Bronze birch borer

Written by V.G. Nealis Revision 2024

French common name: Agrile du bouleau
Scientific name: Agrilus anxius Gory
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Buprestidae

General information and importance

Bronze birch borer and its relatives are members of the insect family known as metallic wood-boring or jewel beetles. Bronze birch borer is native to North America and a specialist feeder on all native and introduced birch (Betula) species in Canada. Larvae feed in tunnels beneath the bark, interrupting the translocation of nutrients and water.

Bronze birch borer generally attacks older and stressed trees, resulting in low population densities in natural forests where there are few susceptible trees. However, recent widespread periods of birch decline, perhaps linked to stress events such as drought, age-related senescence, and spring thaw-freeze events, have been associated with more frequent outbreaks of bronze birch borer. Climate change could exacerbate these conditions, increasing the severity of outbreaks, especially in the southern range of birch.

Distribution and hosts

Although commonly found on white birch (Betula papyrifera), all native birches throughout temperate and boreal North America can be attacked, particularly if they are weakened by old age, drought, or other environmental stresses. Introduced birch species, such as European white birch (B. pendula), appear highly susceptible in comparison. These species have been planted as ornamentals in cities throughout North America, resulting in an increase in the historic range of bronze birch borer to western and southern North America. These novel habitats for the introduced birches may increase their stress and therefore make them more susceptible to bronze birch borer attack.

Host parts affected

Adult bronze birch borers feed briefly on leaves, mostly in the upper canopy, causing little visible damage. Larvae excavate feeding galleries under the bark within the phloem and xylem of branches and the main stem of the tree, disrupting the translocation of water and nutrients.

Symptoms and signs

Adults are slender, olive to copper bronze beetles, ranging from 6 to 12 millimetres in length. They may be confused with several related, similar-looking beetles. Adults are seldom observed, as they are most active in the upper canopy of host trees. Sap flow may be seen on the trunk near where larvae enter, usually on the unshaded side of the tree. The meandering galleries excavated by the feeding larvae under the bark create criss-crossing welts visible on the outer bark of large branches and the main stem. Larvae are legless and slender, except for an enlarged segment behind the head, hence the common name for the related group, flat-headed borers. The larvae have two brown pincer-like structures at the tip of the abdomen. Pupal cells can be found under the bark of the tree. Adults exit from D-shaped holes that may have coarse sawdust deposited nearby. Discolouration and premature drop of foliage from attacked trees begins in the upper crown, followed by dieback of branches.

Life cycle

There is one generation per year in southern portions of the range. Two years are required to complete development in cooler regions or in vigorous trees. Adults emerge during a 6-week period from May to July and feed on foliage before reaching reproductive maturity. They are strong fliers but tend not to disperse if susceptible trees are nearby, causing local and intense infestations. Eggs are laid singly or in clusters in bark crevices, often where smaller branches meet the main stem. Newly hatched larvae bore through the bark into the phloem where larvae complete four or five stages. Pupal cells are constructed just under the bark, although pupation does not occur without a period of chilling. This enables more synchronized early-summer emergence of adults. Following pupation, adults emerge through D-shaped exit holes.

Damage

Damage to foliage by feeding adults is negligible. Tunnels bored by larvae as they feed beneath the bark impede the translocation of water and nutrients in the tree. Eventually, branches are girdled by these tunnels causing dieback. This begins in the upper canopy and continues until the entire tree is affected. Tree death may take several years, depending on local density of borers and tree vigour (i.e., trees weakened by other factors).

Prevention and management

Bronze birch borer is a native species of forested ecosystems throughout North America where birch is present. The insect is considered a secondary pest because it targets old and weakened trees. Impacts in urban landscapes can be more severe where growing conditions are often suboptimal and where birch trees may be under stress. Planting susceptible introduced birch species should be avoided. Maintaining moist, cool soil conditions by mulching and watering is advised. Vigorous trees may be attacked but can produce callus tissue around the feeding gallery, killing the insect and mitigating damage.

Pruning dead branches and removing heavily infested trees reduces local populations of bronze birch borer. Infested material should be chipped, burned, or exposed to full sunlight to kill larvae. In natural woodlands, susceptibility can be reduced by managing for diverse age classes and avoiding the practice of leaving isolated birch during harvest. Following harvest, these residual trees often become weakened or damaged by wind and are susceptible to attack by bronze birch borer. Natural enemies play a small role in reducing bronze birch borer populations. Insecticides have been used against this insect but require multiple drenching or systematic injections.

