Emerald ash borer, Agrillus planipennis
Order: Coleoptera
Family: Buprestidae
Description
Emerald ash borer, abbreviated EAB, is an invasive beetle that was first detected in Colorado in 2013. This pest is highly destructive to all North American ash species (Fraxinus spp.), with costs to municipalities, property owners, nursery operators and forest products industries reaching billions of dollars. Ash trees are among the most widely distributed genera of trees in North American forests. The degree of susceptibility varies among different ash species, and six species of ash are currently listed as critically endangered due to EAB activity: white ash (F. americana), Carolina ash (F. caroliniana), black ash (F. nigra), green ash (F. pennsylvanica), pumpkin ash (F. profunda), and blue ash (F. quadrangulata). Emerald ash borer has not been observed colonizing trees other than ash, even in heavily infested areas. For help identifying ash trees, visit the Purdue University ash tree identification guide.
Adults feed on foliage along leaf margins and tend to seek shelter under leaves or in bark crevices during periods of rain, wind, or cool weather. They are bright metallic green with a rounded abdomen and a slightly flat back. The upper side of the abdomen is a metallic purple-red, but only visible while the insect is in flight. Since EAB is the only Agrilus sp. that has a bright, metallic red upper abdomen in North America, this is one of the simplest features that can be used for identification. The beetles measure 8.5-19 mm (1/3-3/4 inch) long, and males are slightly smaller than females. To a trained eye there are few other wood-boring beetles that look like EAB, but for help distinguishing EAB from other common pests visit the University of Nebraska-Lincoln’s EAB look-alikes page.
The larvae feed underneath tree bark and are the damaging life stage. The larvae are creamy white with 10 flattened abdominal segments and a pair of brown pincer-like protrusions at the rear. Several hind abdominal segments resemble bells which is a distinguishing feature of this species. When mature, larvae measure up to 3.8 cm (1.5 inch). At 1 mm in length, the eggs of EAB are rarely seen.
Quick Facts
- Emerald ash borer is a green flat-headed beetle that develops in ash trees (Fraxinus spp).
- It is an invasive insect that is native to Asia. It was first detected in Detroit in 2002 and was found in Boulder, CO, in September 2013.
- Emerald ash borer wounds ash trees by tunneling under the bark. When wounds become extensive, trees show symptoms of decline and are usually killed.
- Biological control is the primary management approach in North American forests, and insecticides are available to protect high-value ash trees in urban areas. If an infested ash tree is beyond saving, removal is strongly recommended since dead or dying trees pose a safety risk, especially in urban areas.
Adult emerald ash borer. Note the iridescent green wings and characteristic body shape with a flat head and body tapering at the end. Emerald ash borer is an invasive pest in North America. Larvae are the destructive life stage as they feed beneath the bark of ash, eventually killing the trees. Emerald ash borer was detected in Boulder, CO in 2013 and is actively spreading. Restricting the transport of firewood over long distance will help slow the spread of this devastating pest. Image credit: Debbie Miller, USDA Forest Service, Bugwood.org
Larva of EAB. Note the flattened, segmented body with two brown pincer-like protrusions at the end of the abdomen (left), and bell-shaped abdominal segments. Image credit: Pennsylvania Department of Conservation and Natural Resources – Forestry, Bugwood.org
Adults of emerald ash borer and the characteristic D-shaped exit holes they create. Image credit: Debbie Miller, USDA Forest Service, Bugwood.org
EAB tunneling on an ash with bark removed. Note the zig zag appearance of the galleries, produced by feeding of larvae. Image credit: Eric R. Day, Virgina Polytechnic Institute and State University, Bugwood.org
EAB damage on ash tree. Image credit: Steven Katovich, Bugwood.org
Ash tree with epicormic shoots due to EAB infestation. Image Image credit: Steven Katovich, Bugwood.org
Life history and habits
Newly hatched larvae bore into the tree bark and feed on the phloem and cambium, creating S-shaped galleries (tunnels) that disrupt water and nutrient flow. After developing through several instars, larvae excavate pupation chambers in the outer 1-2 cm (2/5-4/5 inch) of sapwood where they overwinter as pre-pupae until the following spring. After pupating, adults remain in pupation chambers for about two weeks, after which they chew D-shaped exit holes in the tree bark. The adults mate and feed on foliage for about two weeks before females lay 40-200 eggs, which typically occurs in June through September. Adult EAB can fly several miles in search of new host trees and can also reinfest the natal tree from which they emerged. Dispersal of EAB naturally occurs at a slow rate but is greatly expedited through the transport of infested wood products and young ash trees. The beetles generally complete their life cycle in one year, but EAB infesting healthy trees will occasionally require two years to complete their development. In this instance, larvae overwinter as earlier instars and resume feeding during the second growing season before emerging the following spring.
