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Daniel A. Herms, Department of Entomology, The Ohio State University
Amy K. Stone, Ohio State University Extension, Lucas County
James A. Chatfield, Ohio State University Extension, Northeast District
Source: Ohio State Extension

Introduction

Economic and Ecological Impact of Emerald Ash Borer

Diagnosing Emerald Ash Borer: Signs and Symptoms

Distinguishing Emerald Ash Borer From Native Borers

The Plan to Eradicate Emerald Ash Borer

Should Trees Outside of Eradication Zones Be Treated with Preventative Insecticide Applications?

But What About Trees in the Immediate Vicinity of Known Infestations?

In Closing

Introduction
Since its accidental importation from Asia, emerald ash borer (Agrilus planipennis) has infested and killed more than 6 million ash trees (Fraxinus spp.) in woodlands, parks, urban forests, street tree plantings, landscapes, and nurseries across several thousand square miles in 13 counties in southeast Michigan and Windsor, Ontario.
All major North American ash species have been killed by emerald ash borer, which infests trees ranging in size from 1½ inch caliper nursery stock to fully mature trees in forests. While most native borers kill only severely weakened trees, emerald ash borer kills healthy trees as well, making it especially devastating. If it is not contained and eradicated, the impact of emerald ash borer on ash in North America will be similar to that of chestnut blight and Dutch elm disease.

Emerald ash borer was unknown in North America until June 2002, when it was discovered killing ash trees in southeast Michigan and neighboring Windsor, Ontario. Native to eastern Russia, northeastern China, Mongolia, Taiwan, Japan, and Korea, where it occurs on several species of ash it was probably imported into Michigan via infested ash crating or pallets.
Localized infestations discovered in Ohio have triggered ongoing eradication efforts in Ohio. An infestation has also been confirmed in Maryland near Washington D.C. Artificial spread of the insect has been traced to movement of infested ash logs, firewood, and nursery stock.

Economic and Ecological Impact of Emerald Ash Borer
The economic and ecological impacts of emerald ash borer have already been substantial, and would be staggering if this exotic pest continues to spread. It clearly has the potential to virtually eliminate ash as a component of North American forests, which will have dramatic effects on ecosystem processes, as well as plant and animal communities. Ash species, which inhabit a variety of soils and ecosystems, are dominant throughout the forests of eastern North America. An Ohio Department of Natural Resources study estimated there to be more than 3.8 billion ash trees in Ohio, with standing timber valued at more than $1 billion.

Diagnosing Emerald Ash Borer: Signs and Symptoms
Infestations of emerald ash borer are difficult to detect until they are severe, because larvae are hidden under bark, they colonize the upper portion of the trunk first, and symptoms resemble other causes of tree decline.

There are few external signs or symptoms of early infestations. When trees are still vigorous, small, vertical splits may form in the bark in response to the growth of wound-periderm (callus) tissue that forces out the bark as it forms over larval galleries in the phloem. To confirm the presence of emerald ash borer, one can widen the splits to reveal larvae and galleries under the bark. Larval galleries are distinctly S-shaped or serpentine, and are packed tightly with frass (mixture of sawdust and excrement). They are also visible on the inner surface of the outer bark when removed.

The presence of small (1/8 inch) D-shaped exit holes in the trunk or main branches by emerging adults is a sure sign of infestation. As infestations progress into the second year, the canopy will start to thin and branch dieback may occur. Decline accelerates rapidly, and trees are generally killed within 2-4 years of infestation. Epicormic shoots often sprout from the main trunk of declining trees. Woodpeckers are proving to be important predators of emerald ash borer. A noticeable increase in woodpecker activity on ash trees can provide an early indication of an infestation, especially during winter.

Distinguishing Emerald Ash Borer From Native Borers
Green Industry professionals and Extension personnel called to inspect declining ash trees may have the first opportunity to detect new emerald ash borer infestations before they become well established. However, there are several native clearwing and roundheaded borers that also commonly infest ash. Hence, the ability to distinguish emerald ash borer infestations from those of native borers is extremely important.

Among the most common of the native ash borers are the banded ash clearwing borer (Podosesia aureocincta) and ash/lilac borer (P. syringae), both of which are the larvae of clearwing moths. The banded ash clearwing borer has become especially common in Ohio’s urban forests. Although many signs and symptoms of native borers resemble those of emerald ash borer, there are several important characteristics that are useful in distinguishing clearwing borer infestations from those of emerald ash borer.

In particular, the nature of the galleries and the shape of the exit holes are distinctly different between emerald ash borer and the native borers. Clearwing larvae bore deep into the sapwood, while galleries of emerald ash borer are confined to the phloem tissue just under the bark. Clearwing borers expel their frass from the tree, which can accumulate in large quantities in bark crevices, branch crotches, and on the ground, providing a good sign of an infestation. Conversely, emerald ash borer larvae pack their frass tightly within their galleries as they feed. Upon emerging, clearwing borers leave behind a pupal case, which is sometimes found protruding from the emergence hole. Flatheaded borers, on the other hand, do not produce a pupal case.

The shape of adult emergence holes in the trunk is the most distinctive diagnostic guide. The emergence holes of emerald ash borer are distinctly D-shaped, while emergence holes of clearwing borers of ash are larger (1/4 inch diameter) and round. Because the galleries of clearwing borers penetrate into the sapwood, one can insert a thin wire through the emergence hole well into the tree. This is not possible with emerald ash borer exit holes, as the galleries wind just under the bark, and are plugged with frass. There is a native species of Agrilus that infests ash, and it probably also produces D-shaped emergence holes. However, it is much smaller than emerald ash borer, and colonizes only small branches and twigs.

