Bronze birch borers are known to attack all native and introduced birch species, although birch susceptibility varies. Generally, the white-barked birches are more susceptible than those without white bark such as river birch, sweet birch and yellow birch. The most serious targets of the Bronze Birch Borer are the European white birch, Betula pendula and the Whitebarked Himalayan birch, Betula jacquemontii. In the United States, it is distributed from Maine, across the Great Lakes region to the Cascade Mountains of Oregon and Washington, and from Maryland to Utah.
The bronze birch borer (order Coleoptera) is a bronzy, iridescent beetle whose larvae can kill birch trees. During its larval stage it attacks paper, yellow, grey, water, and sweet birch and beech trees. The cutleaf weeping birch variety trees are particularly susceptible. Weakened birch trees, such as those suffering birch leafminer damage, are more vulnerable to the bronze birch borer than are healthy trees. Once a branch has been fully girdled by a borer, nutrients are cut off and it dies above the tunneled portion.
Wilting and dying of upper parts of the tree are usually the first indications that a tree is infested with borers. Bronze birch borers are primarily phloem or inner bark borers. Removal of the bark from infested trees reveals irregular, winding, sawdust-packed tunnels, or galleries. The damage interferes with a tree's ability to transport food and water from the soil. Extensive feeding and tunneling girdles the tree, killing the areas above the infestation.
Close examination will reveal ridges and bumps on the limbs and branches as well as occasional D-shaped holes in the bark. If you slice one of these ridges open, the tunneling of the borer should be evidently just beneath the bark.
Life Cycle of Bronze Birch Borer
Bronze birch borers spend the winter as larvae just under the bark of trees. Mature larvae are about 1 inch long, white in color, and rather slender and flat. There is a slight enlargement directly behind the head, and two brownish spines extend from the rear end.
In spring, larvae pupate in their boat-shaped cells in the tree. Adults emerge during late May or early June through characteristic. D-shaped exit holes in the bark. Adults are attractive, greenish-bronze beetles. They are about 3/8 inch long with blunt heads and pointed bodies. They live approximately three weeks.
Adult borers, active during June and may be seen feeding on the foliage of willow, poplar, aspen, and birch, and cause little damage. After feeding and mating, the females look for cracks and breaks in the bark of suitable trees in which to lay eggs. The eggs hatch in a few days and the young larvae chew into the tree, working their way just under the bark. Depending on tree vigor and weather conditions, the life cycle may take one or two years to complete.