Injury
The feeding injury and damage caused EAB larvae can be confused with that of lilac/ash borer, a species of clearwinged moth. Adults of EAB create D-shaped exit holes in the trunk and branches that measure approximately 2-3 mm (~1/10 inch) in diameter. The larvae of EAB feed underneath the tree bark and excavate S-shaped galleries that can reach 10-50 cm (4-20 inches) in length. Canopy thinning and dieback on branches become noticeable on trees heavily infested with EAB, which typically leads to host mortality within 2-4 years. Heavy woodpecker activity can strip layers of bark and leave light or blond sections on trunks or branches. Twig dieback starts in the upper portion of the tree canopy, and yellowing, thinning, or wilting of foliage can also be observed. Vertical splits in bark can also be present, in addition to the growth of epicormic shoots near the bottom of the trunk.
After completing development within the tree, adults chew D-shaped exit holes in bark that often go undetected until other, more visible symptoms are present. Adult EAB feed on leaf margins, chewing small irregular shaped holes.
Monitoring
To help inform decisions related to chemical control, the ash trees in an area should be inventoried based on species composition, size (measured in diameter at breast height), and tree condition prior to EAB detection. Ash trees within 30 miles of a confirmed EAB infestation are considered at risk and should be continuously inspected for signs of an EAB infestation. Community participation and support have improved tree health and EAB management efforts, and it is recommended that observations be shared with management professionals. Some examples of external symptoms include the presence of D-shaped exit holes and heavy woodpecker activity. Canopy thinning in the upper portion of the tree and the presence of S-shaped galleries under bark are also indicators of EAB infestations. Trees growing in stressful conditions are highly at risk and should be given extra attention. However, it should be noted that visual surveys alone are unlikely to reveal new infestations since local population densities of EAB tend to increase before external signs and symptoms become evident. In addition, EAB infestations tend to begin in the upper canopy of trees, which are difficult to inspect from ground level on larger trees.
One of the most effective approaches for monitoring low densities of EAB involves girdling “detection trees” in the spring to attract ovipositing females. The bark is then removed in fall or winter to expose EAB larvae. While this approach has been used by regulatory officials to identify numerous infestations in the United States, it can be labor intensive and is especially challenging in areas where ash trees are scarce or when multi-year surveys are needed.
There are several trap designs for monitoring EAB adults. Multifunnel traps can be re-used annually and do not need to be coated with an adhesive. However, these traps are relatively expensive and require the addition of a preservative, which needs to be replaced periodically throughout the summer. These traps must also be deployed in the mid-canopy and lowered to check for EAB, which can result in the trap getting tangled with foliage and branches. Prism traps are relatively easy to assemble, light weight, and relatively inexpensive. However, prism traps can only be used once and must be coated with an adhesive, which can be messy and result in the catch of nontarget species. Like multifunnel traps, prism traps must also be deployed at mid-canopy and lowered to check for EAB. Double-decker traps include prism traps that do not need to be hung in tree canopies but require additional materials such as PVC pipe that need to be kept in storage when not in use.
