Will ash trees follow fate of chestnut?
By Wood Johnson
A century ago, folks likely looked upon American chestnuts and thought it would be a shame to lose those beautiful, stately trees. That’s what came to my mind recently while looking across a beautiful hardwood bottom heavily stocked with century-old green ash trees.
Decades ago, a nonnative fungal pathogen caused chestnut blight and wiped out the American chestnut. Today, the emerald ash borer (EAB), a nonnative invading beetle from Asia and eastern Russia is on pace to do likewise to ash trees throughout North America.
Introduced near Detroit in the mid to late 1990s likely from infested wood products originating in Asia, EAB has since left millions of dead ash trees in its wake. It is now confirmed in 35 states and the District of Columbia.
In Louisiana, EAB was first detected in Webster Parish in 2015. It is currently established in Bienville, Bossier, Caddo, Claiborne, Jackson, Lincoln, Morehouse, Ouachita and Union parishes. As it continues its inexorable march southward, it may be worthwhile to review the EAB and its status in Louisiana.
The EAB is a woodboring beetle that specializes feeding on and eating on hosts within the Oleaceae family (most notably green and white ash in Louisiana). Although only a pest of stressed trees in its native range, North American Oleaceae did not co-evolve with EAB. Even healthy hosts’ natural chemical defense mechanisms cannot defend against EAB. It may take successive infestations spanning several years, but the repeated injury of larval feeding causes certain death.
Adult EAB measure half an inch long and are a striking green hue. Emerging adults feed briefly on ash foliage before mated females laying eggs in the bark crevices of host trees. The immature larval stages last from approximately June to April of the following year but are cryptic and one must look beneath the bark to observe the white to pale yellow, segmented larva.
Early instar larvae are small and difficult to detect, but as summer wanes, larvae approach their maximum length (about 1.25 inches). Their distinctive bell-shaped segments and characteristic serpentine feeding galleries in the cambium layer can be used to distinguish EAB from cohabiting insects.
Recent inventories estimate ash accounts for only 3 percent of the total trees in Louisiana; however, green ash/pumpkin ash routinely comprise 30 percent or more of our extensive bottomland hardwood forests. The questions, of course are is anything being done about it and is there anything the forest landowner can do in preparation?
The Louisiana Department of Agriculture & Forestry (LDAF), Natural Resource Conservation Service (NRCS), U.S. Forest Service (USFS) and LSU AgCenter personnel conduct annual trapping surveys and routinely investigate ash mortality to delineate EAB spread in Louisiana. Attempts by USDA and university partners to establish three biocontrol agents — parasitic insects that feed specifically on various EAB life stages — that could slow the EAB invasion are underway, but preliminary results indicate these species are having little impact on EAB populations in north Louisiana research sites.
Although several systemic insecticides are registered and very effective for protecting high value trees in an urban setting, such treatments are uneconomical for forest management. Therefore, it is important forest managers know what current ash stocking levels are, what will remain following the wave of the EAB invasion and plan accordingly.
In some areas of north-central Louisiana it is too late. Green ash-rich stands being monitored in Webster Parish already have seen 80 percent or greater mortality. Bottomland forests in the northern Red, Ouachita and Mississippi River Basins are at immediate risk.
Landowners in central and southern Louisiana should be alert for and report any unusual, significant numbers of declining ash trees. Inspect dead and dying ash for D-shaped exit holes (created by the emerging adults) and the serpentine larval galleries beneath the bark.
At present, LDAF is enforcing a quarantine of ash logs and wood products, and all hardwood firewood, restricting the export of ash wood from the 10 known infested parishes listed above (unless these products have been treated to kill EAB) to non-infested parishes. Barring a human-aided introduction of EAB to southern Louisiana, the quarantine should aid forest landowners with economic objectives by slowing EAB invasion and allow ash more time to grow into merchantable classes.
If you would like additional information on the emerald ash borer, several helpful sources are available online. A starting point for general information about the pest, invasion history and status and treatment options for the urban setting is www.emeraldashborer.info.
Consult with foresters, LSU Cooperative Extension parish agents, or Louisiana natural resource professionals if you seek advice on managing the ash in your forest or suspect you may have EAB. Lastly, in this time of global markets and trade, be mindful of transporting any untreated wood products and spreading the pests that may be hiding within.
(Wood Johnson is an entomologist for the U.S. Forest Service at the Alexandria Forestry Center.)