Introduced pests and diseases put the fear of God into plant biologists because these interlopers have not co-evolved with their hosts or predators in a way that keeps their numbers in natural balance.
The classic example of a nightmarish result is the chestnut blight, a fungal disease from Asia that arrived in the early 20th century and proceeded to erase the American chestnut tree as the dominant hardwood species of the Eastern deciduous forest.
Sometimes the anticipated devastation doesn’t pan out. When the gypsy moth arrived in Washington a few decades ago, fears that it would ravage the city’s urban canopy didn’t come to be. The more recent appearance of the brown marmorated stink bug didn’t end fruit and vegetable cultivation as we know it, and even Dutch elm disease hasn’t eradicated the majestic American elm tree, which is making a comeback thanks to scientists who have bred resistant clones. Valiant efforts are underway to restore the chestnut by crossing Asian genes into resistant hybrids, but this has been a slow and plodding task.
We are now in another age of obliteration. The new victim is the ash tree, whose straight grained wood is used for baseball bats and tool handles. Its days in natural areas are numbered, and specimen survivors in gardens will need constant treatment if they are to have a chance of hanging on.
The culprit is the emerald ash borer, a beetle that is just half an inch long but eye-catching in its iridescent green panoply. While the adult is jewel-like, its larva is a grotesque grub that chews its way into the bark and eats through the tree’s vital plumbing. A single tree can have as many as 10,000 of these maggots — it’s enough to make your flesh crawl.
The emerald ash borer — EAB in forester speak — was discovered near Detroit in 2002 and is thought to have arrived as a stowaway on crated cargo from East Asia.
At first, experts believed it could be contained and possibly eradicated by isolating infestations, removing trees in those zones, and banning the hauling of firewood and timber.
When it was first detected in Maryland in 2003, “there was a valiant attempt” to check it, said Patrick Harwood, an urban forester with the Montgomery County Department of Parks. “But we had so little knowledge about the insect.” Entomologists thought the adult could fly one mile but later discovered the distance was more like 15 miles.
At last count, the pest is found in 33 states, the District and three Canadian provinces. The tally of dead trees measures in the hundreds of millions.
In suburban Maryland, the uncontained destructive spread of the borer had become evident by 2015.
I asked Harwood to show me dead trees in the county’s Rock Creek Stream Valley Park, which extends approximately 18 miles north of the District to near Gaithersburg. Here, ash death is manifested three ways. The first is the absence of trees on either side of the highways and trails running through the park (where they’ve been taken down).
The second is the way dead trunks and boughs arch up in the distance, through the tapestry of the woodland canopy — these are trees yet to be felled or that will be left because they are tucked away from where people use the park.
The third iteration is, in its own way, the most poignant.
In new clearings in the woods, huge logs lie in long sections, many in standing water. Some still have their bark, others are naked, laced with the twisting shadows of the tunnels of the larvae. The water is ponding, Harwood said, because the trees can no longer suck the moisture from the land. If there were an elephants’ graveyard for trees, this would be it.
Most of the trees are green ash, but some are of another flood plain species, the black ash. The latter has beautiful blocky bark, like a persimmon’s, but it was never considered pretty enough to make it into the garden. A more familiar species, the towering white ash, is a tree of drier, upland areas.
The parks department has removed more than 4,000 ash trees over the past five years, Harwood said, and knows of another 2,500 due for removal. Some are as tall as 100 feet. But the exact number is unknown. There might be as many as 20,000 more in the heart of the wooded areas that “we aren’t going to touch” because of their remoteness, Harwood said. “We are only removing the trees we absolutely have to.”
It is not unusual for a homeowner next to a park to report a dead tree. When Harwood arrives, he finds dozens of ash trees that are dying or dead.
It can take six months to three years for a tree under attack to die, he said. One sign that a tree is in trouble is the appearance of blond blocks on the bark. This is caused by the chiseling of woodpeckers in search of a meal.
Both white and green ash found their way into our gardens, either as native trees that just showed up and grew, or were planted. Their ability to endure both drought and flood made them a useful shade tree. They became a popular street tree, and hybridizers took advantage of their inherent seedling variability to improve the species. Summit is pyramidal with strong, fall yellow color, Newport has a strong central leader and more lustrous foliage. One prophetically called Emerald has glossy leaves and golden autumn color.
What should homeowners with a prized ash tree do? Get professional help, quickly. Their only hope, if the tree is still healthy, is a recurring regimen of pesticide application. In the end, it may come down to a financial calculation — the cost of annual professional treatment against the bigger one-time price of tree removal. And trees that look healthy may be lousy with borers. If you are swatting away woodpeckers, you may be in the market for a certified arborist.
We clearly are witnessing an ending, not unlike the time in the 1970s in England when miles-long and centuries-old avenues of elm trees were felled on grand estates because of Dutch elm disease.
Harwood can think of nothing positive about the loss of America’s ash trees.
“It’s costing money, it’s reducing diversity, reducing storm water filtration, eroding stream banks, and every insect and animal dependent on ash has now lost its host,” he said. “The forest is a victim of globalization.”