Branch sampling is another monitoring tool for detecting EAB larvae. Videos describing the protocols for deploying prism traps and branch sampling are available. Research is ongoing to determine the most effective trapping protocols for detecting EAB infestations. Observations suggest that detection efforts might improve when focused in residential areas where initial colonization of EAB is more likely to occur on trees grown in open areas. In areas previously uninhabited by EAB, a single beetle capture warrants regulatory action, management, and outreach efforts to increase public awareness. Emerald ash borer findings can be reported to the Colorado Department of Agriculture or emailed to CAPS.program@state.co.us. It is recommended that high quality photos be taken of suspicious insects or feeding injuries.
Prevention
Larvae can remain in firewood underneath the bark for up to two years. To help restrict the spread of EAB, firewood should not be moved. Rather, firewood from local sources should be used.
Cultural control
Asian species of ash are more resistant to EAB than North American species and may provide a source of genetic resistance for ongoing breeding programs. Additionally, surveys conducted in ash stands in Michigan and Ohio have revealed that a small proportion of trees (~1%) survived an EAB infestation, and those surviving the infestation my provide another source of genetic resistance among native populations of ash trees.
Biological control
Natural enemies of EAB include predators, parasitoids, and microbial pathogens. Native woodpeckers are one of the most important biocontrol agents for EAB, and some species of native wasps can attack EAB eggs or larvae. However, neither of these natural enemies has successfully suppressed EAB populations enough to prevent trees from dying. For example, even though woodpeckers can remove up to 90% of larvae within a tree, the tree can still die due to EAB feeding injuries.
Eradication of EAB is highly unlikely due to the mobility of adults and the fact that this pest can quickly kill ash trees. Certain wasp species can kill large numbers of EAB in their native range in Asia, and controlled releases can help protect ash trees in North America. However, these wasps are not commercially available and are only released by professionals. To date, four species of parasitoid wasp species from China and the Russian Far East have been introduced for EAB control in North America: Oobius agrili, Tetrastichus planipennisi, Spathius agrili, and Spathius galinae. Adults of O. agrili parasitize eggs of EAB, while T. planipennisi, S. agrili, and S. galinae are parasites of EAB larvae. As of 2018, the establishment and spread of O. agrili and T. planipennisi has been confirmed in various regions of the United States. Since its release in 2015, S. galinae appears to have become established in Michigan, Connecticut, Massachusetts, and New York, while the establishment of S. agrili has only been confirmed in Maryland. The introduction of a second egg parasitoid of EAB, Oobius primorskyensis, from the Russian Far East has been proposed to enhance egg parasitism in certain regions of North America.
Research indicates that T. planipennis can rapidly spread across areas infested with EAB. However, the parasitism rates of T. planipennis appear to be inversely correlated with host tree diameter due to the parasitoid’s short ovipositor, which limits the accessibility of ovipositing wasps to EAB larvae. For this reason, T. planipennisi is an important biocontrol agent for protecting ash saplings and basal sprouts against EAB. Protecting larger trees is likely to require the successful establishment of S. galinae, which has a longer ovipositor and can attack EAB larvae residing in trees with a larger diameter.
It is worth noting that regional differences are likely to impact the efficacy of these biocontrol agents due to abiotic factors such as temperature. In the southern United States, higher temperatures facilitate more rapid development of EAB. This results in asynchrony of susceptible EAB stages with parasitoid phenology and may prevent the establishment of species not adapted to such environmental conditions. Identifying additional parasitoids in southern regions of the native rage of EAB has been proposed to help address this issue.
Chemical control
Once an EAB infestation is confirmed, certain criteria should be considered to assess whether insecticides can save the tree. If the tree is vigorously growing with more than half its canopy intact, provides value to the owner and landscape, and EAB infestation is not extensive, then the tree can likely be saved through insecticide treatments. When a tree cannot be saved, or in cases where treatment is too costly, tree removal is highly recommended to prevent potential safety hazards to people and property, especially in urban landscapes. Identifying EAB infestations and treating aggressively during the early stages of an infestation is crucial to protecting ash trees, and once the EAB population declines, continuous inspection and treatment of infested trees is necessary, which can be costly and time consuming. A visual representation of the EAB treatment plan depending on the stage of invasion in the region is available here.
