Emerald ash borer, Agrillus planipennis
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. For help identifying ash trees, visit the Purdue University ash tree identification guide.
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.
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.
- 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, ultimately, are usually killed.
- Systemic insecticides are typically applied as either soil drenches, trunk sprays or direct injections into the trunk, and can suppress early emerald ash borer infestations.
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
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 five instars, larvae excavate pupation chambers 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 and infest new ash trees and can also reinfest the same 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, previously uninfested trees will occasionally require two years to complete their development.
The feeding injury and damage caused EAB larvae can be confused with that of lilac/ash borer, a clearwinged moth. Adults of the ash borer produce round exit and while EAB create D-shaped exit holes. 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. 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 production of water sprouts 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.
The presence of D-shaped exit holes and foraging woodpeckers is indicative of an EAB infestation. Canopy thinning in the upper portion of the tree and the presence of S-shaped galleries under bark are also indicators of EAB activity. It is worth noting that new infestations of EAB are very difficult to detect, and often there will not be any observable external symptoms or signs of infestation. However, ash trees within 30 miles of an EAB infestation are likely to be at risk and should be continuously inspected.
Traps with lures can be assembled and deployed in the tree canopy, and branches can be cut from the middle of the tree canopy and stripped of bark at the base to expose any galleries. Videos describing the protocols for trapping and branch sampling are available. Visit the Colorado State Forest Service website for more information on EAB, including its distribution in Colorado.
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.
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 have been introduced for EAB control in North America: Oobius agrili, Tetrastichus planipennisi, Spathius agrili, and Spathius galinae.
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.
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. It is important to note that drought conditions will inhibit the uptake of systemic insecticides, and supplemental irrigation may be necessary during dry periods.
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 noting 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.
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.
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
Emerald Ash Borer Network. (n.d.). Emerald Ash Borer: Biological Control. Emerald Ash Borer Network. Available http://www.emeraldashborer.info/biological-control
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
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