fall, forming
burrows 4 or 5 feet long. The burrows are very crooked and
are filled with frass. They cross and recross, often completely
cutting off the circulation of the sap. The larvae overwinter
in cells constructed at the end of the tunnels. The following
spring, the larvae pupate and emerge as adults through small
D-shaped holes about 1/8 inch in diameter that are cut in
the bark. There is only one generation a year.
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Figure 1. Bronze Birch Borer Adult
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Figure 2. Bronze Birch Borer Damage to Birch
The injury to birch is caused by the larvae tunneling just
under the bark. This tunneling creates feeding galleries that,
in effect, girdle the trunk or branch. The initial result
is a yellowing and thinning of the leaves in the upper crown.
If not checked, the infestation gradually will move downward.
In addition, adventitious growth develops in the lower crown
occasionally, causing twigs to die back in the upper crown.
This is followed by the death of the entire tree.
Signs Of An Infestation
- A yellowing and thinning of leaves in the upper crown.
- Raised bumps and D-shaped holes on upper limbs and the trunk.
- The presence of galleries and tunnels just under the bark.
- Dead and broken branches in the upper crown.
Prevention is the key in preventing borer infestations. Keep plant
material healthy and vigorous by proper watering, mulching, and
fertilizing. The whitespire birch, river birch, and Heritage birch
appear to be less susceptible to borer attacks.
Insecticidal sprays may be applied in early summer when adults are
emerging with a repeat spray three weeks later