Emerald ash borer
- French common name: Agrile du frêne
- Other common names: EAB
- Scientific name: Agrilus planipennis Fairmaire
- Kingdom: Animalia
- Phylum: Arthropoda
- Class: Insecta
- Order: Coleoptera
- Family: Buprestidae
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Partial list of synonyms:
- Agrilus feretrius Obenberger
- Agrilus marcopoli Obenberger
General information and importance
Emerald ash borer is a destructive beetle in the insect family known as metallic wood-boring beetles because of their bright, iridescent colours. Native to the temperate forests of northeast Asia, emerald ash borer was discovered in the United States in Michigan, and nearby southern Ontario in 2002, but was likely introduced in the 1990s via wood-packaging materials. As of 2024, it has been found throughout the eastern North American range of ash (Fraxinus), including southern Ontario and Quebec, and in isolated locations in Atlantic Canada, and in the cities of Winnipeg, Manitoba, and Vancouver, British Columbia. It now occurs in the United States east of the 100th meridian and is expected to occupy the entire range of ash across the American southwest and Pacific coast. Emerald ash borer is so innocuous in its native range it is rarely observed. However, in North America, its effect on native ash trees has been devastating, with most ash trees dying within 10 years of initial infestations. Open grown ash trees growing in urban settings are especially susceptible to attack.
Emerald ash borer is one of the first introduced insects to threaten an entire group of native trees in North America, as did pathogens causing chestnut blight and Dutch elm disease. Significant ecological impacts could be potentially irreversible.
Distribution and hosts
Emerald ash borer is native to northeast Asia including China, Russia, Mongolia, Japan, and Korea. In these areas it attacks weakened or stressed ash species. It was introduced and established in Canada from southern Ontario and Quebec to New Brunswick and Nova Scotia, with isolated infestations west of Lake Superior, and the citiy of Winnipeg, Manitoba. This insect was recently found on ash trees in the City of Vancouver, British Columbia. All species of ash are susceptible and have limited variation among species in their vulnerability. Mountain-ash (Sorbus) is not attacked.
Tree parts affected
Adult emerald ash borers feed on leaves causing little damage. Larvae mining the living vascular tissues of the tree, however, interrupt translocation of water and nutrients and kill trees.
Symptoms and signs
The iridescent, metallic green colour and relatively large size (8 to 14 millimetres long) of emerald ash borer adults make them more conspicuous than many insects, although they favour the upper canopy of the tree and are not easily observed. Adult emerald ash borer cause some defoliation, but the greatest damage is caused by the larvae mining serpentine galleries (10 to 50 centimetres long) under the bark, filled with sawdust and frass. Larvae are white, elongated, and slender, with enlarged segments just behind the head. They reach 25 to 30 millimetres in length. D-shaped holes (3 to 4 millimetres diameter) in the bark reveal where adults have emerged following pupation. These holes are often enlarged by foraging woodpeckers.
Leaves on infested branches turn yellow and branch dieback follows. As the infestation intensifies, vertical cracks develop in the bark. Once weakened, the attacks intensify. Tree death usually occurs within two to four years.
Life cycle
Emerald ash borer has a one-year life cycle but may require two years in cooler climates. The insect mostly overwinters as mature larvae (pre-pupae) in cells constructed in the sapwood under the bark of the host tree. Larvae pupate in spring. Adults emerge in June and feed on foliage for one to two weeks before becoming sexually mature. Mating involves visual cues and females mate several times. They are active fliers, dispersing as far as 15 kilometres. Open-grown and stressed trees are particularly attractive to egg-laying females, suggesting visual and chemical orientation in host location and selection. Females lay 50 to 90 eggs in bark crevices during their lifetimes. Larvae hatch and bore into the inner phloem and cambium, mining serpentine galleries filled with wood and frass as they feed. Larvae go through four instars, completing development by late fall. Final-stage larvae form pupal chambers just beneath the surface of the sapwood and overwinter curled over in a J-shaped position.
Damage
All species of ash (Fraxinus, not mountain-ash, Sorbus) are susceptible to emerald ash borer. Green ash (Fraxinus pennsylvanica), white ash (F. americana), and black ash (F. nigra) are the most vulnerable. Blue ash (F. quadrangulata) appears to be the most resistant of the native ash species. Given the prominence of ash in both urban and natural landscapes, the high mortality rates observed following emerald ash borer infestations will have significant ecological and socio-economic impacts. Ash is a fast-growing tree, able to flourish in a wide range of soil and climatic conditions, and it is generally hardy to urban environments. It has been planted extensively for habitat restoration, shelterbelts, and protection of riverine habitats. Depletion of the formidable inventory of ash will alter forest environments at every level from nutrient cycling to forest succession. Ash is also a valued hardwood species for a variety of wood products and as amenity trees on public and private property.
In its native range, only stressed ash hosts are attacked and the insect is considered secondary. This is not unlike some of our native Buprestids, such as bronze birch borer (Agrilus anxius) on birch (Betula) and twolined chestnut borer (Agrilus bilineatus) on oak (Quercus).
Prevention and management
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.
