A New Exotic Pest Threatens Ohio’s Forests: Emerald Ash Borer

By Robert Long, OSAF Forest Science and Technology Coordinator

A new exotic insect pest has been detected in Michigan and Ontario adding to the growing list of alien intruders that may threaten Ohio’s forests. The insect has been identified as the emerald ash borer (Agrilus planipennis), and it has been found killing ash trees in six Michigan counties near Detroit and in Windsor, Ontario. It is believed that the insect was brought into North America on wood packing materials from the Far East. This Buprestid borer is native to China, Korea, Japan, Mongolia, the Russian Far East and Taiwan.

Life Cycle: The beetle has a one-year life cycle in Michigan. Adults emerge from infested trees beginning in late May and continuing through late June, with peak emergence during the first two weeks of June. Adult females lay 65 to 90 eggs during their lifespan of about 21 to 22 days. Eggs are deposited individually on the bark surface or in cracks, and hatch in 7 to 10 days. Larvae chew through the bark surface and feed on the phloem and outer sapwood. Larval galleries are erratically S-shaped and become wider aslarvae grow larger. The injured area of the sapwood can range from 4 to 20 inches. The insect overwinters as larva after excavating a shallow chamber in the sapwood. Pupation begins in until late April or early May, and may last 1 to 2 weeks. A conspicuous D-shaped exit hole, about 1/8 to 3/16 inch in diameter, is formed by the adult as it chews its way through the bark. Adults are about 1/2 inch long and 1/16 inch wide with a brassy, or golden green color; the wing covers are a darker, metallic or emerald green (there are many good photos available on the web).

Symptoms: Experience in Michigan is that infestations are difficult to detect until canopy dieback begins. The Michigan infestation was initially attributed to ash yellows or ash decline. As much as 30% to 50% of the crown may die in the first year and the tree usually dies by the second or third year of infestation. Infested trees die from the top to lower canopy, and trees of all sizes ranging from stems 2 inches in diameter to mature trees may be attacked. Aside from crown dieback, the most conspicuous sign of infestation is the D-shaped exit hole that the adult makes on the bark surface. These may be difficult to detect on rough barked trees. In addition to the exit holes, there is sometimes vertical splitting of the bark that extends 2 to 4 inches. Serpentine larval galleries are often observed beneath this injured bark. Branches die when they are girdled by encirclement of larval galleries. Sometimes epicormic shoots or branches will form near the margin of living an dead areas on the trunk.

There has been some speculation as to why trees are attacked by this pest. Recent drought years may have stressed trees making them more vulnerable to attack. Also trees already affected by ash yellows or ash dieback could be at risk for attack by the emerald ash borer. However, other evidence suggests that the borer attacked even healthy, vigorous trees that were well watered and fertilized. More research will need to be conducted to resolve this issue.

Hosts: In the Far East hosts are primarily in the genus Fraxinus, however species of Ulmus and Juglans are also attacked. In current outbreak in Michigan, the borer has only been found infesting ash trees. Species affected include white ash, F. Americana, green ash, F. pennsylvanica, black ash, F. nigra, and various ornamental varieties of ash.

Ohio’s Ash Resource: Forest inventory data indicate that there are over 279,000,000 ash trees in Ohio’s forests. Additionally, ash has been widely planted as a street tree and in other urban or suburban environments. In Toledo, about 8% of the street trees are ash trees. The northwest Ohio counties of Williams, Fulton, Lucas and Ottawa may be particularly at risk because of their proximity to the Michigan infestation. State and Federal agencies have been conducting surveys in these areas and no borers have been found in Ohio. The apparent decision by the Animal and Plant Health Inspection Service not to eradicate the borer means that we will likely be dealing with this insect for years to come. To prevent more insects and pathogens from entering the U.S. it will be necessary to assure that wood packing materials are adequately treated or that international shippers use alternative sources of packing materials.

Control Recommendations: Research is in progress to develop control recommendations. Options are presently limited to applying a systemic insecticide such as imidacloprid or bidrin. Imidacloprid can be applied as a soil injection, soil drench or as a tree injection. Bidrin can only be applied by tree injection. Quarantines enacted by the Michigan Department of Agriculture will hopefully slow the spread of the borer.

Web Resources: There are some great resources available on the web that can give you much more information about the borer and the Michigan infestation. Here are two sites with lots of information and photographs.

1.   Michigan Department of Agriculture:
www.michigan.gov/mda/ Use the search feature by typing “ash borer” to get specific information.

2.   USDA Forest Service Northeastern Area State and Private Forestry:
www.na.fs.fed.us/spfo/eab/ This site also has the pest alert publication