Several species of roundheaded borers also infest ash, with the redheaded ash borer (Neoclytus acuminatus) being the most common in Ohio. Roundheaded borers are larvae of longhorned beetles (Order: Coleoptera; Family: Cerambycidae), which derive their name from the long antennae of adults.

As their name implies, the larvae of redheaded ash borer and other roundheaded borers are round in cross section, in contrast to the highly flattened profile of flatheaded borers. Redheaded ash borer infestations can also be distinguished from emerald ash borer by the presence of large (3/8 inch wide), oval exit holes. Galleries initially form just under the bark and are packed with frass, as is the case with emerald ash borer. However, redheaded ash borer galleries are not nearly as serpentine. As larvae mature, they extend their galleries well into the sapwood, usually following the grain of the wood, while emerald ash borer galleries are restricted to the phloem. Furthermore, redheaded ash borer is restricted to severely weakened, dying, and freshly killed trees (e.g. freshly cut timber and firewood), while emerald ash borers colonize healthy trees.

In summary, the presence of serpentine galleries packed tightly with frass just under the outer bark, coupled with one-eighth inch diameter D-shaped emergence holes through the bark of the trunk and main branches, are sure signs of an emerald ash borer infestation.

The Plan to Eradicate Emerald Ash Borer
USDA-APHIS (Animal and Plant Health Inspection Service) and their Canadian counterparts are in the early stages of implementing a program to eradicate emerald ash borer from North America. The plan in the core infestation area of southeast Michigan, where millions of trees are already infested, is to first contain the pest, then reduce beetle density, and finally eradicate the insect.

There are so many infested trees in the core infestation zone in southeast Michigan that it will be physically impossible to remove them all before insects can emerge. Rather the core infestation is being managed following strategies similar to those used to manage large forest fires. A “fire-break” will be created around the core to contain the infestation. Once contained within the fire-break, the infestation should extinguish itself by killing all of its host trees, thereby starving itself out of existence. The plan is for the fire-break to be wide enough to prevent emerald ash borer from being able to cross it in search of new hosts. The fire-break will be constructed by removing ash trees in a zone around the periphery of the infestation, a task that will be facilitated by routing it through areas with naturally low densities of ash such as agricultural land, industrialized areas, and large bodies of water. Research is ongoing to determine just where the fire-break should be located, and how wide it should be.

Should Trees Outside of Eradication Zones Be Treated with Preventive Insecticide Applications?
Ohio State Extension personnel have received many questions from homeowners and Green Industry professionals wondering if preventive insecticide applications are necessary in Ohio to protect ash trees from emerald ash borer. Members of the OSU Extension Nursery and Landscape and Turf Team, in consultation with the Ohio Department of Agriculture, recommend the following:

Currently, the Ohio State Extension Service does not recommend that any ash trees in Ohio be treated with insecticides for emerald ash borer, even if the tree is in the immediate vicinity of a known infestation.

The logic behind this recommendation, which may seem counter-intuitive, is based on the interaction between the biology of the insect and regulatory issues associated with eradicating emerald ash borer from North America. The situation is different in the quarantined counties in Michigan (for reasons discussed below), where many property owners within the infestation zone are protecting their trees with insecticides.

Emerald ash borer is an exotic insect that is currently regulated by USDA-APHIS, and is subject to eradication. Hence, if an infested tree is discovered in Ohio, it will have to be removed and destroyed. Female emerald ash borers are highly mobile and lay eggs on many trees. Infested trees do not show any external symptoms during the first year of the infestation. Therefore, in the vicinity of any tree showing visible signs of infestation, there will be many more trees that are infested but with no external symptoms (asymptomatic carriers). Since there is no way to tell if these trees are infested, all trees in the vicinity of the infested tree will have to be removed and destroyed, as per eradication protocols, before larvae mature and adults can emerge, even if they appear healthy. This will be true even if that tree has been treated previously with insecticide, as research has shown that no insecticide is 100% effective against emerald ash borer.

But What About Trees in the Immediate Vicinity of Known Infestations?
If the eradication programs are successful, it will not be necessary to treat nearby trees with insecticides. People near an eradication zone may be tempted to treat their trees as insurance of an emerald ash borer escaping the eradication program. However, if a borer does escape, it is extremely unlikely that it will lay eggs only on trees that have been treated with insecticides, as they lay many eggs as they move from tree to tree. If it does lay eggs even on one untreated tree in the same neighborhood as the treated tree, eventually the untreated tree will show signs or symptoms of infestation, and will have to be destroyed. In this case, all trees in the vicinity of the infested tree will also have to be destroyed, even if previously treated.

In the core infestation in southeast Michigan, the situation is different. Because there are too many infested trees to cut down as part of the eradication program (discussed above), and because property owners there are financially responsible for removal of dead trees on their property, many people in the core infestation zone are taking steps to protect their ash trees, including preventive insecticide applications.

In Closing
Emerald ash borer represents a lethal threat to ashes throughout their range in North America, and efforts to eradicate this invasive pest are under way in Ontario, Michigan, Ohio, and Maryland. Eradication is possible, but if these efforts are not successful, emerald ash borer will decimate ash in North America, with devastating economic and ecological impacts. The threat cannot be over-estimated.