In the Garden: Ashes to Ashes?

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By Charles Kidder

Ashes, the genus Fraxinus, are widely distributed over the Northern Hemisphere.  Most often medium to large deciduous trees, a few species are shrubby or evergreen.  As members of the family Oleaceae, they’re related to the olive trees of the Mediterranean, as well as forsythias and lilacs.  Useful shade trees with a fine-textured leaf that decomposes readily, ashes aren’t generally grown for their flowers.  (The Flowering Ash, Fraxinus ornus of Asia, would be one exception.)

The most common ash in Virginia, the White Ash (F. Americana), is a large tree found on good upland soils. It is known for attractive yet subtle fall color, often a mix of yellow, purple and red on the same tree. Autumn coloration usually appears earlier than most other tree species and can be very fleeting.

Green Ash (F. pennsylvanica) grows mostly on bottomlands or soils with a higher pH. It is arguably not as handsome as White Ash, with less brilliant fall color. But it is a much more adaptable species, tolerating a wide range of conditions, leading to a tremendous amount of planting in the landscape.

Until recently, that is.

In the early 1990s, an unwelcome invader arrived from Asia. The Emerald Ash Borer, a bright green, half-inch long insect presumably arrived via packing crates made from ash wood of Asian species. It began munching on and ultimately killing ash trees in metro Detroit, although it wasn’t identified as the culprit until 2002. In the following decade, the EAB has spread to approximately 22 states, as well as Ontario and Quebec. The modifier “approximately” is needed, since the bug is advancing rapidly and several states have reported its arrival just in 2013.

Left to its own devices, spread would not be very rapid, but the EAB has apparently moved long distances on firewood, nursery stock, or just by hitching a ride on cars.  Ash trees have now been decimated in the Midwest, and the borer has recently reached the Shenandoah National Park, as reported in the Gazette in September. So for Virginians, is there hope for this important forest and landscape tree?

Currently only a few locations in Virginia have confirmed infestations of EAB, but they are widely scattered throughout the Commonwealth. Therefore, the state and the federal government have declared all of Virginia to be in quarantine, meaning that ash wood can not be transported to non-quarantine areas in other states without proper paperwork.

While the adult EAB consumes ash foliage, this is not where it does the most damage. Eggs laid on the tree’s bark hatch, and the tiny larvae burrow through to reach the cambium, the layers that conduct water and nutrients throughout the tree.  They then proceed to eat their way around the tree, leaving a sinuous trail of destruction. Over the course of two to five years, the ash tree will slowly die. The first sign of trouble will be the loss of some foliage in the canopy, with increasing loss as time goes by.  Other signs of damage are splitting of the bark, increased woodpecker activity, vigorous sprouting of branches from the trunk, and D-shaped emergence holes on the bark.

With tens of millions of ash trees already dead, and with a total estimated U.S. population of eight billion, what can be done to halt the EAB? Other than prevention, i.e. not moving firewood or nursery stock out of a locality, there are two prongs of attack: chemical and biological.

Chemicals can be applied to trees in a variety of ways: to the soil, either by simple drenching or by injection; sprays directly to the lower tree trunk; sprays to the tree’s upper trunk and branches; or by injection through the bark and into the living tissue.  From my research, the latter method seems to be the most effective and also poses a relatively low risk to the environment, although it does slightly wound the tree.  Most of these methods can be performed only by professionals, and that is the only option I would countenance.

But when should you consider treating a tree?  Well, don’t rush out and do it today, thinking you’re going to head off the advancing bug. The trunk injections are effective for only two years, so performing them too soon is just pouring money down holes in the tree. Certainly, if your neighbor’s trees are being attacked, it’s time to act, but if the EAB is more than fifteen miles away, you should wait.

Granted, this is pretty much a game of chicken with the borer. Luckily, a tree can withstand some degree of borer damage, yet still recover when injections are done.  The tree may decline for another year, then start to repair the damage.  If the tree has lost more than 50 percent of its canopy, it may be too late, however.

Chemical treatment may be a viable option for selected trees, but given that there are billions in the forests, biological control should be a better long-term option.  Three species of parasitic wasp have been brought in from Asia, where they normally prey on EAB larvae and eggs.  Tiny and non-stinging, the wasps pose no threat to humans. Currently the wasps are being evaluated for effectiveness in a five-year study concluding in 2014, and preliminary results are encouraging.

Initially the wasps were being reared in labs and being supplied with EAB larvae to attack, and now some are being released into EAB-infested areas. Like any parasite, they need their prey to survive, so releases into “clean” areas will be of no value.  Hopefully, the wasps can follow the borers, build up a self-sustaining population, and keep the borers under control.

Purple traps have been put out to monitor the EAB spread, but if you suspect they are present on your property, call your county extension agent or an arborist.  You can also contact the Virginia Department of Agriculture and Consumer Services at 804-786-3515. I recently spoke with Debra Martin of this office. Although recognizing the gravity of the EAB situation, I would describe her attitude as cautiously hopeful for its control. Let’s all root for those little wasps.