by Dan Crescenzo
While visiting one of Oconee River Land Trust’s conservation easements in Newton County this year, I was struck by the dozens of ash trees that had been killed by the emerald ash borer (Agrilus planipennis). Only a few short years ago, every one of these trees had been healthy. Seeing the damage firsthand, in a forest I was already familiar with and quite fond of, was especially disturbing. Concerned for the other forests ORLT protects, I set out to understand more about the borer and how it might be stopped.
Ash trees are native to Georgia – in the Piedmont, white ash trees (Fraxinus americana) grow as scattered individuals on moister sites above streams, particularly those with more basic soils – and green ash (Fraxinus pennsylvanica) grows on floodplains, in bottomlands along rivers and larger streams, sometimes as a dominant species in the canopy. Both species reach 80 feet at maturity and are recognizable in winter by their opposite branching pattern, and female trees, by clusters of winged seeds that hang on the twigs well into winter.
Ash trees are both useful to human beings and an important part of the ecosystem. Because ash wood has the unique quality of being both lightweight and absorbing shock well, it has famously been used for baseball bats and hockey sticks, as well as tool handles, flooring, and furniture. In the eastern United States, ash supports 150 species of butterflies and moths, as well as hundreds of other native insects, many of which only utilize ash or fringe tree (Chionanthus virginicus), a closely related understory species. Ash also provides food and shelter for a variety of birds.
Since its accidental introduction into the United States in Michigan in 2002, the Emerald ash borer has decimated ash populations in the United States. The adult borer – a metallic green beetle about a half an inch long – lays its eggs beneath the bark of host trees. Its larvae then hatch into small white grubs that eat the nutrient and water carrying layers of the tree, forming an extensive network of tunnels. After 2 - 4 years of this grub activity, infected trees die. The borer colonizes diseased and healthy trees alike, killing all or nearly every ash tree in the forests it attacks. Occasional predation by birds has not been sufficient to control its numbers.
In just under 20 years, this insect has spread to almost every state east of the Mississippi, including Georgia, and as far west as Colorado, and all of the provinces in southeastern Canada, leaving a trail of destruction in its wake. The borer was first observed in DeKalb County in Georgia in 2013 and has since spread to 37 counties in the state, including every county in the Atlanta metro area, most counties in the mountains, as well as Barrow, Newton, and Walton Counties. It is expected to spread throughout the state.
The decline in ash populations has been precipitous enough that in 2017, the International Union for Conservation of Nature (IUNC) moved both white ash and green ash into the conservation status “critically endangered” – where they join another species whose demise from eastern forests is already well-known – the American chestnut. As ash trees disappear from America’s forests, so, too, will the species that depend upon them, leading to a reduction in species richness and diversity of our forests.
The borer’s spread had been facilitated by accidental movement of ash wood, especially firewood containing the beetle eggs or larvae, across county and state lines. Until recently, both the federal Department of Agriculture and the Georgia Forestry Commission (GFC) had quarantines in place to slow the spread of the beetle by prohibiting transport of ash trees and materials. But these efforts have proved largely ineffective, so this year, following the federal Department of Agriculture’s removal of national emerald ash borer quarantine regulations, the statewide quarantine was likewise repealed. GFC’s esources will now be devoted to managing beetle populations in other ways.
Researchers are following two strategies in this regard: cultivating resistant varieties and biological controls. A small percentage of ash trees appear to have some resistance to the beetle. Attempts are already under way to use these individuals to create more beetle-resistant varieties that can then be used to re-populate forests. Several species of parasitic wasps from Asia are also being studied. These have been established in some limited areas, where the wasps effectively control beetle populations in younger ash trees by devouring their eggs or larvae. However, older trees in these areas are less protected by the wasps and continue to decline. It remains to be seen whether a combination of these strategies may prove effective in slowing the decline of ash trees to America’s forests.
In the meantime, although it is no longer required by Georgia or federal regulations, landowners can still take care not to transport any firewood or other woody debris from ash trees off their property or onto it and report any sightings of the borer to GFC. Individual trees can also be preventatively treated with pesticides, but the treatments are too labor and resource-intensive – typically involving trunk injections and high-pressure soil treatments – to be feasible for entire forests. Whether or not the decline of ash in American forests can be reversed, it vividly illustrates the need for continued vigilance against the introduction of further potentially invasive species. The easiest way to prevent invasive species from damaging ecosystems is to not introduce them in the first place.
Check out the rest of the ORLT Fall 2021 Newsletter here.