A five-year U.S. Department of Agriculture study program will monitor the effectiveness of protecting specimen ash trees in Charlottesville from the emerald ash borer at no cost to the city.
Th emerald ash borer is an invasive insect from Asia that damages and kills ash trees in two to three years. It is present in Charlottesville and surrounding areas. The insect’s prevalence is endangering the city’s ash trees.
The metallic-green insects emerge from May through August. Once the adults breed, the eggs are then laid on the bark, and once the insects hatch, they burrow under the bark. As the larvae feed, they create serpentine galleries that disrupt the flow of food and kill the tree.
In May, the USDA approached the city to propose the release of a biological control to help kill emerald ash borers. The USDA is providing parasitoid wasps that lay eggs in the eggs of emerald ash borers, killing them and slowing the spread of the insect.
The study is being conducted on trees in the Ragged Mountain Natural Area.
“USDA reached out to ask if we were interested in biological control of emerald ash borer, and we talked about doing a study. They were looking for, as far as density, forested acres, and we found a good candidate at the Ragged Mountain Natural Area,” said Mike Ronayne, an urban forester with Charlottesville’s Department of Parks and Recreation.
The wasps do not sting and are all female. They only produce clones of themselves. Release of the wasps has been performed in other parts of Virginia and 28 other states and has been evaluated as safe.
“Once a female lays an egg without being fertilized, only females will be reproduced,” Patrick Boos, a plant health safeguarding specialist with the USDA, said during a city Tree Commission meeting June 1.
Boos said the wasps are “specialists” and almost exclusively prey on emerald ash borer eggs.
“USDA has reported that there weren’t any unintended consequences so far, but, I mean, who knows, it’s pretty early in the process. But so far we haven’t had any other insect species or anything else that it was attacking,” he said.
Boos said the goal of this type of treatment is primarily to save younger trees.
“It has the best success on younger trees. So, a lot of the trees that are already infested that are larger … are probably going to die anyways if they’re that far gone. It’s more to protect the younger trees that are going to come up after that,” he said.
In 2016, the parks department handpicked 32 of the city’s ash trees to treat preventatively for emerald ash borer and be retained.
“Back in 2016, we began treating individual trees with a pesticide called emamectin benzoate to preserve 32 of our best trees. It’s kind of cost prohibitive and you have to have the right circumstances to treat trees individually like that,” Ronayne said. “So, this application is more for a forestry application of a forest instead of individual trees.”
“I think we were probably an easy candidate to work with … we had a couple options that are really suitable to study,” he said.
Ronayne said that while a tree sometimes can be saved early on after it’s been infected, it is more likely to survive by preventing the infection from happening in the first place.
“Depending on the infection, if it’s very minor, the tree can be treated with a trunk injection of the emamectin benzoate pesticide. And you can sometimes save the tree, but the tree needs to be in good health to begin with. It’s difficult without really climbing to the canopy and evaluating it to know how bad the infestation is,” he said.
Boos said there previously was a federal quarantine on emerald ash borers to try to limit it the insects to certain areas, but that hasn’t been successful, so the USDA has focused on biocontrol, including using parasitoid wasps.
“The whole idea of the biocontrol is that it’s not going to eradicate the emerald ash borer but it’s going take it to a point where it’s not killing the trees outright,” he said.
Boos said that in emerald ash borers’ native China, the ash trees aren’t as affected by the invasive insect, and this may be due to the presence of parasitoid wasps.
“Chinese ash is rarely affected by any emerald ash borer … it’s just such a small amount of damage compared to what we have, and they have attributed it to these different parasitoid wasps that are attacking emerald ash borer,” he said.
Boos said ash trees in the United States have a disadvantage because there isn’t a native predator to the emerald ash borer like China has, which is why bringing in the wasps may work.
On June 11, the USDA placed the first set of wasps in the trees.
“They had to do it June 11 for biological reasons for the development of the wasp and the relationship they have with developing emerald ash borer,” Ronayne said.
On June 1, the Charlottesville Tree Commission voted to endorse the study. City staffers evaluated the proposition and said they found that the benefits outweigh the risks.
“This isn’t a silver bullet. We’re hoping this is going to be a long-term solution years down the road,” Ronayne said. “The sad reality is we have to deal with a lot of dead, dying ash tees now.”
“Our forests are 1-2% ash trees. Then we have places like the disc golf courses and floodplains where there’s much higher concentrations. When the trees die, impacts are going to be very noticeable,” he said.