Twolined chestnut borer

Written by V.G. Nealis Revision 2025

• French common name: Agrile du châtaignier
• Other common names: Two-lined chestnut borer
• Scientific name: Agrilus bilineatus (Weber)
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Coleoptera
• Family: Buprestidae
• Partial list of synonyms:
  • Agrilus aurolineatus Gory
  • Agrilus flavolineatus Mannerheim
  • Buprestis bilineata Weber

General information and importance

Twolined chestnut borer is indigenous to North America, east of the 100^th meridian. It attacks several species of oak (Quercus) and American chestnut (Castanea dentata). The larvae of this beetle bore feeding galleries beneath the bark of host trees, damaging vascular tissues and impeding the translocation of water and nutrients in the tree. Twolined chestnut borer is considered a secondary pest, as it only successfully attacks trees under stress from drought or previous infestations of defoliating insects.

Distribution and hosts

The insect is indigenous to North America, from Manitoba to New Brunswick in Canada, and in the eastern United States where its hosts are found. Although this insect attacks American chestnut, this tree is now relatively rare because of the impacts of a Chestnut blight, an introduced disease. In Canada, the beetle is most often found attacking weakened red oak (Quercus rubra), white oak (Q. alba), and bur oak (Q. macrocarpa). Oak species in the red oak group are more susceptible than those in the white oak group. Recent (2018) discoveries of twolined chestnut borer in Turkey raise concerns about the possibility of introduction and potential impact on European chestnuts (Castanea) and oak populations. English oak (Q. robur) is a European oak species that is known to be highly susceptible to the twolined chestnut borer.

Tree parts affected

Adults feed on foliage during midseason. Larvae tunnel meandering feeding galleries under the bark in sapwood tissues, interrupting translocation processes in the tree. Branches girdled by tunnels die.

Symptoms and signs

Adults of twolined chestnut borer are elongated beetles that are 4 to 13 millimetres long. They are a dark colour, with two golden stripes along their back and an iridescent underside. Feeding damage to foliage in mid-season by adults is barely noticeable. Mature larvae are up to 25 millimetres long, cream-white and elongated, with a pronounced swelling on thoracic segments behind their head. They bore feeding tunnels in sapwood, filling them with sawdust and frass. D-shaped exit holes (5 millimetres) chewed in the bark demonstrate that adults have emerged.

The first symptom of established feeding galleries is the presence of wilted foliage on scattered branches in late summer. Trees can be killed in the first year of attack, especially if tree health is already compromised. Death of branches occurs first in the upper canopy and progresses lower to the main stem of the tree.

Life cycle

Twolined chestnut borer usually has a 1-year life cycle but may be extended to a 2nd year in vigorous hosts or cooler climates. They overwinter as final (fourth) instar larvae in wood chambers excavated under the bark. Peak emergence of adults occurs from June to July. Adult beetles may be present for most of the summer, as they are relatively long-lived and there is considerable variation in larval development time associated with the vigour of infested hosts. Adults feed on leaves during their maturation period. Mated females locate stressed trees suitable for egg-laying, detected through chemicals (volatiles) emitted by the trees. Eggs are laid in bark crevices and hatch in mid-summer. Larvae bore through the bark and construct feeding galleries in the phloem, cambium, and outer sapwood (xylem). Larvae complete development in late summer and autumn. They burrow into the outer bark to construct individual chambers for overwintering. Pupation occurs in this chamber the following spring. Adults begin to emerge in late spring.

Damage

Feeding damage by adult twolined chestnut borer on foliage is usually minimal. Significant damage is caused by larvae tunnelling in the sapwood, impeding the translocation of water and nutrients. Oak trees are ‘ring porous,’ meaning they conduct water through only a few growth rings in the sapwood. Once this conduit is severed by feeding larvae, the entire branch above the girdling tunnels dies. Trees usually die after 2 to 3 years of attack. Other insects utilizing dying trees often accelerate tree mortality.

Twolined chestnut borer is a secondary pest, hastening rather than initiating decline in stressed or unhealthy trees.

Prevention and management

Twolined chestnut borer is natural component of oak and chestnut forests in eastern North America. The most vulnerable trees are those in poor health because of advanced age, location, damage from other pests, or environmental stress, such as drought.

Larvae under the bark are invulnerable to most interventions short of felling the tree. Management should rely on prevention. The close association between attacks and stressed trees suggests that removing overstocked, less vigorous overstory could benefit the remaining trees. Care should be taken to avoid putting new stress, such as root disturbance or soil compaction, on younger trees. Mulching, watering, and fertilizing urban trees helps maintain tree vigour and reduce susceptibility.

Pruning of high-value, individual trees below the last wilted leaves on each infested branch is a control option in the first year of infestation. Once a tree is infested a 2^nd and 3^rd year, felling of the tree and destroying outer bark becomes the only option. Unless all trees at a particular site are stressed, removal of infested and weakened trees may provide sufficient control at the stand level. Removal of trees should be carried out in early autumn. Bark must be removed before spring to kill larvae prior to emergence. Do not transfer any wood with bark to new areas, as it could harbour fully grown larvae that are ready to emerge.

More aggressive action involves trap trees, where weaker trees are purposefully girdled to attract egg-laying females. Once eggs are laid, the trap tree is removed and destroyed. Natural enemies seem to have little influence on population levels. Woodpeckers may excavate under bark to feed on larvae. However, by this time, most of the damage has been done.

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.

Selected references

Cote, W.A., III; Allen, D.C. 1980. Biology of two-lined chestnut borer, Agrilus bilineatus, in Pennsylvania and New York. Annals of the Entomological Society of America 73(4): 409–413. https://doi.org/10.1093/aesa/73.4.409

Haack, R.A.; Acciavatti, R.E. 1992. Twolined chestnut borer. Forest Insect and Disease Leaflet 168. United States Department of Agriculture, Forest Service. Washington, D.C.  https://www.fs.usda.gov/foresthealth/publications/fidls/index.shtml [Accessed March 2024]

Haack, R.A.; Benjamin, D.M. 1982. The biology and ecology of the twolined chestnut borer, Agrilus bilineatus (Coleoptera: Buprestidae), on oaks, Quercus spp., in Wisconsin. The Canadian Entomologist 114(5): 385–396. https://doi.org/10.4039/Ent114385-5

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

Muzika, R.M.; Liebhold, A.M.; Twery, M.J. 2000. Dynamics of twolined chestnut borer Agrilus bilineatus influenced by defoliation and selection thinning. Agricultural and Forest Entomology 2(4): 283–289. https://doi.org/10.1046/j.1461-9563.2000.00077.x

Paiero, S.M.; Jackson, M.D.; Jewiss-Gaines, A.; Kimoto, T.; Gill, B.D.; Marshall, S.A. 2012. Field guide to the jewel beetles (Coleoptera: Buprestidae) of northeastern North America. Canadian Food Inspection Agency. Ottawa, Ontario. 411 p. https://publications.gc.ca/site/eng/410324/publication.html [Accessed February 2024]

United States Department of Agriculture Forest Service. 1985. Insects of eastern forests. United States Department of Agriculture, Forest Service. Washington, D.C. Miscellaneous Publication 1426. 608 p.

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