Emerald Ash Borer: A New Forest Threat

by Jim McGlone, Urban Forest Conservationist, Virginia Department of Forestry

(Conservation Currents, Northern Virginia Soil and Water Conservation District, Fall 2006)

The emerald ash borer (EAB) or Agrilus planipennis is a non-native beetle that eats and kills ash trees. EAB was first detected in the United States in 2002 near Detroit, Michigan and probably arrived in solid wood packing material from its native Asia. Since then, EAB has killed millions of trees in Michigan, Ontario, Canada, Indiana, and Ohio. EAB has also been detected in Illinois, Maryland and Fairfax County, Virginia.

Adult emerald ash beetles lay eggs on ash bark in the spring. Emerald ash borer adultThe eggs hatch into larvae that bore into the tree and eat the cambium under the bark, leaving S-shaped channels. The larvae pupate and overwinter under the bark or in the sapwood of the tree. In the spring, the adults emerge leaving behind a distinctive D-shaped exit hole in the ash’s bark to start the process again.

Although adults do eat ash foliage, it is the damage the larvae do to the cambium that kills the tree. The cambium is the layer of tissue that generates additional xylem and phloem. Food moves from the leaves to the roots through the phloem, and water and nutrients move from the roots to the leaves through the xylem. As the cambium is destroyed, the xylem and phloem also deteriorate. The leaves and branches in the canopy of the tree begin to die and, eventually, the entire tree succumbs. Infected trees are diagnosed by the canopy die back, distinctive D-shaped exit holes, and S-shaped channels under the bark. Other signs include bark splitting, sprouts at the base of the tree and increased woodpecker feeding.

Although there are reports that EAB attacks walnut and elm species in its native range of northwest Asia, the beetle is only known to attack ash species in the U.S. An exception is the mountain ash, a small tree of the eastern United States that is actually a member of the rose family. Common ashes in Northern Virginia include the green ash, Fraxinus pennsylvanicus, an overstory tree in stream valleys and floodplains, and the white ash, Fraxinus americana.

EAB is thought to spread in three different ways: flight of adult beetles; transport of infected ash timber, particularly firewood; and transport of infected nursery stock. Authorities are targeting all three in order to contain the EAB. Quarantines have been placed to prevent nursery stock and ash timber from being moved from infested areas. Unfortunately, transport continues to expand the range of this pest.

In 2003, nursery stock infected with the emerald ash borer arrived in a retail nursery in Prince George’s County, Maryland. To contain the EAB, the infected trees were destroyed along with all ash trees within one mile of the nursery. (Adult beetles are thought to move less than a mile from the trees from which they emerge.) Some of the infested Prince George’s County nursery stock was, however, planted near Wolf Trap Farm Park leading to a similar eradication effort in Fairfax County.

In Maryland and in Fairfax County, sentinel trees have been established around these eradication zones and are being monitored for EAB activity. Sentinel trees are created by girdling each tree to create stress. The stressed trees give off different chemicals and are a different color in ultraviolet light. If present, the beetles will recognize these signs of stress and be drawn to the sentinel trees because they are less able to fight off infection. In 2006, EAB once again appeared at the Maryland site, but has not been detected again in Fairfax County.

Maryland has extended the quarantine of ash products originating in Prince George’s County. Fairfax County has increased its monitoring for EAB by establishing more sentinel trees near the eradication zone and along the Potomac River.

You can help stop the spread of EAB by not buying firewood except from a known source or transporting firewood into Fairfax County.

Photo: David Cappaert, www.forestryimages.org.

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