Mature healthy trees are often prioritized for treatment since they are more difficult to replace and more costly to remove than young trees. Furthermore, research suggests that green ash should be monitored and treated before white ash since EAB tends to colonize and kill green ash sooner than white ash. Ash trees located in parks, parking lots, and along roads are more likely to be infested by EAB than trees in shaded areas. Chemical treatments with systemic insecticides should begin when the tree’s vascular system is healthy enough to transport the insecticide to leaves and branches. It is important to note that drought conditions will inhibit the uptake of systemic insecticides, and supplemental irrigation may be necessary during dry periods to facilitate the vascular transport of insecticides and help trees produce new growth to compensate for previously sustained EAB injuries. Since chemical treatments for EAB are costly, insecticides should only be applied when monitoring efforts provide evidence of an EAB infestation in the community. The incorporation of a plant growth regulator, paclobutrazol, in EAB management programs can help slow the growth of trees, reducing the volume of insecticide needed over time, and promotes the production of fine roots. Recent research suggests that three-year treatment programs are sufficient to protect susceptible trees. At peak densities of EAB, new injuries may be noticed on trees three years after treatment as insecticide concentrations decrease. Such trees should be prioritized for treatment the following spring. Locations with high densities of ash trees may benefit from emamectin benzoate applications in two-year intervals. Under this approach, not all trees need to be treated. Beetles feeding on trees treated with the insecticide will be killed, while nontreated trees can help support resident populations of parasitoids that attack immature stages of EAB. Parasitoid activity can help provide additional control of EAB after the emamectin benzoate application wanes.
Insecticides for EAB target adults and young larvae and can be applied to healthy ash trees of all sizes. Research suggests that insecticide treatments are more effective in spring than in the fall since mature larvae are less susceptible to insecticides than young larvae. Insecticides can help prevent additional injury to trees but will not reverse injuries that have already been inflicted, and it will take some time for rescued trees to recover from an EAB infestation. Insecticide applications against EAB fall into four categories: systemic insecticides applied as soil drenches or soil injections, systemic insecticides applied as trunk injections, systemic insecticides sprayed on the lower trunk, and insecticides applied as cover sprays on the trunk, main branches, and foliage.
Soil drench or injections
Systemic insecticides applied as soil drenches or soil injections are available for professional applicators and homeowners. However, those available to homeowners have some restrictions, such as the number of applications allowed per year and the application method, which include soil drenches or granules applied to the soil in water. Professionals can inject these insecticides directly into the soil. This requires specialized equipment and delivers the insecticide directly into the root zone of the tree, which also helps reduce the amount of runoff. It is worth mentioning that the rate of uptake is highest near the base of the trunk where the fine tree roots are most dense, and that soil applications are most effective when the soil is moist rather than dry or saturated. To protect pollinators, soil applications should not be made near the roots of flowering plants. To address this, flowering plants can be removed and destroyed, or the insecticide can be applied as a trunk injection.
Trunk injections
Injecting insecticides directly into the base of the tree trunk allows for treatment on sites where soil applications are impractical and allows for quicker uptake of the systemic insecticide. Trunk injections typically require drilling through the bark, which can cause long-term damage to the tree if the application is repeated annually in the late spring. High-pressure injections can also damage the tree if it causes separation of the bark from the cambium.
Systemic basal trunk sprays
Spraying insecticides on the lower five to six feet of the tree trunk can be accomplished with a garden sprayer at low pressure. This application method is quick, easy, and does not harm the tree when done correctly. It is important to calibrate sprayers to ensure the appropriate quantity of product is applied.
Protective cover sprays
This application method is not recommended because of the high incidence of drift, which can affect nontarget species. Thorough coverage is necessary for this application method, which works to kill adult EAB and newly hatched larvae as they chew through bark. This technique will not affect larvae feeding underneath the bark.
