The emerald ash borer, an invasive Asian insect deadly to ash trees, has turned up in trees and traps in three locations in Howard County, accelerating the threat to hundreds of thousands of valuable shade trees in Baltimore and its suburbs.
The beetle's move from Southern Maryland years earlier than expected means the state must step up efforts to slow the spread toward Baltimore, where ash trees make up about 10 percent of the city's tree canopy, and to minimize the costly damage.
"This is a watershed event," said University of Maryland entomologist Mike Raupp. "Baltimore is right up against the decision" of how to manage an infestation. "It has one of the highest populations of ash trees of any major city."
Shady and tolerant of urban abuses, ash trees have long been popular for planting along the city's streets and in its back yards. There are perhaps 300,000 in the city proper, and about the same number in the near suburbs, according to Raupp's estimates. Washington may have 50,000, and Annapolis about 2,000.
The cost of removing and replacing the trees once they're dead can run between $1,000 and $2,000 each.
After the discovery last week, the Maryland Department of Agriculture immediately expanded its ash wood quarantine zone, which now bars the transport of all ash wood, ash nursery stock or any hardwood firewood out of Howard, Charles or Prince George's counties.
"That's one of the primary ways the ash borer can be moved," said Carol Holko, the MDA's program manager for plant protection and weed management. "Frankly, it's just a bad idea to relocate firewood in general, from anywhere. You just don't know what's in it."
Nurseries and plant dealers in Howard County have been notified of the discovery, advised of the quarantine, and asked to remind residents not to transport any firewood from the county.
The first detection was on June 6 in the northwestern part of Howard County, where a homeowner had called in a tree expert to diagnose signs of stress in her trees. The arborist spotted signs of the ash borer, and immediately notified the Department of Agriculture. Maryland officials got confirmation from the U.S. Department of Agriculture two days later.
"That's closer to Baltimore than we'd like it to be," Holko said. "There are a lot of ash trees in Baltimore. It's one of many areas we're trying to protect."
The initial survey of the property found more infested trees. "It looks like two fairly heavily infested trees, and then it fans out to trees that aren't that infested," Holko said. "How widespread it is, we don't know."
But soon after that detection, emerald ash borers were found in two of the state's "purple prism" monitoring traps in Howard — one at the Dorsey Run Business Park in Jessup; the other at the Owen Brown Shopping Center in Columbia.
A more thorough survey in Howard County and beyond is planned, Holko said. "It's fairly early in the trapping season. We trap through August, so we may get additional counties."
But the bad news already means that hundreds of thousands of shade trees in the Baltimore area will be threatened many years earlier than once predicted.
"Last season the data indicated [the pest's spread] had gone from one mile a year to something much more dramatic; it became exponential," Raupp said. Once predicted to reach Baltimore as early as 2052, the new data accelerated that to 2022.
Now, with the discoveries in Howard County, Raupp said, "it's simply going to be there much quicker."
As ash trees die, residents will also lose the benefits they provide in reduced cooling costs, pollution mitigation, carbon sequestration, storm water catchment and added property values. "The benefits of these trees are enormous," said Raupp, who has been developing a computer model to estimate them: "For Baltimore about $20 million a year."
The emerald ash borer is a bright green beetle, about a half-inch long. It first appeared in the United States in 2002, in a Michigan nursery. From there it spread aboard ash trees shipped illegally out of the federal quarantine that was established there.
That's how it reached Maryland, turning up a year later at a nursery in Brandywine, in southern Prince George's County. By 2008 it was found in Charles County. It has also spread to 14 other states, including Pennsylvania, West Virginia and Virginia.
"I don't believe this is a natural rate of spread," Raupp said. "I believe this is human-assisted." That's not to imply it's deliberate, he added. It's more likely to have ridden in a load of firewood or other tree debris transported by truck.
Once released, the adult insects nibble on leaves and lay eggs, but their young — the larvae — burrow beneath the bark, interfering with the trees' ability to transport nutrients. That causes the trees to wither from the top down and die. Infestation is 100 percent lethal.
The state's strategy for fighting the pest once included aggressive tree removal. It has since evolved into one intended to slow the bug's spread with close monitoring and quarantines, allowing more time for new tools to emerge.
"There are some good insecticide products now," Holko said. "And we've had some bio-control efforts in Clinton and Brandywine."
The "bio-controls" are parasitic wasps whose larvae feed on ash borer larvae in their native Asian range. Maryland began deploying them in limited numbers in 2009, and some have begun to survive the winter, reproduce and attack the ash borers.
"It's not going to be a silver bullet," Holko said, "but it is going to be one of our tools."
But it's unlikely anything the state or federal government does will eliminate the pest, or prevent its spread. "What we're doing is trying to slow it down, and let communities and the forest products industry, and anyone affected by this to prepare," she said.
The infested trees in Howard County will eventually be removed, Holko said. But "we're not taking any action until the fall, when they [the insects] are safely tucked up in the trees as larvae. If we removed the trees [now], they'd go flying off looking for new [tree] hosts … That would be counter-productive."
In the meantime, agriculture officials urge residents to learn to identify ash trees in their yards and neighborhoods, and to spot the signs of infestation — die-back at the treetops, the growth of new green shoots at the bottoms, and D-shaped holes in the bark where the larvae emerge. Residents who suspect they have an ash borer infestation, are asked to call the state Department of Agriculture at 410-841-5920.
"If there are other pockets of infestation, we sure want to know where they are," Holko said.
Raupp said the bug's spread to Howard County was inevitable. He likened the spreading infestations to the American chestnut blight and the Dutch elm disease, which eventually wiped out nearly all American chestnuts and elms in the last century.
"The Maryland Department of Agriculture, much to their credit, put up what I would consider an heroic battle to try to keep the lid on since 2003," he said. "What this signifies is that the levee is broken now. … We knew it had to happen, sooner or later. Everybody was hoping it was going to be later rather than sooner, but I think that possibility is gone now."