WHAT IS IT?
The bronze birch borer (Agrilis anxius) is a species of native wood-boring beetle in the family Buprestidae of the order Coleoptera. It is also closely related to the emerald ash borer (Agrilis planipenis). Unlike the emerald ash borer that is native to Asia, the bronze birch borer is a species native to North America. Adult bronze birch borers are typically half an inch in length, and are black with a bronze coloration on their back.
As the name would suggest, the bronze birch borer feeds on birch trees. This beetle seems to be far more attracted and damaging to imported European white-barked varieties that have no natural resistance to this insect at all. However, this beetle had been in North America long prior to the introduction of the European white-barked birch. The beetle survived by attacking native white, paper, and yellow birch species that were weak or stressed. Healthy, vigorously growing native birch resist and fight against bronze birch borer invasions. However, old age and drought will eventually stress these trees, thus attracting the bronze birch borer. Native river birch are quite resistant to this beetle and are seldom affected.
When bronze birch borer eggs hatch, the larva tunnel into the birch looking for the sugary layer of phloem tissue. Unlike the emerald ash borer, which feeds in a back-and-forth serpentine pattern under the bark of ash trees, the bronze birch borer tends to circle the trunk in a girdling pattern which causes significant damage. The harm to this layer prevents the tree from being able to transport nutrients and water from the soil, or move any sugar produced in the leaves to the root system. This can quickly lead to the death of the upper portion of entire stems of infested birch trees when left untreated.
Larvae overwinter under the bark, and then fully developed larvae begin pupation in the xylem tissue in late April to early May. Adults begin to emerge in late May and early June, feed for approximately a week, and then mate and lay eggs. The female will deposit these eggs in cracks and crevices on the bark. Eggs begin hatching about two weeks later. Now, damage to the phloem, cambial tissue, and even xylem tissue begins to occur and will continue throughout larval development. The beetle larvae will go dormant during cold winter months. Larvae that are not fully developed by spring will continue development during the second year before pupating and becoming adults after the second winter. The bronze birch borer larvae will develop faster (in one season compared to two) in less healthy and stressed birch trees.
Preventative insecticide treatments are very successful against this insect. Simple soil drenches of imidacloprid, in addition to a fertilization that increases the vigor of the birch tree, have been the preferred treatment for years.
As a Board Certified Master Arborist, Wayne White strives to stay current with the latest research information shared at National Pollinator conferences. Birch trees are wind pollinated, and are not visited by bees unless there is no other source of local pollen. In the future, possible treatments may move to other bee-friendly insecticides, but at this time, there is no industry concern with birch trees and imidacloprid. Wayne will continue to monitor this situation closely.