Bronze birch borer attacks European species of birch in North America. This observation identified the risk of accidental introduction of the insect into Europe and prompted the European Union to declare bronze birch borer a quarantine pest. The import of birch wood products, including wood packing, require the removal of bark and outer sapwood or treatment with ionizing irradiation under supervision of a national plant protection organization.

Pest management strategies for a particular pest vary depending on several factors. These include:

the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
the value of the host(s) versus the costs of pest management approaches; and
consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

Pesticides registered for use against bronze birch borer under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against this insect. The application of any registered product should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Selected references

Balch, R.E.; Prebble, J.S. 1940. The bronze birch borer and its relation to the dying of birch in New Brunswick forests. The Forestry Chronicle 16(3): 179–201. https://doi.org/10.5558/tfc16179-3

Barter, G.W. 1957. Studies of the bronze birch borer, Agrilus anxius Gory, in New Brunswick. The Canadian Entomologist 89(1): 12–36. https://doi.org/10.4039/Ent8912-1

Bourque, C.P.-A.; Cox, R.M.; Allen, D.J.; Arp, P.A.; Meng, F.-R. 2005. Spatial extent of winter thaw events in eastern North America: historical weather records in relation to yellow birch decline. Global Change Biology 11(9): 1477–1492. https://doi.org/10.1111/j.1365-2486.2005.00956.x

Braathe, P. 1995. Birch dieback – caused by prolonged early spring thaws and subsequent frost. Norwegian Journal of Agricultural Sciences Supplement No. 20. 59 p.

Cerezke, H.F. 1994. Bronze birch borer. Natural Resources Canada, Canadian Forest Service, Northwest Region, Northern Forestry Centre. Edmonton, Alberta. Forestry Leaflet 26. https://ostrnrcan-dostrncan.canada.ca/entities/publication/b9d0ac1e-a4ac-4dff-9aa1-cc209edd9196

Haack, R.A.; Petrice, T. 2019. Historical population increases and related inciting factors of Agrilus anxius, Agrilus bilineatus, and Agrilus granulatus liragus (Coleoptera: Buprestidae) in the Lake States (Michigan, Minnesota, and Wisconsin). The Great Lakes Entomologist 52(1&2): 21–33. https://scholar.valpo.edu/tgle/vol52/iss1/7

Katovich, S.A.; Munson, A.S.; Ball, J.; McCullough, D. 2000. Bronze birch borer. United States Department of Agriculture, Forest Service. Washington, D.C. Forest Insect and Disease Leaflet 111.

Loerch, C.R.; Cameron, E.A. 1984. Within-tree distributions and seasonality of immature stages of the bronze birch borer, Agrilus anxius (Coleoptera: Buprestidae). The Canadian Entomologist 116(2): 147–152. https://doi.org/10.4039/Ent116147-2

Miller, R.O.; Bloese, P.D.; Hanover, J.W.; Haack, R.A. 1991. Paper birch and European white birch vary in growth and resistance to bronze birch borer. Journal of American Society of Horticultural Science 116(3): 580–584. https://doi.org/10.21273/JASHS.116.3.580

Muilenburg, V.L.; Herms, D.A. 2012. A review of bronze birch borer (Coleoptera: Buprestidae) life history, ecology, and management. Environmental Entomology 41(6): 1372–1385. https://doi.org/10.1603/EN12238

Sanchez, N.; Leavell, D. 2023. Homeowner guide to management of the bronze birch borer: an insect killing birches in the PNW. Oregon State University, OSU Extension Service. EM 9189. Available at: https://catalog.extension.oregonstate.edu/em9189/html [Accessed March 2024]

Saskatchewan Ministry of the Environment. (no date). Bronze birch borer. Saskatchewan Ministry of the Environment Forest Pest Fact Sheet. Available at: https://publications.saskatchewan.ca/#/products/76992 [Accessed March 2024]

Schrader, G.; Kinkar, M.; Vos, S. 2020. Pest survey card on Agrilus anxius. European Food Safety Authority (EFSA) Supporting Publications 17(1): 1777E. https://doi.org/10.2903/sp.efsa.2020.EN-1777

Cite this fact sheet

Nealis, V.G. 2024. Bronze birch borer. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.