The emerald ash borer (Agrilus plani-pennis) is an exotic, invasive species that has infested and killed more than 12 million ash trees since its accidental importation from Asia. The infestation is now established across more than 6,000 square miles in southeastern Michigan, northwestern Ohio, and neighboring Essex County, Ontario.
Isolated, localized infestations, termed “outliers,” exist elsewhere in Michigan, Ohio, and northeastern Indiana. All major eastern North American ash species are susceptible to emerald ash borer, which infests trees ranging in size from 1/2-inch-caliper nursery stock to fully mature trees in forests.
While most native borers colonize only weakened trees, emerald ash borer attacks healthy trees as well, making it especially devastating. If it is not contained and eradicated, the impact of emerald ash borer on ash trees in North America will be similar to that of the chestnut blight and Dutch elm disease. These exotic pests devastated natural and urban forests in the 20th century.
However, an aggressive, coordinated containment and eradication program undertaken by federal, state, and Canadian agencies provides reason to be optimistic that North American ash trees can ultimately be spared the fate of the chestnut and elm.
The emerald ash borer was unknown in North America until June 2002, when it was determined to be the cause of unusually widespread ash mortality in southeastern Michigan.
This insect is native to areas of Asia, including eastern Siberia, northeastern China, Mongolia, Japan, and Korea, where it occurs on several species of ash. It was probably imported into Michigan at least 10 to 15 years ago by means of infested ash crating or pallets.
Emerald ash borer was first discovered in Ohio, near Toledo, in February of 2003. Isolated infestations were subsequently found in five additional counties in northwestern Ohio, as well as in suburban Columbus.
In the spring of 2004, two additional outlier infestations were discovered in northeastern Indiana. Most of these outlier infestations have been linked to the artificial spread of the emerald ash borer from southeastern Michigan through the movement of infested nursery stock, logs, or firewood. This largely occurred before emerald ash borer was identified and state and federal quarantines were imposed.
The economic and ecological impacts of the emerald ash borer have already been substantial and will be staggering if the infestation continues to spread. Ash species inhabit a variety of soils and ecosystems and are dominant throughout the forests of eastern North America.
According to USDA Forest Service statistics, there are 3.8 billion ash trees in Ohio, with a standing timber valued at more than $1 billion. Furthermore, ash has been one of the most important nursery and landscape species. According to the USDA, the wholesale value of ash sold in Ohio exceeded $2 million in 1998, while a recent survey conservatively estimated the value of the standing ash crop to exceed $20 million. This market has been decimated since the discovery of emerald ash borer, and many growers are destroying their trees.
Emerald ash borer has already caused tens of millions of dollars of damage to Michigan’s landscapes, urban forests, and woodlots, and this cost is increasing at an exponential rate. The cost of removing dead and dying ash has overwhelmed municipal budgets in affected counties. The quarantine on ash timber and firewood has also had negative economic impacts on sawmills, tool handle factories, and firewood dealers in Michigan and Ohio.
The emerald ash borer is a beetle (Coleoptera) belonging to the family known as metallic wood-borers (Buprestidae). Larvae of these beetles are known as flatheaded borers, deriving their common name from the larval stage, which appears to have a broadly flattened head (it is actually the thorax which mostly conceals the much smaller head).
Figure 2. Emerald ash borer larva.
Figure 1. Emerald ash borer adult. The emerald
ash borer has the potential to decimate ash
throughout their range in North America, and
efforts to eradicate this invasive pest are underway.
For additional information about this serious
threat, see the article titled Emerald Ash Borer: The
Beginning of the End of Ash in North America? in
Special Circular 193.
Emerald ash borer larvae (Figure 2) are white with a long (about one inch when mature) narrow, segmented abdomen that is also flattened, which gives them the appearance of small tapeworms. Adults are elongate, 1/2-inch-long beetles with striking, metallic-green coloration.
Generally, there is one generation per year, although recent studies by Michigan State University (MSU) researchers suggest that development may sometimes take two years in newly infested healthy trees. Adult beetles emerge from infested ash trees in late May through early August, with emergence peaking in mid to late June.
As adults emerge, they leave small (1/8 inch), distinctly D-shaped exit holes (see Figure 1) in the trunk and main branches. Adults may live three to six weeks and nibble on small patches of ash leaves during this period.
Females generally produce 50 to 80 reddish eggs, which are laid individually on the bark surface, or within bark cracks and crevices. When the larvae hatch, they tunnel into the tree, where they feed on the phloem and outer sapwood, excavating S-shaped, serpentine galleries just under the bark. The galleries disrupt the flow of nutrients and water between the canopy and roots. This causes canopy thinning and branch dieback, and ultimately tree death.
Larvae continue to feed through the summer and into the fall. They overwinter in the outer bark or within the outer inch of sapwood. Pupation occurs in mid to late spring. Adults emerge soon thereafter to complete the typical one-year lifecycle.
