Bronze Birch Borer

Need to Know: 

• Bronze birch borer is an infrequent pest of birch and beech in Iowa, but can be serious when present.
• Issues are often secondary to other primary tree health issues.
• Prevention is easier than treatment; a healthy tree is the best defense.

Description and Life History

Bronze birch borer is an occasional serious pest of birch (and occasionally beech) species in Iowa. Adults are small (~1 cm), slender, dark-colored insects that can appear black to green with a hint of bronze iridescence in direct light. Larvae are white, slender, and up to 1.3 cm long with an enlarged head region.

Bronze birch borer overwinters as larvae under the surface of host tree bark. In the spring, once warmer temperatures arrive, the larvae will pupate in the tree and emerge as adults, typically in early June. They will chew their way out of the tree, leaving a D-shaped exit hole in the bark. Adult beetles will mate, and females will lay eggs in cavities on host tree bark, after which the larva will feed and grow throughout the season until it overwinters in the sapwood. Some individuals may take two years to develop from egg to adult. 

Damage

Larval feeding can create winding galleries (tunnels) in the vascular layers of the trunk. The most frequently observed sign of bronze birch borer presence is top-down dieback (dieback of the tree starting at the top and moving down). Bark can also appear to have raised areas or have emergence holes in the shape of the letter "D." Infestations typically start higher up in the tree and work downward. Any area where larvae girdle the vascular tissue will result in the death of the areas above the girdling. 

Management

Carefully monitor your trees consistently to detect potential borer attack as early as possible. If suspected, contact an arborist for damage high in trees. Top-down dieback may have other causes. Don't just assume; confirm the presence of the insect before taking action with chemicals. 

The best offense is a good defense. Bronze birch borer is attracted to already stressed or weakened trees, and they do not survive well in a healthy tree. Keep trees healthy with appropriate watering and fertilization. Prune out any dead or dying branches that can't recover to limit attack and subsequent dieback. 

Insecticides can be used as a last resort, only to manage current attack that can't be solved by swift resolution of tree stress. Insecticide applications must be times appropriately and coincide with the hatching of larvae after egg laying. In Iowa, this is typically in early June. Permethrin and bifenthrin are acceptable insecticides that can be applied as sprays. Dinotefuran is a systemic insecticide that may also be effective if applied to a tree that has yet to experience a lot of dieback. Always check insecticide labels and apply as directed, or contact a professional pesticide applicator for more advice or assistance. 

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect and provide information on what it eats, its life cycle, and the best ways to manage it if it is a pest. Please see our website for instructions on mailing insects and digital images.   

Contact information for each state's diagnostic laboratory for U.S. residents. If you live outside of Iowa, please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.