Emerald ash borer (Agrilus planipennis)

Synonyms: Agrilus marcopoli, Agrilus feretrius

Common Names: EAB

Description:

Emerald ash borer (Agrilus planipennis) is a small, bright metallic-green beetle from northeastern Asia that kills ash trees [1]. It was first found in North America near Detroit, Michigan in 2002, most likely arriving in wood packing material shipped from overseas [1][3]. MISIN tracks it because its larvae tunnel under the bark and cut off the tree's flow of food and water, killing nearly every untreated ash within a few years of attack [3].

Identification:

The adult is a slender, torpedo-shaped beetle about half an inch long ? roughly the length of a grain of cooked rice ? with a brilliant metallic-green back. Lift the wing covers and the body underneath is a coppery purple-red [2]. Adults are out and flying from late spring into mid-summer, so May through August is the best time to spot the beetle itself [2]. The larvae you never see directly: they are flat, cream-colored, and spend the year tunneling under the bark [2].

Most people find this pest by its damage, not the beetle. The clearest sign is a tiny D-shaped exit hole, about one-eighth of an inch wide, where an adult chewed its way out [1][2]. Peel back loose bark and you may see S-shaped tunnels packed with fine sawdust [2]. Damage shows up best in late summer and fall, when the top of the tree thins and dies back; in winter and early spring, woodpeckers strip the outer bark to reach the larvae, leaving pale "blonded" patches that are easy to see from a distance [2].

At-a-glance cues:

1. Slender metallic-green beetle about half an inch long, coppery purple under the wings [2]
2. D-shaped exit holes about one-eighth inch wide in the bark [1][2]
3. S-shaped, sawdust-filled tunnels just under loose or split bark [2]
4. Dieback at the top of the canopy, new sprouts on the trunk, and pale woodpecker "blonding" [2]

Hosts:

Emerald ash borer attacks ash trees (Fraxinus species). In North America it has been found on all native ash ? including green, white, black, and blue ash ? and it can also attack white fringetree, a related ornamental [2]. It will use both healthy and stressed trees, which is part of why it spreads so fast [3].

The beetle is native to northeastern Asia, including parts of China, Korea, Japan, and far-eastern Russia [1][3]. In North America it now turns up anywhere ash grows: forests and swamps, fence rows and woodlots, and the street trees and yards of towns and cities [3]. Because ash was a very popular street and landscape tree, this pest is as much a backyard problem as a woodland one [4].

Life Cycle:

The beetle is a weak flier and spreads only short distances on its own. The long jumps that move it across the map are almost always our fault ? people haul it to new areas in firewood, logs, and nursery stock [1][6]. This is why "don't move firewood" is the single most important thing a homeowner can do.

Emerald ash borer usually completes one generation a year, though in cooler areas or healthier trees a single beetle can take two years to develop [2][3]. In early summer, females lay eggs in cracks and crevices of the bark, and a single female can lay dozens to well over a hundred eggs [3]. The eggs hatch in about two weeks, and the young larvae chew through the bark and feed all summer, carving S-shaped tunnels through the living tissue [2]. They spend the winter under the bark, change into adults in spring, and chew out through D-shaped holes from late spring into mid-summer to start the cycle again [2]. For control, this timing matters: protect a valued tree before larvae have done heavy damage, because once the canopy is thinning the tree is often too far gone to save [3].

Impact and Damage:

For homeowners, the biggest direct risk is safety and cost. Ash killed by this beetle become brittle quickly and start dropping limbs ? and eventually whole trunks ? which is dangerous near houses, driveways, and play areas, and expensive to remove [4]. Replacing a mature shade tree is a real expense for any property owner [4].

For natural areas, the damage is severe. Emerald ash borer kills more than 99% of the ash trees it reaches when they are left untreated, wiping out ash as a forest tree across large regions [3]. When the ash die, they open gaps in the forest canopy that aggressive invasive plants are quick to fill, and they remove a tree that many insects, birds, and other wildlife depend on [3]. Black ash swamps, where ash is often the dominant tree, are especially hard hit [3].

The economic toll is large. One study estimated that treating, removing, and replacing ash trees on developed land in 25 states would cost about $10.7 billion over the ten years from 2009 to 2019 [4]. Since 2002 the beetle has killed tens of millions of ash trees in North America and continues to spread [1][3].

Similar species:

Several metallic or green beetles are mistaken for the emerald ash borer. The surest difference is the beetle's slender shape, its half-inch size, and the D-shaped exit holes it leaves in ash bark [5].

Six-spotted tiger beetle (Cicindela sexguttata) is the closest match ? a bright metallic-green beetle about the same size ? but it has several small white spots along the edges of its wing covers, long legs, and it darts quickly across trails and bare ground rather than resting on ash trunks [5].

Japanese beetle (Popillia japonica) also shines metallic green, but it is rounder and chunkier, has coppery-brown wing covers, and shows small tufts of white hair along the sides of its body [5].

Caterpillar hunter (Calosoma scrutator) is a metallic-green ground beetle, but it is much larger ? close to an inch long ? with gold and red edges, and it hunts on the ground rather than feeding on ash [5].

Two-lined chestnut borer (Agrilus bilineatus) is a native relative with the same slender shape and D-shaped exit holes, but it is darker and more bronze-black, and it attacks stressed oaks, not ash [5].

Monitoring and rapid response:

Signs to watch for:

1. Thinning or dieback starting at the top of an ash tree, often with new sprouts low on the trunk ? most visible in late summer and fall [2]
2. Tiny D-shaped exit holes and split bark on the trunk and larger branches [1][2]
3. Pale "blonded" patches where woodpeckers have stripped bark to reach larvae, easiest to see in winter and early spring [2]

What you can do:

1. Learn to recognize your ash trees and check them each year for the signs above [2].
2. Never move firewood ? buy and burn it where you use it. Moving firewood is the main way this beetle reaches new areas [1][6].
3. Decide early about any high-value ash you want to keep; protection works best before a tree shows decline, while it is still healthy [3].
4. On your own property, have dead or dying ash taken down and the wood chipped or used locally; do not haul infested wood elsewhere [6].
5. Natural area crews can survey ash stands, remove hazard trees, prioritize protecting outlier or high-value trees, and support the release of approved biological-control wasps that attack the beetle [3][6].

When to call a professional:

1. When you want to protect a large or high-value ash tree, since effective treatment and correct timing usually require a licensed applicator [3].
2. When removing large dead or dying ash, which become brittle and dangerous and are best handled by a trained tree service [4].
3. When an infestation covers many trees or a whole woodlot and needs a coordinated management plan [3].
4. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. USDA National Invasive Species Information Center. Emerald Ash Borer (Agrilus planipennis). invasivespeciesinfo.gov
2. NC State Extension. Emerald Ash Borer. content.ces.ncsu.edu
3. Herms DA, McCullough DG (2014). "Emerald ash borer invasion of North America: history, biology, ecology, impacts, and management." Annual Review of Entomology 59:13?30. semanticscholar.org
4. Kovacs KF, Haight RG, McCullough DG, Mercader RJ, Siegert NW, Liebhold AM (2010). "Cost of potential emerald ash borer damage in U.S. communities, 2009?2019." Ecological Economics 69(3):569?578. semanticscholar.org
5. Michigan State University Extension (E2939). Native Borers and Emerald Ash Borer Look-alikes. canr.msu.edu
6. USDA APHIS / Federal Register (2020). Removal of Emerald Ash Borer Domestic Quarantine Regulations (effective Jan 14, 2021; program shift to biological control). federalregister.gov

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