More information on chemical management of EAB, including insecticide recommendations, is available here.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Colorado EAB Response Team. 2015. The Colorado Emerald Ash Borer Management Plan Creation Guide. Colorado Emerald Ash Borer Response Team. Available http://hermes.cde.state.co.us/drupal/islandora/object/co:28488/datastream/OBJ/view
Colorado State Forest Service. (n.d.). Emerald Ash Borer. Colorado State Forest Service: Quick Guide Series. Available https://www.centennialco.gov/files/sharedassets/public/documents/residents/emerald-ash-borer/emerald-ash-borer-quickguide.pdf
Duan, J., L. Bauer, R. Van Driesche, and J. Gould. 2018. Progress and Challenges of Protecting North American Ash Trees from the Emerald Ash Borer using Biological Control. Forests. 9(142): 1-17. Available https://www.mdpi.com/1999-4907/9/3/142
Emerald Ash Borer Network. (n.d.). Emerald Ash Borer: Biological Control. Emerald Ash Borer Network. Available http://www.emeraldashborer.info/biological-control
Herms, D., and D. McCullough. 2014. Emerald Ash Borer Invasion of North America: History, Biology, Ecology, Impacts, and Management. Annual Review of Entomology. 59: 13-30. Available https://www.annualreviews.org/doi/pdf/10.1146/annurev-ento-011613-162051
Herms, D., D. McCullough, D. Smitley, C. Sadof, F. Miller, and W. Cranshaw. (n.d.). Insecticide Options for Protecting Ash Trees from Emerald Ash Borer. Available https://extension.entm.purdue.edu/EAB/PDF/NC-IPM.pdf
Invasive Species Centre. (n.d.). Emerald Ash Borer. Invasive Species Centre. Available https://www.invasivespeciescentre.ca/invasive-species/meet-the-species/invasive-insects/emerald-ash-borer/
NYIS. 2023. Emerald Ash Borer: Biology and Life Cycle. New York Invasive Species Information. Available https://nyis.info/eabbiology/#:~:text=EAB%20generally%20have%20a%20one,of%20black%20locust%2C%20Robina%20pseudoacacia.
Parsons, G. 2008. Emerald Ash Borer: A guide to identification and comparison to similar species. US Department of Agriculture. Available http://www.emeraldashborer.info/documents/eab_id_guide.pdf
Purdue University. (n.d.). Emerald Ash Borer. Purdue University: Extension – Entomology. Available https://extension.entm.purdue.edu/EAB/index.html
Sadof, C., D. McCullough, and M. Ginzel. 2023. Urban ash management and emerald ash borer (Coleoptera: Buprestidae): facts, myths, and an operational synthesis. Journal of Integrated Pest Management. 14(1): 1-13 Available https://academic.oup.com/jipm/article/14/1/14/7235956
Tobin, P. et al. 2021. Evaluation of Trapping Schemes to Detect Emerald Ash Borer (Coleoptera: Buprestidae). Journal of Economic Entomology. 114(3): 1201-1210. Available https://academic.oup.com/jee/article/114/3/1201/6220038
USDA. (n.d.). Emerald Ash Borer Beetle. US Department of Agriculture – Animal and Plant Health Inspection Service. Available https://www.aphis.usda.gov/aphis/resources/pests-diseases/hungry-pests/the-threat/emerald-ash-borer/emerald-ash-borer-beetle#:~:text=The%20emerald%20ash%20borer%20threatens,you%20won’t%20move%20firewood.&text=Click%20here%20to%20learn%20more.&text=The%20Emerald%20Ash%20Borer%20(Agrilus,ash%20trees%20in%2030%20states
Wisconsin Department of Natural Resources. (n.d.). Emerald Ash Borer: Life Cycle and Identification. Wisconsin Department of Natural Resources. Available https://dnr.wisconsin.gov/topic/foresthealth/emeraldashborer/lifecycle