Ash species known to be susceptible include green (Fraxinus pennsylvanica), white (F. americana), black (F. nigra), and blue ash (F. quadrangulata), as well as horticultural cultivars of these species. Only living trees are colonized. Emerald ash borer will not colonize a dead tree.
In China, emerald ash borer colonizes the Asian ash species F. mandshurica (Manchurian ash) and F. chinensis. In Japan, species of Juglans (walnuts and bitternuts), Ulmus (elms), and Pterocarya (wingnuts) have also been recorded as hosts. However, emerald ash borer has not been well studied in Asia (a total of three published pages), and these host records may reflect the existence of subspecies or simply taxonomic confusion.
Furthermore, host records for borers are notoriously unreliable, and often include tree species from which adults were collected, even when the larvae are not able to develop on those species. Research on host range and host preference is underway, and preliminary results from Michigan State University studies strongly suggest that walnut and elm will not be viable hosts for emerald ash borer in North America.
Studies are also underway to investigate the susceptibility of plants related to ash, such as lilacs and privet. To date, these species have not been observed to be infested, even when growing in close proximity to infested ash trees.
USDA-APHIS (Animal Plant Health Inspection Service), the USDA Forest Service, and the Canadian Food Inspection Agency (CFIA), in cooperation with state Departments of Agriculture and Natural Resources have joined forces to implement a long-term program to contain and eventually eradicate emerald ash borer from North America.
The plan, which is in the early stages of implementation, is to (1) locate and promptly eradicate outlier infestations, (2) prevent establishment of new outlier infestations through aggressive enforcement of state and federal quarantines, and (3) contain, suppress, and ultimately eradicate the core infestation.
A key component of the eradication plan is an intensive monitoring program to evaluate the success of outlier eradication efforts, identify existing low-density infestations that have so far escaped detection, and quickly detect new infestations.
Rapid elimination of outlier infestations before they expand and become entrenched is critical. To date, several outlier eradication programs have been implemented in Michigan, Ohio, Maryland, and Virginia, resulting in the destruction of more than 100,000 ash trees. Eradication of outlier infestations involves removal of all visibly infested trees, as well as all other ash trees within a 1/2-mile radius of the visibly infested trees.
Since infested trees do not show external signs or symptoms of attack during the first year, there is no way to determine which trees in the vicinity of infested trees were infested themselves. Consequently, it is necessary to cut even apparently healthy trees to destroy the insects lurking within before they can emerge, disperse, and reproduce. Felled trees are chipped and incinerated at a co-generational power plant, and stumps are treated with herbicide to prevent sprouting.
Three major studies of outlier infestations conducted in 2003 and 2004 by Michigan State University researchers and cooperators provide a science-based rationale for the current eradication strategy. This research involved felling and peeling bark from a large number of ash trees of all sizes occurring within one-half mile of a known point source — e.g., the infested firewood or nursery trees from which the infestation was known to originate.
Intensive sampling showed that 80% of all larvae were in trees within 100 yards of the original point source. At one site, infested trees were found as far as 750 meters (nearly one-half mile) from the point source. But at the other two sites, all larvae were found within 0.38 miles of the point source. Therefore, the cutting of all ash trees within a one-half-mile radius of visibly infested trees should eliminate the vast majority of insects in outlier infestations, if not the entire infestation.
Treating infested trees with insecticides as an alternative to destroying them is not a viable option for eradication sites. While research has shown that preventative insecticide applications can effectively protect shade trees from emerald ash borer in the core infestation in southeastern Michigan, no insecticide program has been effective enough for eradication purposes.
To ensure success, these outlier eradication sites are being monitored for at least three years after cutting to determine if there is a need to “mop up” any beetles that may have slipped the dragnet.
However, monitoring efforts have been complicated by the research that indicates that emerald ash borer apparently does not produce the long-range pheromones that have been so useful in trapping other insect pests such as the gypsy moth. Rather, monitoring is currently being conducted in Michigan, Ohio, and Indiana by means of an extensive grid of “trap trees,” which consists of a girdled green ash tree wrapped with a band of sticky tanglefoot or similar substance.
Research conducted by MSU and USDA Forest Service scientists in 2003 showed that adult beetles were more strongly attracted to girdled trees than unwounded trees, possibly due to host plant volatiles released in the air by girdled trees.
There are so many infested trees in the core infestation in southeastern Michigan and neighboring Essex County, Ontario, that it is physically and economically impossible to remove them all or to treat them with insecticides. Scientists and regulatory officials have developed a plan to surround and contain the core infestation with a Reduced Ash Zone (RAZ) that will likely extend through central and southwestern Michigan and across northeastern Indiana and northwestern Ohio.
