Native to North America, damage caused by the bronze birch borer (BBB), Agrilus anxius, dates back to the late 1800’s and continues to present day. This pest is known to attack all native and non-native species of birch, although susceptibility varies. The BBB is opportunistic and thrives in birch trees that are weakened or stressed by agents such as drought, soil compaction, root injury, other insects, and old age. Birch trees tend to prefer cool and moist growing sites, which are not typically found in most residential and urban areas. With frequent stress caused by dehydration, high temperatures, and compacted soils, landscape birch trees are commonly attacked by this pest.
The best protection from the bronze birch borer is prevention. Because this pest attacks weakened and stressed birch trees, care and attention should be given to maintaining tree health by watering during drought, mulching, and minimizing damage to the root zone that could cause root injury and stress. For new plantings, consider other species that are more stress tolerant.
As a rule of thumb, if more than 30% of the tree is infested the chances of saving it are significantly reduced. Infested branches and deadwood should be removed, so give attention to what the tree will look like before deciding on subsequent treatments.
All non-native birch trees of high value should be targeted for preventive treatment with Transtect or Xytect. Xytect and Transtect are most effective when applied to healthy appearing birch trees that have not been attacked.
Transtect soil applications or systemic basal bark sprays can be applied throughout spring and into early to mid summer. Transtect has the advantage that it moves into the tree within a 2-3 weeks, this gives the practitioner an opportunity to assess the canopy after full leaf out for any signs or symptoms of BBB. Transtect will provide one season of control.
Xytect soil applications are applied in early spring as soon as soils thaw out and no later than the first week of May. Xytect can also be applied in late summer or fall to provide protection the following season. Xytect will provide one season of control.
Mectinite or Xytect 10% as a tree injection treatment for trees that cannot be treated with foliar sprays or other systemic options. These products should also be used on trees that are suspected of being infested or showing symptoms or situations with drought environment/slow uptake.
Other Treatment Practices
- Cambistat improves the health of trees which improves their ability to withstand stress induced insect and disease attacks. For best results, Cambistat should be applied as part of an overall management strategy.
- Adequate water is a key factor in maintaining healthy birch trees. A slow, deep watering event once per week during dry conditions will help maintain soil moisture levels and minimize the stress that invites the bronze birch borer.
- Mulch is very beneficial for trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 2-4 inches of wood chips or shredded bark under the drip line can have a very beneficial effect.
- Avoid pruning birch trees during the peak flight period of the bronze birch borer females. In the north, this is generally from May to July 1. Female borers have been shown to be attracted to fresh pruning wounds.
Birch compartmentalize wounds from tree injection more slowly, consider tree injection treatments when other systemic products cannot be used or only on trees that are suspected of being infested. Preventive treatments are more effective. If >30% of the tree is showing symptoms saving the tree is difficult. If <30% of the tree is showing symptoms treatments may work, but not always. For infested trees, treatments will result in an improvement in tree condition typically in the 2nd full season following treatment as the tree repairs previous damage and prevents future damage.
Signs of Damage
Early symptoms of the BBB include yellow leaves, branch dieback, and an overall thin appearance to the upper part of the canopy. Larvae feed underneath the bark and disrupt the flow of water to the canopy. Wound tissue forms around these disruptions, creating a serpentine lump underneath the bark. Eventually the symptoms progress downward to the lower branches and trunk.
- Small branches in the upper 1/3 of the crown will be thin and begin to die back
- Leaves on infested branches will be stunted, yellow, or show marginal chlorosis in May or June
- Over time dieback continues downward to larger branches •Infested birch trees typically die in 3-4 years
- D-shape exit holes (flat-headed borer)
- Ridges may appear on the bark of the trunk and larger branches in response to larvae feeding beneath the bark in the phloem tissue
- Adult borers are slender, about 3/8 inch (10 mm) long
- Metallic-coppery colored beetles (are rarely seen)
- Larvae occur underneath the bark and are white, segmented, legless grubs with an enlarged area behind the head
- 1/2 inch long when mature
- The larvae of the bronze birch borer generally feed in the phloem tissue, but occasionally bore into the outer sapwood
- Larvae feed laterally around the branch
- Zigzag galleries are often formed
Photo: Whitney Cranshaw
Colorado State University, Bugwood.org
Photo: Steven Katovich
USDA Forest Service, Bugwood.org
Trees At Risk
All species of birch (Betula spp.) may be attacked, but some species are more susceptible than others. The following outlines the relative susceptibility of common birch species:
- Jacquemonti Birch (Betula jacquemontii): High
- European White Birch (Betula pendula): High
- European White Weeping Birch (Betula pendula 'Youngii'): High
- Yellow Birch (Betula alleghaniensis): Moderate
- Sweet Birch (Betula lenta): Moderate
- Paper Birch (Betula papyrifera): Moderate
- Whitespire Birch (Betula platyphylla japonica 'Whitespire'): Moderate
- Gray Birch (Betula populifolia): Moderate
- River Birch (Betula nigra): Very Low
- Heritage Birch (Betula nigra 'Heritage'): Very Low
- Crimson Frost Birch (Betula platyphylla var. szechuanica x Betula pendula 'Purpurea' 'Crimson Frost'): Unknown
- Adults emerge May – June (440-800 GDD), and feed on leaf margins
- Mated females lay eggs in bark crevices
- Larva hatch within 10-14 days and begin feeding in cambium and sapwood
- By late fall mature larva pupate and overwinter
- In colder regions lifecycle may take 2 years
University of Minnesota
Penn State University
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.