A voracious Asian beetle that's killed millions of ash trees across North America has finally been spotted in Baltimore, posing a costly and difficult challenge for a city that stands to lose more than 200,000 of its most common trees to the exotic pest's onslaught. It could denude blocks lined with ash and cost the city millions of dollars to remove dead or dying trees from public lands, while homeowners may be forced to pay hundreds or even thousands to treat or replace their vulnerable trees.

City officials are developing a plan to deal with the emerald ash borer, which was found in June in traps set in ash trees in Druid Hill Park and near Fort McHenry, according to Craig Kuhn, an entomologist with the Maryland Department of Agriculture. The few metallic-green insects caught likely came from infested trees nearby, he said.

The city's tree lovers have been dreading the insects' invasion since they were found in Maryland 11 years ago at a nursery in Prince George's County. The borers have spread across Western, Southern and most of Central Maryland, including Anne Arundel and Howard counties. They showed up for the first time as well this summer in a trap in Carroll County.

"It was inevitable — we knew they would be getting here," said Erik Dihle, the city's arborist. "It's going to be a huge problem," he added, particularly along city streets where many ash trees were planted over the years. And he said it's likely to undercut a long-running effort to boost Baltimore's below-average tree "canopy," the amount of urban land shaded by leaves.

"Now the tidal wave is arching over us," said Sarah Lord, chair of the Baltimore City Forest Conservancy District Board. "Losing tree canopy when Baltimore is working hard to double it will be heartbreaking and expensive."

A 2009 survey by the U.S. Forest Service estimated there were 212,000 ash growing in Baltimore, nearly 9 percent of all the city's trees, said David J. Nowak, the project leader. A new survey is underway this summer, Nowak said.

Emerald ash borers, so named because they attack only ash trees, were discovered in the United States near Detroit in 2002. Experts believe they hitchhiked from Asia in wooden packing material brought in by ship or plane, and likely had been here for years before being detected.

The female beetles lay their eggs on the bark, and newly hatched larvae bore into the tree, where they disrupt its circulation system, starving it of water and nutrients. Trees often die within a few years of infestation.

The borers have been spotted in 24 states and two Canadian provinces. They have killed tens of millions of ash trees in the Midwest and Northeast, with experts estimating that 99 percent of the ash have been lost from forests in southeastern Michigan, where the infestation was first detected.

Besides being popular shade and landscape trees, ash are commercially valuable, providing lumber for flooring, tool handles and baseball bats. The U.S. Department of Agriculture has warned that the borers, if unchecked, could cause tree losses in the tens of billions of dollars nationwide. With 6 million or more ash trees estimated in the Baltimore metro area, federal experts have projected the losses could exceed $200 million.

Efforts to eradicate the borers have failed, including pre-emptively cutting down 42,000 ash trees several years ago around the Prince George's nursery where the first bug was found in Maryland. The beetles can fly a half-mile from tree to tree, and experts believe some have moved farther, perhaps by traveling in firewood. Maryland has imposed a quarantine on transporting firewood and nursery stock from infested counties.

Dihle said the loss of thousands of trees would set the city back from reaching its goal of doubling tree cover by 2037. Baltimore's tree canopy now shades about 27 percent of the city, while nationwide, trees cover 35 percent of urban land, according to the U.S. Forest Service.

Dihle said he's drafting a response plan for Mayor Stephanie Rawlings-Blake.

There's no way to save every ash tree on public land in the city, Dihle said, particularly those in heavily wooded areas like Leakin Park. Instead, he said, he's focusing on what to do about the ash lining city streets and in the more developed parks, a number he put at around 5,000 trees.

Dihle said he's looking at treating the biggest and most visible ash trees along streets and in parks, and would only use a pesticide commonly accepted as safe for bees and other beneficial insects. But he said he's also taking a hard look at gradually replacing the city's ash trees, since there's no known way to eradicate the borers.

"Do we want to artificially keep a population of ash trees in Baltimore, knowing very well that an insect that devastates these trees is here to stay?" he asked.

But cutting down a mature tree can cost as much as $1,000, meaning the city would need up to $2.5 million to take out 5,000 ash trees.

"We're looking into the millions of dollars in removal and replanting," Dihle said. Though the cost could be spread over a few years, Dihle said his division's budget is $3.2 million, so the city would likely need to seek help from the state and private sector.

Private arborists and tree-care experts say there's hope for saving many if not all the city's ash, though it requires a prompt, concerted campaign to treat seemingly healthy and even moderately infested trees with pesticide every year or two.

"We know how to stop this pest from destroying urban landscapes," said Rob Gorden, director of urban forestry for Arborjet, a Massachusetts-based company that sells an injectable tree pesticide under the trade name TREE-age.