Early attempts to eradicate emerald ash borer by removal of infested trees were abandoned once the magnitude of the initial introductions became apparent. Subsequent rapid rate of spread was aided by inadvertent movement of nursery stock, sawlogs, and firewood. The regulatory strategy shifted to containment, restricting the movement of these wood products out of infested areas.
There has been an active campaign to introduce natural enemies from the home range of emerald ash borer. It will likely take several years for the true impact of these natural enemies to become apparent.
Removal of infested trees has been the most common approach to minimize the impacts and spread of emerald ash borer. Efficacy of removal methods rely on early detection of emeral ash borer in an area. This is challenging. Visual surveys are usually unable to detect populations until they are well-established and already producing dispersing adults. Adults do not seem to produce any long-range pheromones, so baited traps are not useful for monitoring. A common operational method is to create “trap” trees by girdling the trunk in spring to attract egg-laying female beetles, and debarking the trees in the fall or winter to expose larvae. This is a labour intensive and expensive method. Much of the initial economic impact of emerald ash borer has been the cost of removal and disposal of infected trees, as well as the cost of delimiting quarantine zones and enforcing regulatory restrictions, such as the transport of ash products with bark intact (particularly firewood).
Seeds of various provenances of ash are being conserved in different repositories in anticipation of widespread extirpation of ash on the landscape.
The efficacy of systemic chemical insecticides has been tested and is preferably applied by injection to individual trees. These intensive methods may be an option because of the high cost of tree removal in urban settings. Pesticides registered for use against emerald ash 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.
Photos
Emerald ash borer adult on the bark of an ash.
D.B. Lyons
Emerald ash borer pupa.
D.B. Lyons
Emerald ash borer adult on an ash leaflet.
D.B. Lyons
Ash trees killed by emerald ash borer.
D.B. Lyons
Emerald ash borer adult.
D.B. Lyons
Tunnelled galleries cut into the wood of ash by emerald ash borer larvae. These are visible on the trunk after the bark of the host has been removed.
D.B. Lyons
Urban trees infested by emerald ash borer.
D.B. Lyons
Emerald ash borer larva on ash trunk. Note the flattened head of the larva.
D.B. Lyons
Selected references
Cipollini, D.; Peterson, D.L. 2018. The potential for host switching via ecological fitting in the emerald ash borer-host plant system. Oecologia 187: 507–519. https://doi.org/10.1007/s00442-018-4089-3
de Groot, P.; Biggs, W.D.; Lyons, D.B.; Scarr, T.; Czerwinski, E.; Evans, H.J.; Ingram, W.; Marchant, K. 2006. A visual guide to detecting emerald ash borer damage. Natural Resources Canada, Canadian Forest Service. Sault Ste-Marie, Ontario. 16 p.
Government of Canada. Emerald Ash Borer – Agrilus planipennis. Government of Canada, Canadian Food Inspection Agency. Available at: https://inspection.canada.ca/plant-health/invasive-species/insects/emerald-ash-borer/eng/1337273882117/1337273975030 [Accessed January 2022]
Government of Canada. Emerald Ash Borer – Questions and Answers. Government of Canada, Canadian Food Inspection Agency. Available at: https://inspection.canada.ca/plant-health/invasive-species/insects/emerald-ash-borer/faq/eng/1337355937903/1337356019017 [Accessed January 2022]
Herms, D.A.; McCullough, D.G. 2014. Emerald ash borer invasion of North America: history, biology, ecology, impacts, and management. Annual Review of Entomology 59: 13–30. https://doi.org/10.1146/annurev-ento-011613-162051
McCullough, D.G.; Poland, T.M.; Tluczek, A.R.; Anulewicz, A.; Wieferich, J.; Siegart, N.W. 2019. Emerald ash borer (Coleoptera: Buprestidae) densities over a 6-yr period on untreated trees and trees treated with systemic insecticides at 1-, 2-, and 3-yr intervals in a Central Michigan forest. Journal of Economic Entomology 112(1): 201–212. https://doi.org/10.1093/jee/toy282
McKenney, D.; Pedlar, J.; Lyons, B.; Campbell, K.; Lawrence, K. 2013. Emerald ash borer: economic models for homeowners and municipalities. Natural Resources Canada, Canadian Forest Service. Sault Ste-Marie, Ontario. Frontline Express, Bulletin 70. 2p.
Parsons, G.L. 2008. Emerald Ash Borer. A guide to identification and comparison to similar species. Department of Entomology, Michigan State University. Prepared for USDA, National Plant Diagnostic Network, and North Central Plant Diagnostic Network. 56 p.
Poland, T.M.; Chen, Y.; Koch, J.; Pureswaran, D. 2015. Review of the emerald ash borer (Coleoptera: Buprestidae), life history, mating behaviours, host plant selection, and host resistance. The Canadian Entomologist 147: 252–262.
Ryall, K. 2017. Release of parasitic wasps for biological control of the emerald ash borer in Canada. Natural Resources Canada, Canadian Forest Service. Great Lakes Forestry Centre. Sault Ste-Marie, Ontario. Frontline Express, Bulletin 82. 2p.