Emerald ash borer

Emerald ash borer (Agrilus planipennis) is an invasive metallic-green beetle whose larvae tunnel beneath the bark of ash trees, disrupting phloem and cambial function. Infestations may remain hidden until canopy dieback, epicormic shoots, bark splits, woodpecker activity, S-shaped galleries or D-shaped exit holes appear. Early confirmation and a site-specific management decision are essential because severely declining trees may be hazardous and difficult to save.

Emerald ash borer (EAB) is a destructive, nonnative wood-boring beetle in the family Buprestidae. In the United States, its principal hosts are ash trees in the genus Fraxinus. Both stressed and apparently healthy trees can be attacked, including landscape trees, street trees and forest ash.

Adults are typically metallic green and about one-half inch long. The damaging stage is the larva, which develops under the bark. Early infestations are difficult to detect because the galleries are concealed and canopy decline may lag behind the initial attack.

Diagnosis should be confirmed carefully because D-shaped holes, bark cracks, dieback and woodpecker activity can also occur with other borers, diseases or environmental stresses. Contact a county Extension office, state forestry or agriculture agency, or a qualified arborist when confirmation is uncertain.

• Progressive thinning and dieback, often beginning in the upper canopy or individual branches.
• Smaller leaves, reduced shoot growth and increased epicormic shoots or sprouts on the trunk and branches.
• Vertical bark splits that may expose an S-shaped larval gallery beneath the bark.
• Winding, S-shaped galleries packed with frass in the phloem and outer sapwood.
• Small D-shaped adult emergence holes, approximately one-eighth inch across.
• Unusually heavy woodpecker activity or patches of bark removed while birds forage for larvae.
• In advanced cases, major canopy loss, bark peeling, branch failure and tree death.

Visible symptoms often indicate an established infestation rather than a newly arrived one. Similar signs may be caused by native wood borers, ash decline, drought or other disorders.

Larvae damage the tissues immediately beneath the bark as they feed and tunnel. Numerous galleries can girdle branches and the trunk, blocking phloem transport and impairing the movement of water and stored carbohydrates.

Natural spread occurs as adults fly to nearby ash trees. Long-distance spread is strongly associated with human movement of infested ash firewood, logs, nursery stock, wood packaging and other untreated ash material. Do not move ash firewood across state lines or within restricted areas unless it meets applicable state or federal requirements.

Adults generally emerge from May through August in the United States, with timing varying by latitude, elevation and weather. They feed on ash foliage, mate and females lay eggs in bark crevices, commonly producing dozens to more than 100 eggs during their short adult life.

After hatching, larvae bore through the bark and feed in the phloem and outer sapwood, passing through several instars. Larvae overwinter beneath the bark, then pupate and emerge as adults through D-shaped holes. The cycle is commonly completed in one year but may require two years, especially in colder climates or in vigorous trees.

Management depends on tree size, health, infestation stage, location and the cost and risks of preservation. A qualified arborist should assess trees with substantial dieback, cracked bark or weakened branches because dead ash can become hazardous.

Healthy or lightly infested landscape ash may sometimes be protected with a professionally selected systemic insecticide program. Effectiveness is best when treatment begins before extensive canopy loss and is repeated according to the product label and tree condition. Application methods may include soil injection, soil drench or trunk injection, but suitability depends on the product, tree size, site and local regulations.

Trees with severe canopy decline, extensive galleries, major structural defects or poor prospects for recovery may need removal by a qualified tree professional. Dispose of or process infested wood in accordance with state and local requirements so that adults cannot emerge and spread.

Biological control is an important long-term component of the U.S. EAB program. Federal and state partners release and monitor specialized parasitoid wasps, including species such as Spathius galinae, Tetrastichus planipennisi and Oobius agrili. These agents are not a quick rescue treatment for an individual backyard tree, but established populations may contribute to suppression in forests and natural areas.

Native predators, especially woodpeckers, can remove and consume larvae. Their activity is useful as a warning sign but is rarely sufficient to protect a heavily infested tree. Cultural measures remain essential: do not move infested wood, promptly process or debark ash material, remove hazardous trees responsibly, and replace lost ash with a diverse selection of suitable non-ash species.

• Plant a diverse mix of non-ash tree species rather than replacing ash with a single tree type.
• Buy firewood where it will be burned, or use firewood certified as heat-treated under applicable U.S. requirements.
• Never transport untreated ash firewood, logs or nursery stock unless state and federal rules explicitly allow it.
• Inspect ash trees at least annually, especially from late spring through summer, for canopy thinning, bark splits, epicormic shoots, D-shaped holes and woodpecker activity.
• Maintain appropriate watering and avoid unnecessary trunk injury, soil compaction and severe root disturbance; good vigor does not prevent EAB but can improve management options.
• Before planting or treating an ash, confirm its identity and assess whether the tree is structurally sound and worth preserving.

• EAB was first detected in North America near Detroit, Michigan, and Windsor, Ontario, in 2002.
• It has caused extensive mortality among North American ash and has been detected in numerous U.S. states and the District of Columbia.
• The unrelated “mountain ash” trees, which are species of Sorbus, are not true ash and are not normal EAB hosts.
• White fringetree, Chionanthus virginicus, has been documented as an alternative reproductive host in the United States, although its overall importance is much less than that of Fraxinus.
• Adult beetles are short-lived, while most of the insect’s life is spent hidden beneath ash bark.
• Blue ash may show greater tolerance than several other North American ash species, but it should not be considered immune.