PORTLAND, Maine — Yellow-clad workers with chainsaws dangling from their belts ascended into the tallest branches of a pair of stately ash trees just off Park Avenue on Monday morning. Then they began cutting: sawdust flew, limbs fell and the decades-old trees came down.
It was a sad sight to see, but for the best.
The imposing hardwoods were dying and not going to leaf out this spring due to an infestation of emerald ash borer. The invasive pest is currently wreaking havoc on the city’s 600-or-so inventoried ash trees, and officials expect hundreds will die. Many other ash trees will be cut down in an effort to keep the devastating insects from spreading.
In all, the city’s Forestry Division expects 400 ash trees — about two-thirds of them — will be gone within two years.
“It’s a tough pill to swallow,” said Portland City Arborist Mark Reiland. “And a lot of the ash trees were planted to replace elms wiped out in the 1970s.”
At that time, a fungus commonly referred to as Dutch elm disease swept through Maine. The emerald ash borer, while also devastating to infected trees, is an insect, rather than a fungus.
First discovered 22 years ago in the U.S. in Michigan, the native Asian beetle made its way to Maine in 2019. The borers are now found mostly in southern Maine and Aroostook County. They feed almost exclusively on members of the ash genus.
Adult beetles consume the foliage while the larvae bore into the tree and devour vascular tissues within the sapwood. The larval feeding leads to rapid decline and death of affected trees. Mortality rates exceed 90 percent within about five years of the initial infestation.
Reiland said he’s fighting the ash borers by cutting down infected trees and also by trying experimental treatments on others.
Before cutting ash trees along Portland’s streets and in city-owned parks, Forestry Division workers first mark them with signs. The laminated placards warn people of the tree’s fate and explain why it has to come down. Then, once the cutting is done, a similar, past-tense sign reveals why there’s nothing but a stump where a tree used to stand.
“We’ve gotten quite a few angry phone calls about missing trees, which is good,” Reiland said. “It means people are paying attention to the urban forest.”
Ash trees cut down this year likely won’t be replaced until next year, he said.
Instead of replacing them with one species, as was often done when ash supplanted city elms, a variety of tree species will be used — in case another pest or disease shows up with a taste for a single kind of tree.
“There’s no silver bullet, it’ll be a whole smorgasbord,” he said, “including poplar, black gum and hackberry.”
The new plan will follow the 10-20-30 biodiversity rule. It states that an urban tree population should include no more than 10 percent of any one species, 20 percent of any one genus or 30 percent of any family.
According to a city press release, Portland hopes to receive federal funding to replace the trees.
Along with the cut-down plan, the city is also experimenting with two scientific measures to save some ash trees, mostly along the Presumpscot River at the Riverton Trolley Park.
Last spring, city forestry workers chose 142 ash trees for stem-injected insecticide treatments. Of those, 114 responded well to the treatments. But, Reiland said, the injections will have to continue, every two or three years, for the lifetime of the tree. Thus, it’s not a practical solution for all of the inventoried ash trees. But forestry workers can likely continue to do so for the Trolley Park ashes.
“We already do the same thing for a handful of legacy elm trees,” Reiland said.
The insecticide Reiland’s crew used is emamectin benzoate, which is widely used in the U.S. against the ash borer.
Along with the pesticide, city forestry workers also released non-stinging Asian wasps with a taste for the ash borers on seven more trees. But it’s too soon to tell how much of an effect the insects have had in saving them.