The optimal location of the RAZ is being determined by analyzing a combination of aerial photos, land-use data, and ground surveys to estimate ash density and emerald ash borer distribution. The RAZ will be routed as much as possible through areas with naturally low densities of ash such as agricultural land, industrialized areas, and large bodies of water.
Incentive-based programs and ash markets will be developed to encourage property owners to remove and sell ash trees of all sizes before they are killed by emerald ash borer or removed in an eradication action. An aggressive emerald ash borer suppression program will occur just inside the RAZ to relieve pressure on the leading edge and minimize emerald ash borer breakouts.
An intensive monitoring program within and beyond the periphery of the RAZ will be implemented to rapidly detect the spot infestations that will inevitably breach the RAZ so that they can be quickly extinguished. It is important to realize that all of the ash trees in the RAZ inevitably will be killed by emerald ash borer, as will billions more, if emerald ash borer is allowed to spread unchecked across North America.
Preventing the artificial spread of emerald ash borer is another major component of the eradication plan. Accordingly, federal, state, and Canadian quarantines have been enacted to prohibit the movement of firewood, ash nursery stock, logs, wood chips, and untreated lumber.
Preventing the movement of firewood presents a particularly tough challenge. Many federal, state, and provincial agencies, along with Extension personnel at Ohio State, Michigan State, and Purdue Universities, and state Department of Natural Resources are assisting regulatory officials by actively spreading the word about the emerald ash borer and particularly the dangers posed by transporting firewood.
A multi-media, multi-agency publicity campaign featuring television, radio, and newspaper ads, billboards, press releases, fliers, posters, and bulletins has been launched to inform people about the firewood quarantine. Highway signs warn motorists of substantial fines for moving firewood outside the quarantine zone.
A stepped-up inspection and enforcement program has targeted violators at rest areas, along highways, and at campgrounds at critical times such as major holidays and during hunting season. Regulatory and law-enforcement officials have even established check-points on highways at the Ohio-Michigan border to stop vehicles and intercept firewood. Canadian officials have been ticketing violators at the camp sites. These and related outreach and enforcement efforts will continue to expand to minimize the artificial spread of the emerald ash borer.
Since the first discovery of Emerald Ash Borer in Ohio, satellite infestations have been identified in Defiance, Franklin, Fulton, Henry, Lucas, and Wood counties. To date, 17 sites throughout these counties in northwestern Ohio and in Franklin County have been marked for eradication by the Ohio Department of Agriculture (ODA) to eliminate the pest and to protect the state’s 3.8 billion ash trees from devastation. Five of these sites have been eradicated, and nearly 38,000 ash trees have been destroyed. These locations are indicated with dots in the map shown in Figure 3. The eradication process continues at the remaining sites, indicated with the numbered dots.
Figure 3. Nearly 38,000 ash trees have been destroyed in five sites in Ohio in efforts to eliminate this pest and to protect the state’s 3.8 billion ash trees from devastation.
In 2004, eradications occurred in Rossford in Wood County (424 trees destroyed), Columbus in Franklin County (17,713 trees destroyed), Whitehouse in Lucas County (6,000), Toledo Express Airport in Lucas County (10,000), and Hicksville in Defiance County (3,379 trees destroyed). ODA also removed ash trees in North Baltimore in Wood County, where approximately 15,000 trees were cut, chipped, and incinerated.
Additional infestations slated for eradication include Swanton in Fulton County; Maumee State Forest in Henry County; Maumee State Forest, Oak Openings Metropark, Resurrection Cemetery, Ottawa Park, Greenwood Park, Secor Road and I-475, Point Place, and Pickle Road in Lucas County; and Pioneer in Williams County, which stems from a Michigan-based infestation.
ODA also has regulated areas in northwestern Ohio that restrict the movement of ash trees, branches, firewood, bark, wood chips, and other ash materials. Restricted items in these areas can only move out of the regulated areas after a compliance agreement has been done with the department.
Currently, regulated areas include Hicksville Township in Defiance County and the area formed by Lucas County, north of the Maumee River; Fulton County, east of State Route 109; and Henry County, east of State Route 109 and north of the Maumee River. An Ohio ban also prohibits movement of any ash materials from Michigan into Ohio.
Though ash firewood is the only firewood named in the quarantine, the state is asking every citizen to refrain from moving unidentified firewood, which could inadvertently contribute to the spread of emerald ash borer. To strengthen its regulations, ODA is currently changing its quarantine to include all “non-coniferous” firewood.
Agency collaboration in Ohio is key as we battle the borer. ODA’s role is that of regulatory; Extension’s includes research and educational outreach; and the Department of Natural Resources is focusing efforts on the reforestation aspect.
For additional information on the emerald ash borer, check out these sites:
Ohio State University Extension http://ashalert.osu.edu
Ohio Department of Agriculture http://www.ohioagriculture.gov/eab/
Ohio Department of Natural Resources http://www.dnr.ohio.gov/forestry/eab/default.htm