Bronze Birch Borer

Bronze birch borers are known to attack all native and introduced birch species, although birch susceptibility varies. Many varieties of birch species as well as numerous crosses between species are currently planted as ornamentals in North America. Although some varieties are more resistant than others, none are immune to birch borer attack. Generally, the white-barked birches are more susceptible than those without white bark such as river birch, sweet birch and yellow birch.

Life Cycle: bronze birch borer is a serious native insect pest. The adult is a black beetle about 1/2 inch long and has bronzy iridescence on the back. It is a sun-loving insect and may be found crawling on the sunny side of the trunk during late May and early June.

The female deposits her eggs under cracks and crevices of the bark. Eggs hatch in 2 weeks or less, and the slender larvae tunnel immediately into the phloem tissue to construct their galleries. They may occasionally tunnel into the xylem (wood) to molt and overwinter. One or two years may be required to complete larval development. The larvae pupate in the xylem in late April or early May.

Damages: Bronze birch borer attacks paper, European white , gray, yellow, and other birches, as well as poplar, cottonwood and willow. The larva bores in the trunk and larger limbs, often girdling them. Trees weakened by drought or injured are most susceptible to attack.Tree injury is caused by larval tunneling in the inner bark or cambium. The girdling of the trunk or branches interrupts sap flow downward to the roots and destroys the tree’s cambium tissue. The interruption and subsequent accumulation of sap flow above larval tunnels often causes characteristic swollen bands or vein-like ridges in trunks and affected branches. The galleries range to approximately 1.5 meters in length and are packed with a dark brown sawdust-like frass..

Control: Sufficient watering is probably the most important factor in maintaining a healthy birch tree. During the growing season, a slow (2-3 hours), deep (8-18 inches) watering once per week is a general rule for maintaining adequate soil moisture.Mulching (2-4 inches deep) over the tree’s root system will help moderate soil temperatures and keep soils moist.

Good materials are wood chips, shredded bark, or leaf compost. Likewise, planting ground covers using shade tolerant plants in the tree’s root zone will also help keep soils moist and cool.Fertilizing is beneficial only when nutrients are lacking. Thus, unless your tree is deficient of nutrients, fertilizing will have little effect on reducing bronze birch borer attacks. If fertilizer is needed, it is important to provide sufficient water when applying.

Bronze Birch Borer Damage

Pruning should not be done from May 1 to July 1. This is during the borer flight period and it has been shown that female borers are attracted to fresh pruning wounds. Wound dressings should not be used; they are ineffective at repelling borers and do not promote closing of wounds. In addition, excessive pruning (more than 25% of the canopy) should be avoided as this increases light penetration to the root zone and can increase soil temperatures and reduce moisture levels.